// Copyright 2015-2020 Parity Technologies (UK) Ltd.
// This file is part of OpenEthereum.

// OpenEthereum is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.

// OpenEthereum is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// You should have received a copy of the GNU General Public License
// along with OpenEthereum.  If not, see <http://www.gnu.org/licenses/>.

//! A blockchain engine that supports a non-instant BFT proof-of-authority.
//!
//! It is recommended to use the `two_thirds_majority_transition` option, to defend against the
//! ["Attack of the Clones"](https://arxiv.org/pdf/1902.10244.pdf). Newly started networks can
//! set this option to `0`, to use a 2/3 quorum from the beginning.
//!
//! To support on-chain governance, the [ValidatorSet] is pluggable: Aura supports simple
//! constant lists of validators as well as smart contract-based dynamic validator sets.
//! Misbehavior is reported to the [ValidatorSet] as well, so that e.g. governance contracts
//! can penalize or ban attacker's nodes.
//!
//! * "Benign" misbehavior are faults that can happen in normal operation, like failing
//!   to propose a block in your slot, which could be due to a temporary network outage, or
//!   wrong timestamps (due to out-of-sync clocks).
//! * "Malicious" reports are made only if the sender misbehaved deliberately (or due to a
//!   software bug), e.g. if they proposed multiple blocks with the same step number.

use std::{
    cmp,
    collections::{BTreeMap, BTreeSet, HashSet},
    fmt,
    iter::{self, FromIterator},
    ops::Deref,
    sync::{
        atomic::{AtomicBool, AtomicU64, Ordering as AtomicOrdering},
        Arc, Weak,
    },
    time::{Duration, UNIX_EPOCH},
    u64,
};

use self::finality::RollingFinality;
use super::{
    signer::EngineSigner,
    validator_set::{new_validator_set_posdao, SimpleList, ValidatorSet},
    EthEngine,
};
use block::*;
use bytes::Bytes;
use client::{
    traits::{ForceUpdateSealing, TransactionRequest},
    EngineClient,
};
use crypto::publickey::{self, Signature};
use engines::{
    block_reward,
    block_reward::{BlockRewardContract, RewardKind},
    ConstructedVerifier, Engine, EngineError, Seal, SealingState,
};
use error::{BlockError, Error, ErrorKind};
use ethereum_types::{Address, H256, H520, U128, U256};

use ethjson::{self, uint::Uint};
use hash::keccak;
use io::{IoContext, IoHandler, IoService, TimerToken};
use itertools::{self, Itertools};
use lru_cache::LruCache;
use machine::{AuxiliaryData, Call, EthereumMachine};
use parking_lot::{Mutex, RwLock};
use rand::rngs::OsRng;
use rlp::{encode, Decodable, DecoderError, Encodable, Rlp, RlpStream};
use time_utils::CheckedSystemTime;
use types::{
    ancestry_action::AncestryAction,
    header::{ExtendedHeader, Header},
    ids::BlockId,
    transaction::SignedTransaction,
    BlockNumber,
};
use unexpected::{Mismatch, OutOfBounds};

//mod block_gas_limit as crate_block_gas_limit;
mod finality;
mod randomness;
pub(crate) mod util;

/// `AuthorityRound` params.
pub struct AuthorityRoundParams {
    /// A map defining intervals of blocks with the given times (in seconds) to wait before next
    /// block or authority switching. The keys in the map are steps of starting blocks of those
    /// periods. The entry at `0` should be defined.
    ///
    /// Wait times (durations) are additionally required to be less than 65535 since larger values
    /// lead to slow block issuance.
    pub step_durations: BTreeMap<u64, u64>,
    /// Starting step,
    pub start_step: Option<u64>,
    /// Valid validators.
    pub validators: Box<dyn ValidatorSet>,
    /// Chain score validation transition block.
    pub validate_score_transition: u64,
    /// Monotonic step validation transition block.
    pub validate_step_transition: u64,
    /// Immediate transitions.
    pub immediate_transitions: bool,
    /// Block reward in base units.
    pub block_reward: BTreeMap<BlockNumber, U256>,
    /// Block reward contract addresses with their associated starting block numbers.
    pub block_reward_contract_transitions: BTreeMap<u64, BlockRewardContract>,
    /// Number of accepted uncles transition block.
    pub maximum_uncle_count_transition: u64,
    /// Number of accepted uncles.
    pub maximum_uncle_count: usize,
    /// Empty step messages transition block.
    pub empty_steps_transition: u64,
    /// First block for which a 2/3 quorum (instead of 1/2) is required.
    pub two_thirds_majority_transition: BlockNumber,
    /// Number of accepted empty steps.
    pub maximum_empty_steps: usize,
    /// Transition block to strict empty steps validation.
    pub strict_empty_steps_transition: u64,
    /// If set, enables random number contract integration. It maps the transition block to the contract address.
    pub randomness_contract_address: BTreeMap<u64, Address>,
    /// The addresses of contracts that determine the block gas limit with their associated block
    /// numbers.
    pub block_gas_limit_contract_transitions: BTreeMap<u64, Address>,
    /// If set, this is the block number at which the consensus engine switches from AuRa to AuRa
    /// with POSDAO modifications.
    pub posdao_transition: Option<BlockNumber>,
}

const U16_MAX: usize = ::std::u16::MAX as usize;

/// The number of recent block hashes for which the gas limit override is memoized.
const GAS_LIMIT_OVERRIDE_CACHE_CAPACITY: usize = 10;

impl From<ethjson::spec::AuthorityRoundParams> for AuthorityRoundParams {
    fn from(p: ethjson::spec::AuthorityRoundParams) -> Self {
        let map_step_duration = |u: ethjson::uint::Uint| {
            let mut step_duration_usize: usize = u.into();
            if step_duration_usize == 0 {
                panic!("AuthorityRoundParams: step duration cannot be 0");
            }
            if step_duration_usize > U16_MAX {
                warn!(target: "engine", "step duration is too high ({}), setting it to {}", step_duration_usize, U16_MAX);
                step_duration_usize = U16_MAX;
            }
            step_duration_usize as u64
        };
        let step_durations: BTreeMap<_, _> = match p.step_duration {
            ethjson::spec::StepDuration::Single(u) => {
                iter::once((0, map_step_duration(u))).collect()
            }
            ethjson::spec::StepDuration::Transitions(tr) => {
                if tr.is_empty() {
                    panic!("AuthorityRoundParams: step duration transitions cannot be empty");
                }
                tr.into_iter()
                    .map(|(timestamp, u)| (timestamp.into(), map_step_duration(u)))
                    .collect()
            }
        };
        let transition_block_num = p.block_reward_contract_transition.map_or(0, Into::into);
        let mut br_transitions: BTreeMap<_, _> = p
            .block_reward_contract_transitions
            .unwrap_or_default()
            .into_iter()
            .map(|(block_num, address)| {
                (
                    block_num.into(),
                    BlockRewardContract::new_from_address(address.into()),
                )
            })
            .collect();
        if (p.block_reward_contract_code.is_some() || p.block_reward_contract_address.is_some())
            && br_transitions
                .keys()
                .next()
                .map_or(false, |&block_num| block_num <= transition_block_num)
        {
            let s = "blockRewardContractTransition";
            panic!("{} should be less than any of the keys in {}s", s, s);
        }
        if let Some(code) = p.block_reward_contract_code {
            br_transitions.insert(
                transition_block_num,
                BlockRewardContract::new_from_code(Arc::new(code.into())),
            );
        } else if let Some(address) = p.block_reward_contract_address {
            br_transitions.insert(
                transition_block_num,
                BlockRewardContract::new_from_address(address.into()),
            );
        }
        let randomness_contract_address =
            p.randomness_contract_address
                .map_or_else(BTreeMap::new, |transitions| {
                    transitions
                        .into_iter()
                        .map(|(ethjson::uint::Uint(block), addr)| (block.as_u64(), addr.into()))
                        .collect()
                });
        let block_gas_limit_contract_transitions: BTreeMap<_, _> = p
            .block_gas_limit_contract_transitions
            .unwrap_or_default()
            .into_iter()
            .map(|(block_num, address)| (block_num.into(), address.into()))
            .collect();
        AuthorityRoundParams {
            step_durations,
            validators: new_validator_set_posdao(p.validators, p.posdao_transition.map(Into::into)),
            start_step: p.start_step.map(Into::into),
            validate_score_transition: p.validate_score_transition.map_or(0, Into::into),
            validate_step_transition: p.validate_step_transition.map_or(0, Into::into),
            immediate_transitions: p.immediate_transitions.unwrap_or(false),
            block_reward: p.block_reward.map_or_else(
                || {
                    let mut ret = BTreeMap::new();
                    ret.insert(0, U256::zero());
                    ret
                },
                |reward| match reward {
                    ethjson::spec::BlockReward::Single(reward) => {
                        let mut ret = BTreeMap::new();
                        ret.insert(0, reward.into());
                        ret
                    }
                    ethjson::spec::BlockReward::Multi(mut multi) => {
                        if multi.is_empty() {
                            panic!("No block rewards are found in config");
                        }
                        // add block reward from genesis and put reward to zero.
                        multi
                            .entry(Uint(U256::from(0)))
                            .or_insert(Uint(U256::from(0)));
                        multi
                            .into_iter()
                            .map(|(block, reward)| (block.into(), reward.into()))
                            .collect()
                    }
                },
            ),
            block_reward_contract_transitions: br_transitions,
            maximum_uncle_count_transition: p.maximum_uncle_count_transition.map_or(0, Into::into),
            maximum_uncle_count: p.maximum_uncle_count.map_or(0, Into::into),
            empty_steps_transition: p
                .empty_steps_transition
                .map_or(u64::max_value(), |n| ::std::cmp::max(n.into(), 1)),
            maximum_empty_steps: p.maximum_empty_steps.map_or(0, Into::into),
            two_thirds_majority_transition: p
                .two_thirds_majority_transition
                .map_or_else(BlockNumber::max_value, Into::into),
            strict_empty_steps_transition: p.strict_empty_steps_transition.map_or(0, Into::into),
            randomness_contract_address,
            block_gas_limit_contract_transitions,
            posdao_transition: p.posdao_transition.map(Into::into),
        }
    }
}

/// A triple containing the first step number and the starting timestamp of the given step duration.
#[derive(Clone, Copy, Debug)]
struct StepDurationInfo {
    transition_step: u64,
    transition_timestamp: u64,
    step_duration: u64,
}

/// Helper for managing the step.
#[derive(Debug)]
struct Step {
    calibrate: bool, // whether calibration is enabled.
    inner: AtomicU64,
    /// Planned durations of steps.
    durations: Vec<StepDurationInfo>,
}

impl Step {
    fn load(&self) -> u64 {
        self.inner.load(AtomicOrdering::SeqCst)
    }

    fn duration_remaining(&self) -> Duration {
        self.opt_duration_remaining().unwrap_or_else(|| {
            let ctr = self.load();
            error!(target: "engine", "Step counter under- or overflow: {}, aborting", ctr);
            panic!("step counter under- or overflow: {}", ctr)
        })
    }

    /// Finds the remaining duration of the current step. Returns `None` if there was a counter
    /// under- or overflow.
    fn opt_duration_remaining(&self) -> Option<Duration> {
        let next_step = self.load().checked_add(1)?;
        let StepDurationInfo {
            transition_step,
            transition_timestamp,
            step_duration,
        } = self
            .durations
            .iter()
            .take_while(|info| info.transition_step < next_step)
            .last()
            .expect("durations cannot be empty")
            .clone();
        let next_time = transition_timestamp.checked_add(
            next_step
                .checked_sub(transition_step)?
                .checked_mul(step_duration)?,
        )?;
        Some(Duration::from_secs(
            next_time.saturating_sub(unix_now().as_secs()),
        ))
    }

    /// Increments the step number.
    ///
    /// Panics if the new step number is `u64::MAX`.
    fn increment(&self) {
        // fetch_add won't panic on overflow but will rather wrap
        // around, leading to zero as the step counter, which might
        // lead to unexpected situations, so it's better to shut down.
        if self.inner.fetch_add(1, AtomicOrdering::SeqCst) == u64::MAX {
            error!(target: "engine", "Step counter is too high: {}, aborting", u64::MAX);
            panic!("step counter is too high: {}", u64::MAX);
        }
    }

    fn calibrate(&self) {
        if self.calibrate {
            if self.opt_calibrate().is_none() {
                let ctr = self.load();
                error!(target: "engine", "Step counter under- or overflow: {}, aborting", ctr);
                panic!("step counter under- or overflow: {}", ctr)
            }
        }
    }

    /// Calibrates the AuRa step number according to the current time.
    fn opt_calibrate(&self) -> Option<()> {
        let now = unix_now().as_secs();
        let StepDurationInfo {
            transition_step,
            transition_timestamp,
            step_duration,
        } = self
            .durations
            .iter()
            .take_while(|info| info.transition_timestamp < now)
            .last()
            .expect("durations cannot be empty")
            .clone();
        let new_step = (now.checked_sub(transition_timestamp)? / step_duration)
            .checked_add(transition_step)?;
        self.inner.store(new_step, AtomicOrdering::SeqCst);
        Some(())
    }

    fn check_future(&self, given: u64) -> Result<(), Option<OutOfBounds<u64>>> {
        const REJECTED_STEP_DRIFT: u64 = 4;

        // Verify if the step is correct.
        if given <= self.load() {
            return Ok(());
        }

        // Make absolutely sure that the given step is incorrect.
        self.calibrate();
        let current = self.load();

        // reject blocks too far in the future
        if given > current + REJECTED_STEP_DRIFT {
            Err(None)
        // wait a bit for blocks in near future
        } else if given > current {
            let d = self
                .durations
                .iter()
                .take_while(|info| info.transition_step <= current)
                .last()
                .expect("Duration map has at least a 0 entry.")
                .step_duration;
            Err(Some(OutOfBounds {
                min: None,
                max: Some(d * current),
                found: d * given,
            }))
        } else {
            Ok(())
        }
    }
}

// Chain scoring: total weight is sqrt(U256::max_value())*height - step
fn calculate_score(parent_step: u64, current_step: u64, current_empty_steps: usize) -> U256 {
    U256::from(U128::max_value()) + U256::from(parent_step) - U256::from(current_step)
        + U256::from(current_empty_steps)
}

struct EpochManager {
    epoch_transition_hash: H256,
    epoch_transition_number: BlockNumber,
    finality_checker: RollingFinality,
    force: bool,
}

impl EpochManager {
    fn blank(two_thirds_majority_transition: BlockNumber) -> Self {
        EpochManager {
            epoch_transition_hash: H256::default(),
            epoch_transition_number: 0,
            finality_checker: RollingFinality::blank(Vec::new(), two_thirds_majority_transition),
            force: true,
        }
    }

    /// Zooms to the epoch after the header with the given hash. Returns true if succeeded, false otherwise.
    fn zoom_to_after(
        &mut self,
        client: &dyn EngineClient,
        machine: &EthereumMachine,
        validators: &dyn ValidatorSet,
        hash: H256,
    ) -> bool {
        let last_was_parent = self.finality_checker.subchain_head() == Some(hash);

        // early exit for current target == chain head, but only if the epochs are
        // the same.
        if last_was_parent && !self.force {
            return true;
        }

        self.force = false;
        debug!(target: "engine", "Zooming to epoch after block {}", hash);

        // epoch_transition_for can be an expensive call, but in the absence of
        // forks it will only need to be called for the block directly after
        // epoch transition, in which case it will be O(1) and require a single
        // DB lookup.
        let last_transition = match client.epoch_transition_for(hash) {
            Some(t) => t,
            None => {
                // this really should never happen unless the block passed
                // hasn't got a parent in the database.
                debug!(target: "engine", "No genesis transition found.");
                return false;
            }
        };

        // extract other epoch set if it's not the same as the last.
        if last_transition.block_hash != self.epoch_transition_hash {
            let (signal_number, set_proof, _) = destructure_proofs(&last_transition.proof)
                .expect("proof produced by this engine; therefore it is valid; qed");

            trace!(
                target: "engine",
                "extracting epoch validator set for epoch ({}, {}) signalled at #{}",
                last_transition.block_number, last_transition.block_hash, signal_number
            );

            let first = signal_number == 0;
            let (list, _) = validators
                .epoch_set(
                    first,
                    machine,
                    signal_number, // use signal number so multi-set first calculation is correct.
                    set_proof,
                )
                .expect("proof produced by this engine; therefore it is valid; qed");
            trace!(
                target: "engine",
                "Updating finality checker with new validator set extracted from epoch ({}, {}): {:?}",
                last_transition.block_number, last_transition.block_hash, &list
            );
            let epoch_set = list.into_inner();
            let two_thirds_majority_transition =
                self.finality_checker.two_thirds_majority_transition();
            self.finality_checker =
                RollingFinality::blank(epoch_set, two_thirds_majority_transition);
        }

        self.epoch_transition_hash = last_transition.block_hash;
        self.epoch_transition_number = last_transition.block_number;

        true
    }

    // note new epoch hash. this will force the next block to re-load
    // the epoch set
    // TODO: optimize and don't require re-loading after epoch change.
    fn note_new_epoch(&mut self) {
        self.force = true;
    }

    /// Get validator set. Zoom to the correct epoch first.
    fn validators(&self) -> &SimpleList {
        self.finality_checker.validators()
    }
}

/// A message broadcast by authorities when it's their turn to seal a block but there are no
/// transactions. Other authorities accumulate these messages and later include them in the seal as
/// proof.
///
/// An empty step message is created _instead of_ a block if there are no pending transactions.
/// It cannot itself be a parent, and `parent_hash` always points to the most recent block. E.g.:
/// * Validator A creates block `bA`.
/// * Validator B has no pending transactions, so it signs an empty step message `mB`
///   instead whose hash points to block `bA`.
/// * Validator C also has no pending transactions, so it also signs an empty step message `mC`
///   instead whose hash points to block `bA`.
/// * Validator D creates block `bD`. The parent is block `bA`, and the header includes `mB` and `mC`.
#[derive(Clone, Debug, PartialEq, Eq)]
struct EmptyStep {
    /// The signature of the other two fields, by the message's author.
    signature: H520,
    /// This message's step number.
    step: u64,
    /// The hash of the most recent block.
    parent_hash: H256,
}

impl PartialOrd for EmptyStep {
    fn partial_cmp(&self, other: &Self) -> Option<cmp::Ordering> {
        Some(self.cmp(other))
    }
}

impl Ord for EmptyStep {
    fn cmp(&self, other: &Self) -> cmp::Ordering {
        self.step
            .cmp(&other.step)
            .then_with(|| self.parent_hash.cmp(&other.parent_hash))
            .then_with(|| self.signature.cmp(&other.signature))
    }
}

impl EmptyStep {
    fn from_sealed(sealed_empty_step: SealedEmptyStep, parent_hash: &H256) -> EmptyStep {
        let signature = sealed_empty_step.signature;
        let step = sealed_empty_step.step;
        let parent_hash = parent_hash.clone();
        EmptyStep {
            signature,
            step,
            parent_hash,
        }
    }

    /// Returns `true` if the message has a valid signature by the expected proposer in the message's step.
    fn verify(&self, validators: &dyn ValidatorSet) -> Result<bool, Error> {
        let message = keccak(empty_step_rlp(self.step, &self.parent_hash));
        let correct_proposer = step_proposer(validators, &self.parent_hash, self.step);

        publickey::verify_address(&correct_proposer, &self.signature.into(), &message)
            .map_err(|e| e.into())
    }

    fn author(&self) -> Result<Address, Error> {
        let message = keccak(empty_step_rlp(self.step, &self.parent_hash));
        let public = publickey::recover(&self.signature.into(), &message)?;
        Ok(publickey::public_to_address(&public))
    }

    fn sealed(&self) -> SealedEmptyStep {
        let signature = self.signature;
        let step = self.step;
        SealedEmptyStep { signature, step }
    }
}

impl fmt::Display for EmptyStep {
    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
        write!(
            f,
            "({:x}, {}, {:x})",
            self.signature, self.step, self.parent_hash
        )
    }
}

impl Encodable for EmptyStep {
    fn rlp_append(&self, s: &mut RlpStream) {
        let empty_step_rlp = empty_step_rlp(self.step, &self.parent_hash);
        s.begin_list(2)
            .append(&self.signature)
            .append_raw(&empty_step_rlp, 1);
    }
}

impl Decodable for EmptyStep {
    fn decode(rlp: &Rlp) -> Result<Self, DecoderError> {
        let signature = rlp.val_at(0)?;
        let empty_step_rlp = rlp.at(1)?;

        let step = empty_step_rlp.val_at(0)?;
        let parent_hash = empty_step_rlp.val_at(1)?;

        Ok(EmptyStep {
            signature,
            step,
            parent_hash,
        })
    }
}

pub fn empty_step_full_rlp(signature: &H520, empty_step_rlp: &[u8]) -> Vec<u8> {
    let mut s = RlpStream::new_list(2);
    s.append(signature).append_raw(empty_step_rlp, 1);
    s.out()
}

pub fn empty_step_rlp(step: u64, parent_hash: &H256) -> Vec<u8> {
    let mut s = RlpStream::new_list(2);
    s.append(&step).append(parent_hash);
    s.out()
}

/// An empty step message that is included in a seal, the only difference is that it doesn't include
/// the `parent_hash` in order to save space. The included signature is of the original empty step
/// message, which can be reconstructed by using the parent hash of the block in which this sealed
/// empty message is included.
struct SealedEmptyStep {
    signature: H520,
    step: u64,
}

impl Encodable for SealedEmptyStep {
    fn rlp_append(&self, s: &mut RlpStream) {
        s.begin_list(2).append(&self.signature).append(&self.step);
    }
}

impl Decodable for SealedEmptyStep {
    fn decode(rlp: &Rlp) -> Result<Self, DecoderError> {
        let signature = rlp.val_at(0)?;
        let step = rlp.val_at(1)?;

        Ok(SealedEmptyStep { signature, step })
    }
}

struct PermissionedStep {
    inner: Step,
    can_propose: AtomicBool,
}

/// Engine using `AuthorityRound` proof-of-authority BFT consensus.
pub struct AuthorityRound {
    transition_service: IoService<()>,
    step: Arc<PermissionedStep>,
    client: Arc<RwLock<Option<Weak<dyn EngineClient>>>>,
    signer: RwLock<Option<Box<dyn EngineSigner>>>,
    validators: Box<dyn ValidatorSet>,
    validate_score_transition: u64,
    validate_step_transition: u64,
    empty_steps: Mutex<BTreeSet<EmptyStep>>,
    epoch_manager: Mutex<EpochManager>,
    immediate_transitions: bool,
    block_reward: BTreeMap<BlockNumber, U256>,
    block_reward_contract_transitions: BTreeMap<u64, BlockRewardContract>,
    maximum_uncle_count_transition: u64,
    maximum_uncle_count: usize,
    empty_steps_transition: u64,
    strict_empty_steps_transition: u64,
    two_thirds_majority_transition: BlockNumber,
    maximum_empty_steps: usize,
    machine: EthereumMachine,
    /// History of step hashes recently received from peers.
    received_step_hashes: RwLock<BTreeMap<(u64, Address), H256>>,
    /// If set, enables random number contract integration. It maps the transition block to the contract address.
    randomness_contract_address: BTreeMap<u64, Address>,
    /// The addresses of contracts that determine the block gas limit.
    block_gas_limit_contract_transitions: BTreeMap<u64, Address>,
    /// Memoized gas limit overrides, by block hash.
    gas_limit_override_cache: Mutex<LruCache<H256, Option<U256>>>,
    /// The block number at which the consensus engine switches from AuRa to AuRa with POSDAO
    /// modifications. For details about POSDAO, see the whitepaper:
    /// https://www.xdaichain.com/for-validators/posdao-whitepaper
    posdao_transition: Option<BlockNumber>,
}

// header-chain validator.
struct EpochVerifier {
    step: Arc<PermissionedStep>,
    subchain_validators: SimpleList,
    empty_steps_transition: u64,
    /// First block for which a 2/3 quorum (instead of 1/2) is required.
    two_thirds_majority_transition: BlockNumber,
    eip1559_transition: BlockNumber,
}

impl super::EpochVerifier<EthereumMachine> for EpochVerifier {
    fn verify_light(&self, header: &Header) -> Result<(), Error> {
        // Validate the timestamp
        verify_timestamp(
            &self.step.inner,
            header_step(header, self.empty_steps_transition)?,
        )?;
        // always check the seal since it's fast.
        // nothing heavier to do.
        verify_external(
            header,
            &self.subchain_validators,
            self.empty_steps_transition,
        )
    }

    fn check_finality_proof(&self, proof: &[u8]) -> Option<Vec<H256>> {
        let signers = self.subchain_validators.clone().into_inner();
        let mut finality_checker =
            RollingFinality::blank(signers, self.two_thirds_majority_transition);
        let mut finalized = Vec::new();

        let proof_rlp = Rlp::new(proof);
        let headers: Vec<Header> =
            Header::decode_rlp_list(&proof_rlp, self.eip1559_transition).ok()?;

        {
            let mut push_header = |parent_header: &Header, header: Option<&Header>| {
                // ensure all headers have correct number of seal fields so we can `verify_external`
                // and get `empty_steps` without panic.
                if parent_header.seal().len()
                    != header_expected_seal_fields(parent_header, self.empty_steps_transition)
                {
                    return None;
                }
                if header.iter().any(|h| {
                    h.seal().len() != header_expected_seal_fields(h, self.empty_steps_transition)
                }) {
                    return None;
                }

                // `verify_external` checks that signature is correct and author == signer.
                verify_external(
                    parent_header,
                    &self.subchain_validators,
                    self.empty_steps_transition,
                )
                .ok()?;

                let mut signers = match header {
                    Some(header) => {
                        header_empty_steps_signers(header, self.empty_steps_transition).ok()?
                    }
                    _ => Vec::new(),
                };
                signers.push(*parent_header.author());

                let newly_finalized = finality_checker
                    .push_hash(parent_header.hash(), parent_header.number(), signers)
                    .ok()?;
                finalized.extend(newly_finalized);

                Some(())
            };

            for window in headers.windows(2) {
                push_header(&window[0], Some(&window[1]))?;
            }

            if let Some(last) = headers.last() {
                push_header(last, None)?;
            }
        }

        if finalized.is_empty() {
            None
        } else {
            Some(finalized)
        }
    }
}

fn header_seal_hash(header: &Header, empty_steps_rlp: Option<&[u8]>) -> H256 {
    match empty_steps_rlp {
        Some(empty_steps_rlp) => {
            let mut message = header.bare_hash().as_bytes().to_vec();
            message.extend_from_slice(empty_steps_rlp);
            keccak(message)
        }
        None => header.bare_hash(),
    }
}

fn header_expected_seal_fields(header: &Header, empty_steps_transition: u64) -> usize {
    if header.number() >= empty_steps_transition {
        3
    } else {
        2
    }
}

fn header_step(header: &Header, empty_steps_transition: u64) -> Result<u64, ::rlp::DecoderError> {
    Rlp::new(&header.seal().get(0).unwrap_or_else(||
		panic!("was either checked with verify_block_basic or is genesis; has {} fields; qed (Make sure the spec \
            file has a correct genesis seal)", header_expected_seal_fields(header, empty_steps_transition))
	))
	.as_val()
}

fn header_signature(
    header: &Header,
    empty_steps_transition: u64,
) -> Result<Signature, ::rlp::DecoderError> {
    Rlp::new(&header.seal().get(1).unwrap_or_else(|| {
        panic!(
            "was checked with verify_block_basic; has {} fields; qed",
            header_expected_seal_fields(header, empty_steps_transition)
        )
    }))
    .as_val::<H520>()
    .map(Into::into)
}

// extracts the raw empty steps vec from the header seal. should only be called when there are 3 fields in the seal
// (i.e. header.number() >= self.empty_steps_transition)
fn header_empty_steps_raw(header: &Header) -> &[u8] {
    header
        .seal()
        .get(2)
        .expect("was checked with verify_block_basic; has 3 fields; qed")
}

// extracts the empty steps from the header seal. should only be called when there are 3 fields in the seal
// (i.e. header.number() >= self.empty_steps_transition).
fn header_empty_steps(header: &Header) -> Result<Vec<EmptyStep>, ::rlp::DecoderError> {
    let empty_steps = Rlp::new(header_empty_steps_raw(header)).as_list::<SealedEmptyStep>()?;
    Ok(empty_steps
        .into_iter()
        .map(|s| EmptyStep::from_sealed(s, header.parent_hash()))
        .collect())
}

// gets the signers of empty step messages for the given header, does not include repeated signers
fn header_empty_steps_signers(
    header: &Header,
    empty_steps_transition: u64,
) -> Result<Vec<Address>, Error> {
    if header.number() >= empty_steps_transition {
        let mut signers = HashSet::new();
        for empty_step in header_empty_steps(header)? {
            signers.insert(empty_step.author()?);
        }

        Ok(Vec::from_iter(signers.into_iter()))
    } else {
        Ok(Vec::new())
    }
}

fn step_proposer(validators: &dyn ValidatorSet, bh: &H256, step: u64) -> Address {
    let proposer = validators.get(bh, step as usize);
    trace!(target: "engine", "Fetched proposer for step {}: {}", step, proposer);
    proposer
}

fn is_step_proposer(
    validators: &dyn ValidatorSet,
    bh: &H256,
    step: u64,
    address: &Address,
) -> bool {
    step_proposer(validators, bh, step) == *address
}

fn verify_timestamp(step: &Step, header_step: u64) -> Result<(), BlockError> {
    match step.check_future(header_step) {
        Err(None) => {
            trace!(target: "engine", "verify_timestamp: block from the future");
            Err(BlockError::InvalidSeal.into())
        }
        Err(Some(oob)) => {
            // NOTE This error might be returned only in early stage of verification (Stage 1).
            // Returning it further won't recover the sync process.
            trace!(target: "engine", "verify_timestamp: block too early");

            let found = CheckedSystemTime::checked_add(UNIX_EPOCH, Duration::from_secs(oob.found))
                .ok_or(BlockError::TimestampOverflow)?;
            let max = oob
                .max
                .and_then(|m| CheckedSystemTime::checked_add(UNIX_EPOCH, Duration::from_secs(m)));
            let min = oob
                .min
                .and_then(|m| CheckedSystemTime::checked_add(UNIX_EPOCH, Duration::from_secs(m)));

            let new_oob = OutOfBounds { min, max, found };

            Err(BlockError::TemporarilyInvalid(new_oob).into())
        }
        Ok(_) => Ok(()),
    }
}

fn verify_external(
    header: &Header,
    validators: &dyn ValidatorSet,
    empty_steps_transition: u64,
) -> Result<(), Error> {
    let header_step = header_step(header, empty_steps_transition)?;

    let proposer_signature = header_signature(header, empty_steps_transition)?;
    let correct_proposer = validators.get(header.parent_hash(), header_step as usize);
    let is_invalid_proposer = *header.author() != correct_proposer || {
        let empty_steps_rlp = if header.number() >= empty_steps_transition {
            Some(header_empty_steps_raw(header))
        } else {
            None
        };

        let header_seal_hash = header_seal_hash(header, empty_steps_rlp);
        !publickey::verify_address(&correct_proposer, &proposer_signature, &header_seal_hash)?
    };

    if is_invalid_proposer {
        trace!(target: "engine", "verify_block_external: bad proposer for step: {}", header_step);
        Err(EngineError::NotProposer(Mismatch {
            expected: correct_proposer,
            found: *header.author(),
        }))?
    } else {
        Ok(())
    }
}

fn combine_proofs(signal_number: BlockNumber, set_proof: &[u8], finality_proof: &[u8]) -> Vec<u8> {
    let mut stream = RlpStream::new_list(3);
    stream
        .append(&signal_number)
        .append(&set_proof)
        .append(&finality_proof);
    stream.out()
}

fn destructure_proofs(combined: &[u8]) -> Result<(BlockNumber, &[u8], &[u8]), Error> {
    let rlp = Rlp::new(combined);
    Ok((rlp.at(0)?.as_val()?, rlp.at(1)?.data()?, rlp.at(2)?.data()?))
}

trait AsMillis {
    fn as_millis(&self) -> u64;
}

impl AsMillis for Duration {
    fn as_millis(&self) -> u64 {
        self.as_secs() * 1_000 + (self.subsec_nanos() / 1_000_000) as u64
    }
}

// A type for storing owned or borrowed data that has a common type.
// Useful for returning either a borrow or owned data from a function.
enum CowLike<'a, A: 'a + ?Sized, B> {
    Borrowed(&'a A),
    Owned(B),
}

impl<'a, A: ?Sized, B> Deref for CowLike<'a, A, B>
where
    B: AsRef<A>,
{
    type Target = A;
    fn deref(&self) -> &A {
        match self {
            CowLike::Borrowed(b) => b,
            CowLike::Owned(o) => o.as_ref(),
        }
    }
}

impl AuthorityRound {
    /// Create a new instance of AuthorityRound engine.
    pub fn new(
        our_params: AuthorityRoundParams,
        machine: EthereumMachine,
    ) -> Result<Arc<Self>, Error> {
        if !our_params.step_durations.contains_key(&0) {
            error!(target: "engine", "Authority Round step 0 duration is undefined, aborting");
            return Err(Error::from_kind(ErrorKind::Engine(EngineError::Custom(
                String::from("step 0 duration is undefined"),
            ))));
        }
        if our_params.step_durations.values().any(|v| *v == 0) {
            error!(target: "engine", "Authority Round step duration cannot be 0");
            return Err(Error::from_kind(ErrorKind::Engine(EngineError::Custom(
                String::from("step duration cannot be 0"),
            ))));
        }

        let should_timeout = our_params.start_step.is_none();

        let initial_step = our_params.start_step.unwrap_or(0);

        let mut durations = Vec::new();
        {
            let mut dur_info = StepDurationInfo {
                transition_step: 0u64,
                transition_timestamp: 0u64,
                step_duration: our_params.step_durations[&0],
            };
            durations.push(dur_info);
            for (time, dur) in our_params.step_durations.iter().skip(1) {
                let (step, time) = next_step_time_duration(dur_info, *time)
                    .ok_or(BlockError::TimestampOverflow)?;
                dur_info.transition_step = step;
                dur_info.transition_timestamp = time;
                dur_info.step_duration = *dur;
                durations.push(dur_info);
            }
        }

        let step = Step {
            inner: AtomicU64::new(initial_step),
            calibrate: our_params.start_step.is_none(),
            durations,
        };
        step.calibrate();

        let engine = Arc::new(AuthorityRound {
            transition_service: IoService::<()>::start("AuRa")?,
            step: Arc::new(PermissionedStep {
                inner: step,
                can_propose: AtomicBool::new(true),
            }),
            client: Arc::new(RwLock::new(None)),
            signer: RwLock::new(None),
            validators: our_params.validators,
            validate_score_transition: our_params.validate_score_transition,
            validate_step_transition: our_params.validate_step_transition,
            empty_steps: Default::default(),
            epoch_manager: Mutex::new(EpochManager::blank(
                our_params.two_thirds_majority_transition,
            )),
            immediate_transitions: our_params.immediate_transitions,
            block_reward: our_params.block_reward,
            block_reward_contract_transitions: our_params.block_reward_contract_transitions,
            maximum_uncle_count_transition: our_params.maximum_uncle_count_transition,
            maximum_uncle_count: our_params.maximum_uncle_count,
            empty_steps_transition: our_params.empty_steps_transition,
            maximum_empty_steps: our_params.maximum_empty_steps,
            two_thirds_majority_transition: our_params.two_thirds_majority_transition,
            strict_empty_steps_transition: our_params.strict_empty_steps_transition,
            machine: machine,
            received_step_hashes: RwLock::new(Default::default()),
            randomness_contract_address: our_params.randomness_contract_address,
            block_gas_limit_contract_transitions: our_params.block_gas_limit_contract_transitions,
            gas_limit_override_cache: Mutex::new(LruCache::new(GAS_LIMIT_OVERRIDE_CACHE_CAPACITY)),
            posdao_transition: our_params.posdao_transition,
        });

        // Do not initialize timeouts for tests.
        if should_timeout {
            let handler = TransitionHandler {
                step: engine.step.clone(),
                client: engine.client.clone(),
            };
            engine
                .transition_service
                .register_handler(Arc::new(handler))?;
        }
        Ok(engine)
    }

    // fetch correct validator set for epoch at header, taking into account
    // finality of previous transitions.
    fn epoch_set<'a>(
        &'a self,
        header: &Header,
    ) -> Result<(CowLike<dyn ValidatorSet, SimpleList>, BlockNumber), Error> {
        Ok(if self.immediate_transitions {
            (CowLike::Borrowed(&*self.validators), header.number())
        } else {
            let mut epoch_manager = self.epoch_manager.lock();
            let client = self.upgrade_client_or("Unable to verify sig")?;

            if !epoch_manager.zoom_to_after(
                &*client,
                &self.machine,
                &*self.validators,
                *header.parent_hash(),
            ) {
                debug!(target: "engine", "Unable to zoom to epoch.");
                return Err(EngineError::RequiresClient.into());
            }

            (
                CowLike::Owned(epoch_manager.validators().clone()),
                epoch_manager.epoch_transition_number,
            )
        })
    }

    fn empty_steps(&self, from_step: u64, to_step: u64, parent_hash: H256) -> Vec<EmptyStep> {
        let from = EmptyStep {
            step: from_step + 1,
            parent_hash,
            signature: Default::default(),
        };
        let to = EmptyStep {
            step: to_step,
            parent_hash: Default::default(),
            signature: Default::default(),
        };

        if from >= to {
            return vec![];
        }

        self.empty_steps
            .lock()
            .range(from..to)
            .filter(|e| e.parent_hash == parent_hash)
            .cloned()
            .collect()
    }

    fn clear_empty_steps(&self, step: u64) {
        // clear old `empty_steps` messages
        let mut empty_steps = self.empty_steps.lock();
        *empty_steps = empty_steps.split_off(&EmptyStep {
            step: step + 1,
            parent_hash: Default::default(),
            signature: Default::default(),
        });
    }

    fn handle_empty_step_message(&self, empty_step: EmptyStep) {
        self.empty_steps.lock().insert(empty_step);
    }

    fn generate_empty_step(&self, parent_hash: &H256) {
        let step = self.step.inner.load();
        let empty_step_rlp = empty_step_rlp(step, parent_hash);

        if let Ok(signature) = self.sign(keccak(&empty_step_rlp)).map(Into::into) {
            let message_rlp = empty_step_full_rlp(&signature, &empty_step_rlp);

            let parent_hash = *parent_hash;
            let empty_step = EmptyStep {
                signature,
                step,
                parent_hash,
            };

            trace!(target: "engine", "broadcasting empty step message: {:?}", empty_step);
            self.broadcast_message(message_rlp);
            self.handle_empty_step_message(empty_step);
        } else {
            warn!(target: "engine", "generate_empty_step: FAIL: accounts secret key unavailable");
        }
    }

    fn broadcast_message(&self, message: Vec<u8>) {
        if let Ok(c) = self.upgrade_client_or(None) {
            c.broadcast_consensus_message(message);
        }
    }

    fn report_skipped(
        &self,
        header: &Header,
        current_step: u64,
        parent_step: u64,
        validators: &dyn ValidatorSet,
        set_number: u64,
    ) {
        // we're building on top of the genesis block so don't report any skipped steps
        if header.number() == 1 {
            return;
        }

        if let (true, Some(me)) = (
            current_step > parent_step + 1,
            self.signer.read().as_ref().map(|s| s.address()),
        ) {
            debug!(target: "engine", "Author {} built block with step gap. current step: {}, parent step: {}",
				   header.author(), current_step, parent_step);
            let mut reported = HashSet::new();
            for step in parent_step + 1..current_step {
                let skipped_primary = step_proposer(validators, header.parent_hash(), step);
                // Do not report this signer.
                if skipped_primary != me {
                    // Stop reporting once validators start repeating.
                    if !reported.insert(skipped_primary) {
                        break;
                    }
                    trace!(
                        target: "engine",
                        "Reporting benign misbehaviour (cause: skipped step) at block #{}, \
                        epoch set number {}, step proposer={:#x}. Own address: {}",
                        header.number(), set_number, skipped_primary, me
                    );
                    self.validators
                        .report_benign(&skipped_primary, set_number, header.number());
                }
            }
        }
    }

    // Returns the hashes of all ancestor blocks that are finalized by the given `chain_head`.
    fn build_finality(
        &self,
        chain_head: &Header,
        ancestry: &mut dyn Iterator<Item = Header>,
    ) -> Vec<H256> {
        if self.immediate_transitions {
            return Vec::new();
        }

        let client = match self.upgrade_client_or("Unable to apply ancestry actions") {
            Ok(client) => client,
            Err(_) => return Vec::new(),
        };

        let mut epoch_manager = self.epoch_manager.lock();
        if !epoch_manager.zoom_to_after(
            &*client,
            &self.machine,
            &*self.validators,
            *chain_head.parent_hash(),
        ) {
            return Vec::new();
        }

        if epoch_manager.finality_checker.subchain_head() != Some(*chain_head.parent_hash()) {
            // build new finality checker from unfinalized ancestry of chain head, not including chain head itself yet.
            trace!(target: "finality", "Building finality up to parent of {} ({})",
				   chain_head.hash(), chain_head.parent_hash());

            // the empty steps messages in a header signal approval of the
            // parent header.
            let mut parent_empty_steps_signers = match header_empty_steps_signers(
                &chain_head,
                self.empty_steps_transition,
            ) {
                Ok(empty_step_signers) => empty_step_signers,
                Err(_) => {
                    warn!(target: "finality", "Failed to get empty step signatures from block {}", chain_head.hash());
                    return Vec::new();
                }
            };

            let epoch_transition_hash = epoch_manager.epoch_transition_hash;
            let ancestry_iter = ancestry.map(|header| {
				let mut signers = vec![*header.author()];
				signers.extend(parent_empty_steps_signers.drain(..));

				if let Ok(empty_step_signers) = header_empty_steps_signers(&header, self.empty_steps_transition) {
					let res = (header.hash(), header.number(), signers);
					trace!(target: "finality", "Ancestry iteration: yielding {:?}", res);

					parent_empty_steps_signers = empty_step_signers;

					Some(res)

				} else {
					warn!(target: "finality", "Failed to get empty step signatures from block {}", header.hash());
					None
				}
			})
				.while_some()
				.take_while(|&(h, _, _)| h != epoch_transition_hash);

            if let Err(e) = epoch_manager
                .finality_checker
                .build_ancestry_subchain(ancestry_iter)
            {
                debug!(target: "engine", "inconsistent validator set within epoch: {:?}", e);
                return Vec::new();
            }
        }

        let finalized = epoch_manager.finality_checker.push_hash(
            chain_head.hash(),
            chain_head.number(),
            vec![*chain_head.author()],
        );
        finalized.unwrap_or_default()
    }

    fn address(&self) -> Option<Address> {
        self.signer.read().as_ref().map(|s| s.address())
    }

    /// Make calls to the randomness contract.
    fn run_randomness_phase(&self, block: &ExecutedBlock) -> Result<Vec<SignedTransaction>, Error> {
        let contract_addr = match self
            .randomness_contract_address
            .range(..=block.header.number())
            .last()
        {
            Some((_, &contract_addr)) => contract_addr,
            None => return Ok(Vec::new()), // No randomness contract.
        };

        let opt_signer = self.signer.read();
        let signer = match opt_signer.as_ref() {
            Some(signer) => signer,
            None => return Ok(Vec::new()), // We are not a validator, so we shouldn't call the contracts.
        };
        let our_addr = signer.address();
        let client = self.upgrade_client_or("Unable to prepare block")?;
        let full_client = client.as_full_client().ok_or_else(|| {
            EngineError::FailedSystemCall("Failed to upgrade to BlockchainClient.".to_string())
        })?;

        // Random number generation
        let contract = util::BoundContract::new(&*client, BlockId::Latest, contract_addr);
        let phase = randomness::RandomnessPhase::load(&contract, our_addr)
            .map_err(|err| EngineError::Custom(format!("Randomness error in load(): {:?}", err)))?;
        let data = match phase
            .advance(&contract, &mut OsRng, signer.as_ref())
            .map_err(|err| {
                EngineError::Custom(format!("Randomness error in advance(): {:?}", err))
            })? {
            Some(data) => data,
            None => return Ok(Vec::new()), // Nothing to commit or reveal at the moment.
        };

        let nonce = block.state.nonce(&our_addr)?;
        let tx_request = TransactionRequest::call(contract_addr, data)
            .gas_price(U256::zero())
            .nonce(nonce);
        Ok(vec![full_client.create_transaction(tx_request)?])
    }

    /// Returns the reference to the client, if registered.
    fn upgrade_client_or<'a, T>(
        &self,
        opt_error_msg: T,
    ) -> Result<Arc<dyn EngineClient>, EngineError>
    where
        T: Into<Option<&'a str>>,
    {
        self.client
            .read()
            .as_ref()
            .and_then(|weak| weak.upgrade())
            .ok_or_else(|| {
                if let Some(error_msg) = opt_error_msg.into() {
                    debug!(target: "engine", "{}: missing client ref.", error_msg);
                }
                EngineError::RequiresClient
            })
    }

    fn run_posdao(
        &self,
        block: &ExecutedBlock,
        nonce: Option<U256>,
    ) -> Result<Vec<SignedTransaction>, Error> {
        // Skip the rest of the function unless there has been a transition to POSDAO AuRa.
        if self
            .posdao_transition
            .map_or(true, |posdao_block| block.header.number() < posdao_block)
        {
            trace!(target: "engine", "Skipping POSDAO calls to validator set contracts");
            return Ok(Vec::new());
        }

        let opt_signer = self.signer.read();
        let signer = match opt_signer.as_ref() {
            Some(signer) => signer,
            None => return Ok(Vec::new()), // We are not a validator, so we shouldn't call the contracts.
        };
        let our_addr = signer.address();
        let client = self.upgrade_client_or("Unable to prepare block")?;
        let full_client = client.as_full_client().ok_or_else(|| {
            EngineError::FailedSystemCall("Failed to upgrade to BlockchainClient.".to_string())
        })?;

        // Makes a constant contract call.
        let mut call = |to: Address, data: Bytes| {
            full_client
                .call_contract(BlockId::Latest, to, data)
                .map_err(|e| format!("{}", e))
        };

        // Our current account nonce. The transactions must have consecutive nonces, starting with this one.
        let mut tx_nonce = if let Some(tx_nonce) = nonce {
            tx_nonce
        } else {
            block.state.nonce(&our_addr)?
        };
        let mut transactions = Vec::new();

        // Creates and signs a transaction with the given contract call.
        let mut make_transaction = |to: Address, data: Bytes| -> Result<SignedTransaction, Error> {
            let tx_request = TransactionRequest::call(to, data)
                .gas_price(U256::zero())
                .nonce(tx_nonce);
            tx_nonce += U256::one(); // Increment the nonce for the next transaction.
            Ok(full_client.create_transaction(tx_request)?)
        };

        // Genesis is never a new block, but might as well check.
        let first = block.header.number() == 0;
        for (addr, data) in
            self.validators
                .generate_engine_transactions(first, &block.header, &mut call)?
        {
            transactions.push(make_transaction(addr, data)?);
        }

        Ok(transactions)
    }
}

fn unix_now() -> Duration {
    UNIX_EPOCH
        .elapsed()
        .expect("Valid time has to be set in your system.")
}

struct TransitionHandler {
    step: Arc<PermissionedStep>,
    client: Arc<RwLock<Option<Weak<dyn EngineClient>>>>,
}

const ENGINE_TIMEOUT_TOKEN: TimerToken = 23;

impl IoHandler<()> for TransitionHandler {
    fn initialize(&self, io: &IoContext<()>) {
        let remaining = AsMillis::as_millis(&self.step.inner.duration_remaining());
        io.register_timer_once(ENGINE_TIMEOUT_TOKEN, Duration::from_millis(remaining))
            .unwrap_or_else(
                |e| warn!(target: "engine", "Failed to start consensus step timer: {}.", e),
            )
    }

    fn timeout(&self, io: &IoContext<()>, timer: TimerToken) {
        if timer == ENGINE_TIMEOUT_TOKEN {
            // NOTE we might be lagging by couple of steps in case the timeout
            // has not been called fast enough.
            // Make sure to advance up to the actual step.
            while AsMillis::as_millis(&self.step.inner.duration_remaining()) == 0 {
                self.step.inner.increment();
                self.step.can_propose.store(true, AtomicOrdering::SeqCst);
                if let Some(ref weak) = *self.client.read() {
                    if let Some(c) = weak.upgrade() {
                        c.update_sealing(ForceUpdateSealing::No);
                    }
                }
            }

            let next_run_at = Duration::from_millis(
                AsMillis::as_millis(&self.step.inner.duration_remaining()) >> 2,
            );
            io.register_timer_once(ENGINE_TIMEOUT_TOKEN, next_run_at)
                .unwrap_or_else(
                    |e| warn!(target: "engine", "Failed to restart consensus step timer: {}.", e),
                )
        }
    }
}

impl Engine<EthereumMachine> for AuthorityRound {
    fn name(&self) -> &str {
        "AuthorityRound"
    }

    fn machine(&self) -> &EthereumMachine {
        &self.machine
    }

    /// Three fields - consensus step and the corresponding proposer signature, and a list of empty
    /// step messages (which should be empty if no steps are skipped)
    fn seal_fields(&self, header: &Header) -> usize {
        header_expected_seal_fields(header, self.empty_steps_transition)
    }

    fn step(&self) {
        self.step.inner.increment();
        self.step.can_propose.store(true, AtomicOrdering::SeqCst);
        if let Ok(c) = self.upgrade_client_or(None) {
            c.update_sealing(ForceUpdateSealing::No);
        }
    }

    /// Additional engine-specific information for the user/developer concerning `header`.
    fn extra_info(&self, header: &Header) -> BTreeMap<String, String> {
        if header.seal().len() < header_expected_seal_fields(header, self.empty_steps_transition) {
            return BTreeMap::default();
        }

        let step = header_step(header, self.empty_steps_transition)
            .as_ref()
            .map(ToString::to_string)
            .unwrap_or_default();
        let signature = header_signature(header, self.empty_steps_transition)
            .as_ref()
            .map(ToString::to_string)
            .unwrap_or_default();

        let mut info = map![
            "step".into() => step,
            "signature".into() => signature
        ];

        if header.number() >= self.empty_steps_transition {
            let empty_steps = if let Ok(empty_steps) = header_empty_steps(header).as_ref() {
                format!(
                    "[{}]",
                    empty_steps
                        .iter()
                        .fold("".to_string(), |acc, e| if acc.len() > 0 {
                            acc + ","
                        } else {
                            acc
                        } + &e.to_string())
                )
            } else {
                "".into()
            };

            info.insert("emptySteps".into(), empty_steps);
        }

        info
    }

    fn maximum_uncle_count(&self, block: BlockNumber) -> usize {
        if block >= self.maximum_uncle_count_transition {
            self.maximum_uncle_count
        } else {
            // fallback to default value
            2
        }
    }

    fn populate_from_parent(&self, header: &mut Header, parent: &Header) {
        let parent_step = header_step(parent, self.empty_steps_transition)
            .expect("Header has been verified; qed");
        let current_step = self.step.inner.load();

        let current_empty_steps_len = if header.number() >= self.empty_steps_transition {
            self.empty_steps(parent_step, current_step, parent.hash())
                .len()
        } else {
            0
        };

        let score = calculate_score(parent_step, current_step, current_empty_steps_len);
        header.set_difficulty(score);
        if let Some(gas_limit) = self.gas_limit_override(header) {
            trace!(target: "engine", "Setting gas limit to {} for block {}.", gas_limit, header.number());
            let parent_gas_limit = *parent.gas_limit();
            header.set_gas_limit(gas_limit);
            if parent_gas_limit != gas_limit {
                info!(target: "engine", "Block gas limit was changed from {} to {}.", parent_gas_limit, gas_limit);
            }
        }
    }

    // Mostly is the same as `fn sealing_state(&self)` except that it does not
    // check whether the node is a step proposer.
    fn is_allowed_to_seal(&self) -> bool {
        let our_addr = match *self.signer.read() {
            Some(ref signer) => signer.address(),
            None => return false,
        };

        let client = match self.upgrade_client_or("Not preparing block") {
            Ok(client) => client,
            Err(_) => return false,
        };

        let parent = match client.as_full_client() {
            Some(full_client) => full_client.best_block_header(),
            None => {
                return false;
            }
        };

        let validators = if self.immediate_transitions {
            CowLike::Borrowed(&*self.validators)
        } else {
            let mut epoch_manager = self.epoch_manager.lock();
            if !epoch_manager.zoom_to_after(
                &*client,
                &self.machine,
                &*self.validators,
                parent.hash(),
            ) {
                return false;
            }
            CowLike::Owned(epoch_manager.validators().clone())
        };
        validators.contains(&parent.hash(), &our_addr)
    }

    fn sealing_state(&self) -> SealingState {
        let our_addr = match *self.signer.read() {
            Some(ref signer) => signer.address(),
            None => {
                warn!(target: "engine", "Not preparing block; cannot sign.");
                return SealingState::NotReady;
            }
        };

        let client = match self.upgrade_client_or("Not preparing block") {
            Ok(client) => client,
            Err(_) => return SealingState::NotReady,
        };

        let parent = match client.as_full_client() {
            Some(full_client) => full_client.best_block_header(),
            None => {
                debug!(target: "engine", "Not preparing block: not a full client.");
                return SealingState::NotReady;
            }
        };

        let validators = if self.immediate_transitions {
            CowLike::Borrowed(&*self.validators)
        } else {
            let mut epoch_manager = self.epoch_manager.lock();
            if !epoch_manager.zoom_to_after(
                &*client,
                &self.machine,
                &*self.validators,
                parent.hash(),
            ) {
                debug!(target: "engine", "Not preparing block: Unable to zoom to epoch.");
                return SealingState::NotReady;
            }
            CowLike::Owned(epoch_manager.validators().clone())
        };

        let step = self.step.inner.load();

        if !is_step_proposer(&*validators, &parent.hash(), step, &our_addr) {
            trace!(target: "engine", "Not preparing block: not a proposer for step {}. (Our address: {})",
               step, our_addr);
            return SealingState::NotReady;
        }

        SealingState::Ready
    }

    fn handle_message(&self, rlp: &[u8]) -> Result<(), EngineError> {
        fn fmt_err<T: fmt::Debug>(x: T) -> EngineError {
            EngineError::MalformedMessage(format!("{:?}", x))
        }

        let rlp = Rlp::new(rlp);
        let empty_step: EmptyStep = rlp.as_val().map_err(fmt_err)?;

        if empty_step.verify(&*self.validators).unwrap_or(false) {
            if self.step.inner.check_future(empty_step.step).is_ok() {
                trace!(target: "engine", "handle_message: received empty step message {:?}", empty_step);
                self.handle_empty_step_message(empty_step);
            } else {
                trace!(target: "engine", "handle_message: empty step message from the future {:?}", empty_step);
            }
        } else {
            trace!(target: "engine", "handle_message: received invalid step message {:?}", empty_step);
        };

        Ok(())
    }

    /// Attempt to seal the block internally.
    ///
    /// This operation is synchronous and may (quite reasonably) not be available, in which case
    /// `Seal::None` will be returned.
    fn generate_seal(&self, block: &ExecutedBlock, parent: &Header) -> Seal {
        // first check to avoid generating signature most of the time
        // (but there's still a race to the `compare_exchange`)
        if !self.step.can_propose.load(AtomicOrdering::SeqCst) {
            trace!(target: "engine", "Aborting seal generation. Can't propose.");
            return Seal::None;
        }

        let header = &block.header;
        let parent_step = header_step(parent, self.empty_steps_transition)
            .expect("Header has been verified; qed");

        let step = self.step.inner.load();

        // filter messages from old and future steps and different parents
        let empty_steps = if header.number() >= self.empty_steps_transition {
            self.empty_steps(parent_step.into(), step.into(), *header.parent_hash())
        } else {
            Vec::new()
        };

        let expected_diff = calculate_score(parent_step, step.into(), empty_steps.len().into());

        if header.difficulty() != &expected_diff {
            debug!(target: "engine", "Aborting seal generation. The step or empty_steps have changed in the meantime. {:?} != {:?}",
				   header.difficulty(), expected_diff);
            return Seal::None;
        }

        if parent_step > step.into() {
            warn!(target: "engine", "Aborting seal generation for invalid step: {} > {}", parent_step, step);
            return Seal::None;
        }

        let (validators, set_number) = match self.epoch_set(header) {
            Err(err) => {
                warn!(target: "engine", "Unable to generate seal: {}", err);
                return Seal::None;
            }
            Ok(ok) => ok,
        };

        if is_step_proposer(&*validators, header.parent_hash(), step, header.author()) {
            // this is guarded against by `can_propose` unless the block was signed
            // on the same step (implies same key) and on a different node.
            if parent_step == step {
                warn!("Attempted to seal block on the same step as parent. Is this authority sealing with more than one node?");
                return Seal::None;
            }

            // if there are no transactions to include in the block, we don't seal and instead broadcast a signed
            // `EmptyStep(step, parent_hash)` message. If we exceed the maximum amount of `empty_step` rounds we proceed
            // with the seal.
            if header.number() >= self.empty_steps_transition
                && block.transactions.is_empty()
                && empty_steps.len() < self.maximum_empty_steps
            {
                if self
                    .step
                    .can_propose
                    .compare_exchange(true, false, AtomicOrdering::SeqCst, AtomicOrdering::SeqCst)
                    .is_ok()
                {
                    self.generate_empty_step(header.parent_hash());
                }

                return Seal::None;
            }

            let empty_steps_rlp = if header.number() >= self.empty_steps_transition {
                let empty_steps: Vec<_> = empty_steps.iter().map(|e| e.sealed()).collect();
                Some(::rlp::encode_list(&empty_steps))
            } else {
                None
            };

            if let Ok(signature) = self.sign(header_seal_hash(
                header,
                empty_steps_rlp.as_ref().map(|e| &**e),
            )) {
                trace!(target: "engine", "generate_seal: Issuing a block for step {}.", step);

                // only issue the seal if we were the first to reach the compare_exchange.
                if self
                    .step
                    .can_propose
                    .compare_exchange(true, false, AtomicOrdering::SeqCst, AtomicOrdering::SeqCst)
                    .is_ok()
                {
                    // we can drop all accumulated empty step messages that are
                    // older than the parent step since we're including them in
                    // the seal
                    self.clear_empty_steps(parent_step);

                    // report any skipped primaries between the parent block and
                    // the block we're sealing, unless we have empty steps enabled
                    if header.number() < self.empty_steps_transition {
                        self.report_skipped(header, step, parent_step, &*validators, set_number);
                    }

                    let mut fields =
                        vec![encode(&step), encode(&(H520::from(signature).as_bytes()))];

                    if let Some(empty_steps_rlp) = empty_steps_rlp {
                        fields.push(empty_steps_rlp);
                    }

                    return Seal::Regular(fields);
                }
            } else {
                warn!(target: "engine", "generate_seal: FAIL: Accounts secret key unavailable.");
            }
        } else {
            trace!(target: "engine", "generate_seal: {} not a proposer for step {}.",
				header.author(), step);
        }

        Seal::None
    }

    fn verify_local_seal(&self, _header: &Header) -> Result<(), Error> {
        Ok(())
    }

    // t_nb 8.1.5
    fn on_new_block(
        &self,
        block: &mut ExecutedBlock,
        epoch_begin: bool,
        _ancestry: &mut dyn Iterator<Item = ExtendedHeader>,
    ) -> Result<(), Error> {
        // with immediate transitions, we don't use the epoch mechanism anyway.
        // the genesis is always considered an epoch, but we ignore it intentionally.
        if self.immediate_transitions || !epoch_begin {
            return Ok(());
        }

        // genesis is never a new block, but might as well check.
        let header = block.header.clone();
        let first = header.number() == 0;

        let mut call = |to, data| {
            let result = self.machine.execute_as_system(
                block,
                to,
                U256::max_value(), // unbounded gas? maybe make configurable.
                Some(data),
            );

            result.map_err(|e| format!("{}", e))
        };

        self.validators.on_epoch_begin(first, &header, &mut call)
    }

    /// Apply the block reward on finalisation of the block.
    fn on_close_block(&self, block: &mut ExecutedBlock) -> Result<(), Error> {
        let mut beneficiaries = Vec::new();

        if block.header.number() == self.two_thirds_majority_transition {
            info!(target: "engine", "Block {}: Transitioning to 2/3 quorum.", self.two_thirds_majority_transition);
        }

        if block.header.number() >= self.empty_steps_transition {
            let empty_steps = if block.header.seal().is_empty() {
                // this is a new block, calculate rewards based on the empty steps messages we have accumulated
                let client = match self.client.read().as_ref().and_then(|weak| weak.upgrade()) {
                    Some(client) => client,
                    None => {
                        debug!(target: "engine", "Unable to close block: missing client ref.");
                        return Err(EngineError::RequiresClient.into());
                    }
                };

                let parent = client
                    .block_header(::client::BlockId::Hash(*block.header.parent_hash()))
                    .expect("hash is from parent; parent header must exist; qed")
                    .decode(self.params().eip1559_transition)?;

                let parent_step = header_step(&parent, self.empty_steps_transition)?;
                let current_step = self.step.inner.load();
                self.empty_steps(parent_step.into(), current_step.into(), parent.hash())
            } else {
                // we're verifying a block, extract empty steps from the seal
                header_empty_steps(&block.header)?
            };

            for empty_step in empty_steps {
                let author = empty_step.author()?;
                beneficiaries.push((author, RewardKind::EmptyStep));
            }
        }

        let author = *block.header.author();
        let number = block.header.number();
        beneficiaries.push((author, RewardKind::Author));

        let block_reward_contract_transition = self
            .block_reward_contract_transitions
            .range(..=block.header.number())
            .last();
        let rewards: Vec<_> = if let Some((_, contract)) = block_reward_contract_transition {
            let mut call = crate::engines::default_system_or_code_call(&self.machine, block);
            let rewards = contract.reward(&beneficiaries, &mut call)?;
            rewards
                .into_iter()
                .map(|(author, amount)| (author, RewardKind::External, amount))
                .collect()
        } else {
            let (_, reward) = self
                .block_reward
                .iter()
                .rev()
                .find(|&(block, _)| *block <= number)
                .expect(
                    "Current block's reward is not found; this indicates a chain config error; qed",
                );
            let reward = *reward;

            beneficiaries
                .into_iter()
                .map(|(author, reward_kind)| (author, reward_kind, reward))
                .collect()
        };

        if let Some(signer) = self.signer.read().as_ref() {
            let our_addr = signer.address();
            self.validators.on_close_block(&block.header, &our_addr)?
        }

        block_reward::apply_block_rewards(&rewards, block, &self.machine)
    }

    fn generate_engine_transactions(
        &self,
        block: &ExecutedBlock,
    ) -> Result<Vec<SignedTransaction>, Error> {
        let mut transactions = self.run_randomness_phase(block)?;
        let nonce = transactions.last().map(|tx| tx.tx().nonce + U256::one());
        transactions.extend(self.run_posdao(block, nonce)?);
        Ok(transactions)
    }

    /// Check the number of seal fields.
    fn verify_block_basic(&self, header: &Header) -> Result<(), Error> {
        if header.number() >= self.validate_score_transition
            && *header.difficulty() >= U256::from(U128::max_value())
        {
            return Err(From::from(BlockError::DifficultyOutOfBounds(OutOfBounds {
                min: None,
                max: Some(U256::from(U128::max_value())),
                found: *header.difficulty(),
            })));
        }

        match verify_timestamp(
            &self.step.inner,
            header_step(header, self.empty_steps_transition)?,
        ) {
            Err(BlockError::InvalidSeal) => {
                // This check runs in Phase 1 where there is no guarantee that the parent block is
                // already imported, therefore the call to `epoch_set` may fail. In that case we
                // won't report the misbehavior but this is not a concern because:
                // - Only authorities can report and it's expected that they'll be up-to-date and
                //   importing, therefore the parent header will most likely be available
                // - Even if you are an authority that is syncing the chain, the contract will most
                //   likely ignore old reports
                // - This specific check is only relevant if you're importing (since it checks
                //   against wall clock)
                if let Ok((_, set_number)) = self.epoch_set(header) {
                    self.validators
                        .report_benign(header.author(), set_number, header.number());
                }

                Err(BlockError::InvalidSeal.into())
            }
            Err(e) => Err(e.into()),
            Ok(()) => Ok(()),
        }
    }

    /// Do the step and gas limit validation.
    fn verify_block_family(&self, header: &Header, parent: &Header) -> Result<(), Error> {
        let step = header_step(header, self.empty_steps_transition)?;
        let parent_step = header_step(parent, self.empty_steps_transition)?;

        let (validators, set_number) = self.epoch_set(header)?;

        // Ensure header is from the step after parent.
        if step == parent_step
            || (header.number() >= self.validate_step_transition && step <= parent_step)
        {
            trace!(target: "engine", "Multiple blocks proposed for step {}.", parent_step);

            self.validators.report_malicious(
                header.author(),
                set_number,
                header.number(),
                Default::default(),
            );
            Err(EngineError::DoubleVote(*header.author()))?;
        }

        // Report malice if the validator produced other sibling blocks in the same step.
        let received_step_key = (step, *header.author());
        let new_hash = header.hash();
        if self
            .received_step_hashes
            .read()
            .get(&received_step_key)
            .map_or(false, |h| *h != new_hash)
        {
            trace!(target: "engine", "Validator {} produced sibling blocks in the same step", header.author());
            self.validators.report_malicious(
                header.author(),
                set_number,
                header.number(),
                Default::default(),
            );
        } else {
            self.received_step_hashes
                .write()
                .insert(received_step_key, new_hash);
        }

        // Remove hash records older than two full rounds of steps (picked as a reasonable trade-off between
        // memory consumption and fault-tolerance).
        let sibling_malice_detection_period = 2 * validators.count(&parent.hash()) as u64;
        let oldest_step = parent_step.saturating_sub(sibling_malice_detection_period);
        if oldest_step > 0 {
            let mut rsh = self.received_step_hashes.write();
            let new_rsh = rsh.split_off(&(oldest_step, Address::zero()));
            *rsh = new_rsh;
        }

        // If empty step messages are enabled we will validate the messages in the seal, missing messages are not
        // reported as there's no way to tell whether the empty step message was never sent or simply not included.
        let empty_steps_len = if header.number() >= self.empty_steps_transition {
            let validate_empty_steps = || -> Result<usize, Error> {
                let strict_empty_steps = header.number() >= self.strict_empty_steps_transition;
                let empty_steps = header_empty_steps(header)?;
                let empty_steps_len = empty_steps.len();
                let mut prev_empty_step = 0;

                for empty_step in empty_steps {
                    if empty_step.step <= parent_step || empty_step.step >= step {
                        Err(EngineError::InsufficientProof(format!(
                            "empty step proof for invalid step: {:?}",
                            empty_step.step
                        )))?;
                    }

                    if empty_step.parent_hash != *header.parent_hash() {
                        Err(EngineError::InsufficientProof(format!(
                            "empty step proof for invalid parent hash: {:?}",
                            empty_step.parent_hash
                        )))?;
                    }

                    if !empty_step.verify(&*validators).unwrap_or(false) {
                        Err(EngineError::InsufficientProof(format!(
                            "invalid empty step proof: {:?}",
                            empty_step
                        )))?;
                    }

                    if strict_empty_steps {
                        if empty_step.step <= prev_empty_step {
                            Err(EngineError::InsufficientProof(format!(
                                "{} empty step: {:?}",
                                if empty_step.step == prev_empty_step {
                                    "duplicate"
                                } else {
                                    "unordered"
                                },
                                empty_step
                            )))?;
                        }

                        prev_empty_step = empty_step.step;
                    }
                }

                Ok(empty_steps_len)
            };

            match validate_empty_steps() {
                Ok(len) => len,
                Err(err) => {
                    trace!(
                        target: "engine",
                        "Reporting benign misbehaviour (cause: invalid empty steps) \
                        at block #{}, epoch set number {}. Own address: {}",
                        header.number(), set_number, self.address().unwrap_or_default()
                    );
                    self.validators
                        .report_benign(header.author(), set_number, header.number());
                    return Err(err);
                }
            }
        } else {
            self.report_skipped(header, step, parent_step, &*validators, set_number);

            0
        };

        if header.number() >= self.validate_score_transition {
            let expected_difficulty =
                calculate_score(parent_step.into(), step.into(), empty_steps_len.into());
            if header.difficulty() != &expected_difficulty {
                return Err(From::from(BlockError::InvalidDifficulty(Mismatch {
                    expected: expected_difficulty,
                    found: header.difficulty().clone(),
                })));
            }
        }

        Ok(())
    }

    // t_nb 6.4 Check the validators.
    fn verify_block_external(&self, header: &Header) -> Result<(), Error> {
        let (validators, set_number) = self.epoch_set(header)?;

        // verify signature against fixed list, but reports should go to the
        // contract itself.
        let res = verify_external(header, &*validators, self.empty_steps_transition);
        match res {
            Err(Error(ErrorKind::Engine(EngineError::NotProposer(_)), _)) => {
                trace!(
					target: "engine",
					"Reporting benign misbehaviour (cause: block from incorrect proposer) \
					at block #{}, epoch set number {}. Own address: {}",
					header.number(), set_number, self.address().unwrap_or_default());
                self.validators
                    .report_benign(header.author(), set_number, header.number());
            }
            Ok(_) => {
                // we can drop all accumulated empty step messages that are older than this header's step
                let header_step = header_step(header, self.empty_steps_transition)?;
                self.clear_empty_steps(header_step.into());
            }
            _ => {}
        }
        res
    }

    fn genesis_epoch_data(&self, header: &Header, call: &Call) -> Result<Vec<u8>, String> {
        self.validators
            .genesis_epoch_data(header, call)
            .map(|set_proof| combine_proofs(0, &set_proof, &[]))
    }

    fn signals_epoch_end(
        &self,
        header: &Header,
        aux: AuxiliaryData,
    ) -> super::EpochChange<EthereumMachine> {
        if self.immediate_transitions {
            return super::EpochChange::No;
        }

        let first = header.number() == 0;
        self.validators.signals_epoch_end(first, header, aux)
    }

    fn is_epoch_end(
        &self,
        chain_head: &Header,
        finalized: &[H256],
        chain: &super::Headers<Header>,
        transition_store: &super::PendingTransitionStore,
    ) -> Option<Vec<u8>> {
        // epochs only matter if we want to support light clients.
        if self.immediate_transitions {
            return None;
        }

        let first = chain_head.number() == 0;

        // Apply transitions that don't require finality and should be enacted immediately (e.g from chain spec)
        if let Some(change) = self.validators.is_epoch_end(first, chain_head) {
            info!(
                target: "engine",
                "Immediately applying validator set change signalled at block {}",
                chain_head.number()
            );
            self.epoch_manager.lock().note_new_epoch();
            let change = combine_proofs(chain_head.number(), &change, &[]);
            return Some(change);
        }

        // check transition store for pending transitions against recently finalized blocks
        for finalized_hash in finalized {
            if let Some(pending) = transition_store(*finalized_hash) {
                // walk the chain backwards from current head until finalized_hash
                // to construct transition proof. author == ec_recover(sig) known
                // since the blocks are in the DB.
                let mut hash = chain_head.hash();
                let mut finality_proof: Vec<_> = itertools::repeat_call(move || {
                    chain(hash).and_then(|header| {
                        hash = *header.parent_hash();
                        if header.number() == 0 {
                            None
                        } else {
                            Some(header)
                        }
                    })
                })
                .while_some()
                .take_while(|h| h.hash() != *finalized_hash)
                .collect();

                let finalized_header = if *finalized_hash == chain_head.hash() {
                    // chain closure only stores ancestry, but the chain head is also unfinalized.
                    chain_head.clone()
                } else {
                    chain(*finalized_hash).expect(
                        "header is finalized; finalized headers must exist in the chain; qed",
                    )
                };

                let signal_number = finalized_header.number();
                info!(target: "engine", "Applying validator set change signalled at block {}", signal_number);

                finality_proof.push(finalized_header);
                finality_proof.reverse();

                let finality_proof = ::rlp::encode_list(&finality_proof);

                self.epoch_manager.lock().note_new_epoch();

                // We turn off can_propose here because upon validator set change there can
                // be two valid proposers for a single step: one from the old set and
                // one from the new.
                //
                // This way, upon encountering an epoch change, the proposer from the
                // new set will be forced to wait until the next step to avoid sealing a
                // block that breaks the invariant that the parent's step < the block's step.
                self.step.can_propose.store(false, AtomicOrdering::SeqCst);
                return Some(combine_proofs(
                    signal_number,
                    &pending.proof,
                    &*finality_proof,
                ));
            }
        }

        None
    }

    fn epoch_verifier<'a>(
        &self,
        _header: &Header,
        proof: &'a [u8],
    ) -> ConstructedVerifier<'a, EthereumMachine> {
        let (signal_number, set_proof, finality_proof) = match destructure_proofs(proof) {
            Ok(x) => x,
            Err(e) => return ConstructedVerifier::Err(e),
        };

        let first = signal_number == 0;
        match self
            .validators
            .epoch_set(first, &self.machine, signal_number, set_proof)
        {
            Ok((list, finalize)) => {
                let verifier = Box::new(EpochVerifier {
                    step: self.step.clone(),
                    subchain_validators: list,
                    empty_steps_transition: self.empty_steps_transition,
                    two_thirds_majority_transition: self.two_thirds_majority_transition,
                    eip1559_transition: self.params().eip1559_transition,
                });

                match finalize {
                    Some(finalize) => {
                        ConstructedVerifier::Unconfirmed(verifier, finality_proof, finalize)
                    }
                    None => ConstructedVerifier::Trusted(verifier),
                }
            }
            Err(e) => ConstructedVerifier::Err(e),
        }
    }

    fn register_client(&self, client: Weak<dyn EngineClient>) {
        *self.client.write() = Some(client.clone());
        self.validators.register_client(client);
    }

    fn set_signer(&self, signer: Option<Box<dyn EngineSigner>>) {
        *self.signer.write() = signer;
    }

    fn sign(&self, hash: H256) -> Result<Signature, Error> {
        Ok(self
            .signer
            .read()
            .as_ref()
            .ok_or(publickey::Error::InvalidAddress)?
            .sign(hash)?)
    }

    fn snapshot_components(&self) -> Option<Box<dyn crate::snapshot::SnapshotComponents>> {
        if self.immediate_transitions {
            None
        } else {
            Some(Box::new(::snapshot::PoaSnapshot))
        }
    }

    fn fork_choice(&self, new: &ExtendedHeader, current: &ExtendedHeader) -> super::ForkChoice {
        super::total_difficulty_fork_choice(new, current)
    }

    fn ancestry_actions(
        &self,
        header: &Header,
        ancestry: &mut dyn Iterator<Item = ExtendedHeader>,
    ) -> Vec<AncestryAction> {
        let finalized = self.build_finality(
            header,
            &mut ancestry.take_while(|e| !e.is_finalized).map(|e| e.header),
        );

        if !finalized.is_empty() {
            debug!(target: "finality", "Finalizing blocks: {:?}", finalized);
        }

        finalized
            .into_iter()
            .map(AncestryAction::MarkFinalized)
            .collect()
    }

    fn gas_limit_override(&self, header: &Header) -> Option<U256> {
        let (_, &address) = self
            .block_gas_limit_contract_transitions
            .range(..=header.number())
            .last()?;
        let client = self.upgrade_client_or("Unable to prepare block").ok()?;
        let full_client = match client.as_full_client() {
            Some(full_client) => full_client,
            None => {
                error!(target: "engine", "Failed to upgrade to BlockchainClient.");
                return None;
            }
        };
        if let Some(limit) = self.gas_limit_override_cache.lock().get_mut(&header.hash()) {
            return *limit;
        }
        let limit = util::block_gas_limit(full_client, header, address);
        self.gas_limit_override_cache
            .lock()
            .insert(header.hash(), limit);
        limit
    }
}

/// A helper accumulator function mapping a step duration and a step duration transition timestamp
/// to the corresponding step number and the correct starting second of the step.
fn next_step_time_duration(info: StepDurationInfo, time: u64) -> Option<(u64, u64)> {
    let step_diff = time
        .checked_add(info.step_duration)?
        .checked_sub(1)?
        .checked_sub(info.transition_timestamp)?
        .checked_div(info.step_duration)?;
    let time_diff = step_diff.checked_mul(info.step_duration)?;
    Some((
        info.transition_step.checked_add(step_diff)?,
        info.transition_timestamp.checked_add(time_diff)?,
    ))
}

#[cfg(test)]
mod tests {
    use super::{
        calculate_score, next_step_time_duration, util::BoundContract, AuthorityRound,
        AuthorityRoundParams, EmptyStep, SealedEmptyStep, StepDurationInfo,
    };
    use accounts::AccountProvider;
    use block::*;
    use crypto::publickey::Signature;
    use engines::{
        block_reward::BlockRewardContract,
        validator_set::{SimpleList, TestSet},
        Engine, EngineError, EthEngine, Seal,
    };
    use error::{Error, ErrorKind};
    use ethabi_contract::use_contract;
    use ethereum_types::{Address, H256, H520, U256};
    use ethjson;
    use hash::keccak;
    use miner::{Author, MinerService};
    use rlp::encode;
    use spec::Spec;
    use std::{
        collections::BTreeMap,
        str::FromStr,
        sync::{
            atomic::{AtomicU64, AtomicUsize, Ordering as AtomicOrdering},
            Arc,
        },
        time::Duration,
    };
    use test_helpers::{
        generate_dummy_client_with_spec, generate_dummy_client_with_spec_and_data,
        get_temp_state_db, TestNotify,
    };
    use types::{
        header::Header,
        ids::BlockId,
        transaction::{Action, Transaction, TypedTransaction},
    };

    fn aura<F>(f: F) -> Arc<AuthorityRound>
    where
        F: FnOnce(&mut AuthorityRoundParams),
    {
        let mut params = AuthorityRoundParams {
            step_durations: [(0, 1)].to_vec().into_iter().collect(),
            start_step: Some(1),
            validators: Box::new(TestSet::default()),
            validate_score_transition: 0,
            validate_step_transition: 0,
            immediate_transitions: true,
            maximum_uncle_count_transition: 0,
            maximum_uncle_count: 0,
            empty_steps_transition: u64::max_value(),
            maximum_empty_steps: 0,
            block_reward: Default::default(),
            block_reward_contract_transitions: Default::default(),
            strict_empty_steps_transition: 0,
            two_thirds_majority_transition: 0,
            randomness_contract_address: BTreeMap::new(),
            block_gas_limit_contract_transitions: BTreeMap::new(),
            posdao_transition: Some(0),
        };

        // mutate aura params
        f(&mut params);

        // create engine
        let mut c_params = ::spec::CommonParams::default();
        c_params.gas_limit_bound_divisor = 5.into();
        let machine = ::machine::EthereumMachine::regular(c_params, Default::default());
        AuthorityRound::new(params, machine).unwrap()
    }

    #[test]
    fn test_next_step_time_duration() {
        // At step 7 (time 1000), we transitioned to step duration 10.
        let info = StepDurationInfo {
            step_duration: 10,
            transition_step: 7,
            transition_timestamp: 1000,
        };
        // So the next transition can happen e.g. at step 12 (time 1050) or 13 (time 1060).
        assert_eq!(Some((12, 1050)), next_step_time_duration(info, 1050));
        assert_eq!(Some((13, 1060)), next_step_time_duration(info, 1051));
        assert_eq!(Some((13, 1060)), next_step_time_duration(info, 1055));
        // The next transition could also happen immediately.
        assert_eq!(Some((7, 1000)), next_step_time_duration(info, 1000));
    }

    #[test]
    fn has_valid_metadata() {
        let engine = Spec::new_test_round().engine;
        assert!(!engine.name().is_empty());
    }

    #[test]
    fn can_return_schedule() {
        let engine = Spec::new_test_round().engine;
        let schedule = engine.schedule(10000000);

        assert!(schedule.stack_limit > 0);
    }

    #[test]
    fn can_do_signature_verification_fail() {
        let engine = Spec::new_test_round().engine;
        let mut header: Header = Header::default();
        header.set_seal(vec![encode(&H520::default())]);

        let verify_result = engine.verify_block_external(&header);
        assert!(verify_result.is_err());
    }

    #[test]
    fn generates_seal_and_does_not_double_propose() {
        let tap = Arc::new(AccountProvider::transient_provider());
        let addr1 = tap.insert_account(keccak("1").into(), &"1".into()).unwrap();

        let spec = Spec::new_test_round();
        let engine = &*spec.engine;
        let genesis_header = spec.genesis_header();
        let db1 = spec
            .ensure_db_good(get_temp_state_db(), &Default::default())
            .unwrap();
        let last_hashes = Arc::new(vec![genesis_header.hash()]);
        let b1 = OpenBlock::new(
            engine,
            Default::default(),
            false,
            db1,
            &genesis_header,
            last_hashes.clone(),
            addr1,
            (3141562.into(), 31415620.into()),
            vec![],
            false,
            None,
        )
        .unwrap();
        let b1 = b1.close_and_lock().unwrap();

        engine.set_signer(Some(Box::new((tap.clone(), addr1, "1".into()))));
        if let Seal::Regular(seal) = engine.generate_seal(&b1, &genesis_header) {
            assert!(b1.clone().try_seal(engine, seal).is_ok());
            // Second proposal is forbidden.
            assert!(engine.generate_seal(&b1, &genesis_header) == Seal::None);
        } else {
            panic!("block 1 not sealed");
        }
    }

    #[test]
    fn generates_seal_iff_sealer_is_set() {
        let tap = Arc::new(AccountProvider::transient_provider());
        let addr1 = tap.insert_account(keccak("1").into(), &"1".into()).unwrap();
        let spec = Spec::new_test_round();
        let engine = &*spec.engine;
        let genesis_header = spec.genesis_header();
        let db1 = spec
            .ensure_db_good(get_temp_state_db(), &Default::default())
            .unwrap();
        let last_hashes = Arc::new(vec![genesis_header.hash()]);
        let b1 = OpenBlock::new(
            engine,
            Default::default(),
            false,
            db1,
            &genesis_header,
            last_hashes.clone(),
            addr1,
            (3141562.into(), 31415620.into()),
            vec![],
            false,
            None,
        )
        .unwrap()
        .close_and_lock()
        .unwrap();
        // Not a signer. A seal cannot be generated.
        assert!(engine.generate_seal(&b1, &genesis_header) == Seal::None);
        // Become a signer.
        engine.set_signer(Some(Box::new((tap.clone(), addr1, "1".into()))));
        if let Seal::Regular(seal) = engine.generate_seal(&b1, &genesis_header) {
            assert!(b1.clone().try_seal(engine, seal).is_ok());
            // Second proposal is forbidden.
            assert!(engine.generate_seal(&b1, &genesis_header) == Seal::None);
        } else {
            panic!("block 1 not sealed");
        }
        // Stop being a signer.
        engine.set_signer(None);
        // Make a step first and then create a new block in that new step.
        engine.step();
        let addr2 = tap.insert_account(keccak("0").into(), &"0".into()).unwrap();
        let mut header2 = genesis_header.clone();
        header2.set_number(2);
        header2.set_author(addr2);
        header2.set_parent_hash(header2.hash());
        let db2 = spec
            .ensure_db_good(get_temp_state_db(), &Default::default())
            .unwrap();
        let b2 = OpenBlock::new(
            engine,
            Default::default(),
            false,
            db2,
            &header2,
            last_hashes,
            addr2,
            (3141562.into(), 31415620.into()),
            vec![],
            false,
            None,
        )
        .unwrap()
        .close_and_lock()
        .unwrap();
        // Not a signer. A seal cannot be generated.
        assert!(engine.generate_seal(&b2, &header2) == Seal::None);
        // Become a signer once more.
        engine.set_signer(Some(Box::new((tap, addr2, "0".into()))));
        if let Seal::Regular(seal) = engine.generate_seal(&b2, &header2) {
            assert!(b2.clone().try_seal(engine, seal).is_ok());
            // Second proposal is forbidden.
            assert!(engine.generate_seal(&b2, &header2) == Seal::None);
        } else {
            panic!("block 2 not sealed");
        }
    }

    #[test]
    fn checks_difficulty_in_generate_seal() {
        let tap = Arc::new(AccountProvider::transient_provider());
        let addr1 = tap.insert_account(keccak("1").into(), &"1".into()).unwrap();
        let addr2 = tap.insert_account(keccak("0").into(), &"0".into()).unwrap();

        let spec = Spec::new_test_round();
        let engine = &*spec.engine;

        let genesis_header = spec.genesis_header();
        let db1 = spec
            .ensure_db_good(get_temp_state_db(), &Default::default())
            .unwrap();
        let db2 = spec
            .ensure_db_good(get_temp_state_db(), &Default::default())
            .unwrap();
        let last_hashes = Arc::new(vec![genesis_header.hash()]);

        let b1 = OpenBlock::new(
            engine,
            Default::default(),
            false,
            db1,
            &genesis_header,
            last_hashes.clone(),
            addr1,
            (3141562.into(), 31415620.into()),
            vec![],
            false,
            None,
        )
        .unwrap();
        let b1 = b1.close_and_lock().unwrap();
        let b2 = OpenBlock::new(
            engine,
            Default::default(),
            false,
            db2,
            &genesis_header,
            last_hashes,
            addr2,
            (3141562.into(), 31415620.into()),
            vec![],
            false,
            None,
        )
        .unwrap();
        let b2 = b2.close_and_lock().unwrap();

        engine.set_signer(Some(Box::new((tap.clone(), addr1, "1".into()))));
        match engine.generate_seal(&b1, &genesis_header) {
            Seal::None | Seal::Proposal(_) => panic!("wrong seal"),
            Seal::Regular(_) => {
                engine.step();

                engine.set_signer(Some(Box::new((tap.clone(), addr2, "0".into()))));
                match engine.generate_seal(&b2, &genesis_header) {
                    Seal::Regular(_) | Seal::Proposal(_) => {
                        panic!("sealed despite wrong difficulty")
                    }
                    Seal::None => {}
                }
            }
        }
    }

    #[test]
    fn proposer_switching() {
        let tap = AccountProvider::transient_provider();
        let addr = tap.insert_account(keccak("0").into(), &"0".into()).unwrap();
        let mut parent_header: Header = Header::default();
        parent_header.set_seal(vec![encode(&0usize)]);
        parent_header.set_gas_limit("222222".parse::<U256>().unwrap());
        let mut header: Header = Header::default();
        header.set_number(1);
        header.set_gas_limit("222222".parse::<U256>().unwrap());
        header.set_author(addr);

        let engine = Spec::new_test_round().engine;

        // Two validators.
        // Spec starts with step 2.
        header.set_difficulty(calculate_score(0, 2, 0));
        let signature = tap
            .sign(addr, Some("0".into()), header.bare_hash())
            .unwrap();
        header.set_seal(vec![encode(&2usize), encode(&(&*signature as &[u8]))]);
        assert!(engine.verify_block_family(&header, &parent_header).is_ok());
        assert!(engine.verify_block_external(&header).is_err());
        header.set_difficulty(calculate_score(0, 1, 0));
        let signature = tap
            .sign(addr, Some("0".into()), header.bare_hash())
            .unwrap();
        header.set_seal(vec![encode(&1usize), encode(&(&*signature as &[u8]))]);
        assert!(engine.verify_block_family(&header, &parent_header).is_ok());
        assert!(engine.verify_block_external(&header).is_ok());
    }

    #[test]
    fn rejects_future_block() {
        let tap = AccountProvider::transient_provider();
        let addr = tap.insert_account(keccak("0").into(), &"0".into()).unwrap();

        let mut parent_header: Header = Header::default();
        parent_header.set_seal(vec![encode(&0usize)]);
        parent_header.set_gas_limit("222222".parse::<U256>().unwrap());
        let mut header: Header = Header::default();
        header.set_number(1);
        header.set_gas_limit("222222".parse::<U256>().unwrap());
        header.set_author(addr);

        let engine = Spec::new_test_round().engine;

        // Two validators.
        // Spec starts with step 2.
        header.set_difficulty(calculate_score(0, 1, 0));
        let signature = tap
            .sign(addr, Some("0".into()), header.bare_hash())
            .unwrap();
        header.set_seal(vec![encode(&1usize), encode(&(&*signature as &[u8]))]);
        assert!(engine.verify_block_family(&header, &parent_header).is_ok());
        assert!(engine.verify_block_external(&header).is_ok());
        header.set_seal(vec![encode(&5usize), encode(&(&*signature as &[u8]))]);
        assert!(engine.verify_block_basic(&header).is_err());
    }

    #[test]
    fn rejects_step_backwards() {
        let tap = AccountProvider::transient_provider();
        let addr = tap.insert_account(keccak("0").into(), &"0".into()).unwrap();

        let mut parent_header: Header = Header::default();
        parent_header.set_seal(vec![encode(&4usize)]);
        parent_header.set_gas_limit("222222".parse::<U256>().unwrap());
        let mut header: Header = Header::default();
        header.set_number(1);
        header.set_gas_limit("222222".parse::<U256>().unwrap());
        header.set_author(addr);

        let engine = Spec::new_test_round().engine;

        let signature = tap
            .sign(addr, Some("0".into()), header.bare_hash())
            .unwrap();
        // Two validators.
        // Spec starts with step 2.
        header.set_seal(vec![encode(&5usize), encode(&(&*signature as &[u8]))]);
        header.set_difficulty(calculate_score(4, 5, 0));
        assert!(engine.verify_block_family(&header, &parent_header).is_ok());
        header.set_seal(vec![encode(&3usize), encode(&(&*signature as &[u8]))]);
        header.set_difficulty(calculate_score(4, 3, 0));
        assert!(engine.verify_block_family(&header, &parent_header).is_err());
    }

    #[test]
    fn reports_skipped() {
        let last_benign = Arc::new(AtomicUsize::new(0));
        let aura = aura(|p| {
            p.validators = Box::new(TestSet::new(Default::default(), last_benign.clone()));
        });

        let mut parent_header: Header = Header::default();
        parent_header.set_seal(vec![encode(&1usize)]);
        parent_header.set_gas_limit("222222".parse::<U256>().unwrap());
        let mut header: Header = Header::default();
        header.set_difficulty(calculate_score(1, 3, 0));
        header.set_gas_limit("222222".parse::<U256>().unwrap());
        header.set_seal(vec![encode(&3usize)]);

        // Do not report when signer not present.
        assert!(aura.verify_block_family(&header, &parent_header).is_ok());
        assert_eq!(last_benign.load(AtomicOrdering::SeqCst), 0);

        aura.set_signer(Some(Box::new((
            Arc::new(AccountProvider::transient_provider()),
            Default::default(),
            "".into(),
        ))));

        // Do not report on steps skipped between genesis and first block.
        header.set_number(1);
        assert!(aura.verify_block_family(&header, &parent_header).is_ok());
        assert_eq!(last_benign.load(AtomicOrdering::SeqCst), 0);

        // Report on skipped steps otherwise.
        header.set_number(2);
        assert!(aura.verify_block_family(&header, &parent_header).is_ok());
        assert_eq!(last_benign.load(AtomicOrdering::SeqCst), 2);
    }

    #[test]
    fn reports_multiple_blocks_per_step() {
        let tap = AccountProvider::transient_provider();
        let addr0 = tap.insert_account(keccak("0").into(), &"0".into()).unwrap();
        let addr1 = tap.insert_account(keccak("1").into(), &"1".into()).unwrap();

        let validator_set = TestSet::from_validators(vec![addr0, addr1]);
        let aura = aura(|p| p.validators = Box::new(validator_set.clone()));

        aura.set_signer(Some(Box::new((Arc::new(tap), addr0, "0".into()))));

        let mut parent_header: Header = Header::default();
        parent_header.set_number(2);
        parent_header.set_seal(vec![encode(&1usize)]);
        parent_header.set_gas_limit("222222".parse::<U256>().unwrap());
        let mut header: Header = Header::default();
        header.set_number(3);
        header.set_difficulty(calculate_score(1, 2, 0));
        header.set_gas_limit("222222".parse::<U256>().unwrap());
        header.set_seal(vec![encode(&2usize)]);
        header.set_author(addr1);

        // First sibling block.
        assert!(aura.verify_block_family(&header, &parent_header).is_ok());
        assert_eq!(validator_set.last_malicious(), 0);

        // Second sibling block: should be reported.
        header.set_gas_limit("222223".parse::<U256>().unwrap());
        assert!(aura.verify_block_family(&header, &parent_header).is_ok());
        assert_eq!(validator_set.last_malicious(), 3);
    }

    #[test]
    fn test_uncles_transition() {
        let aura = aura(|params| {
            params.maximum_uncle_count_transition = 1;
        });

        assert_eq!(aura.maximum_uncle_count(0), 2);
        assert_eq!(aura.maximum_uncle_count(1), 0);
        assert_eq!(aura.maximum_uncle_count(100), 0);
    }

    #[test]
    #[should_panic(expected = "counter is too high")]
    fn test_counter_increment_too_high() {
        use super::Step;
        let step = Step {
            calibrate: false,
            inner: AtomicU64::new(::std::u64::MAX),
            durations: [StepDurationInfo {
                transition_step: 0,
                transition_timestamp: 0,
                step_duration: 1,
            }]
            .to_vec()
            .into_iter()
            .collect(),
        };
        step.increment();
    }

    #[test]
    #[should_panic(expected = "step counter under- or overflow")]
    fn test_counter_duration_remaining_too_high() {
        use super::Step;
        let step = Step {
            calibrate: false,
            inner: AtomicU64::new(::std::u64::MAX),
            durations: [StepDurationInfo {
                transition_step: 0,
                transition_timestamp: 0,
                step_duration: 1,
            }]
            .to_vec()
            .into_iter()
            .collect(),
        };
        step.duration_remaining();
    }

    #[test]
    fn test_change_step_duration() {
        use super::Step;
        use std::thread;

        let now = super::unix_now().as_secs();
        let step = Step {
            calibrate: true,
            inner: AtomicU64::new(::std::u64::MAX),
            durations: [
                StepDurationInfo {
                    transition_step: 0,
                    transition_timestamp: 0,
                    step_duration: 1,
                },
                StepDurationInfo {
                    transition_step: now,
                    transition_timestamp: now,
                    step_duration: 2,
                },
                StepDurationInfo {
                    transition_step: now + 1,
                    transition_timestamp: now + 2,
                    step_duration: 4,
                },
            ]
            .to_vec()
            .into_iter()
            .collect(),
        };
        // calibrated step `now`
        step.calibrate();
        let duration_remaining = step.duration_remaining();
        assert_eq!(step.inner.load(AtomicOrdering::SeqCst), now);
        assert!(duration_remaining <= Duration::from_secs(2));
        thread::sleep(duration_remaining);
        step.increment();
        // calibrated step `now + 1`
        step.calibrate();
        let duration_remaining = step.duration_remaining();
        assert_eq!(step.inner.load(AtomicOrdering::SeqCst), now + 1);
        assert!(duration_remaining > Duration::from_secs(2));
        assert!(duration_remaining <= Duration::from_secs(4));
    }

    #[test]
    #[should_panic]
    fn test_step_duration_zero() {
        aura(|params| {
            params.step_durations = [(0, 0)].to_vec().into_iter().collect();
        });
    }

    fn setup_empty_steps() -> (Spec, Arc<AccountProvider>, Vec<Address>) {
        let spec = Spec::new_test_round_empty_steps();
        let tap = Arc::new(AccountProvider::transient_provider());

        let addr1 = tap.insert_account(keccak("1").into(), &"1".into()).unwrap();
        let addr2 = tap.insert_account(keccak("0").into(), &"0".into()).unwrap();

        let accounts = vec![addr1, addr2];

        (spec, tap, accounts)
    }

    fn empty_step(engine: &dyn EthEngine, step: u64, parent_hash: &H256) -> EmptyStep {
        let empty_step_rlp = super::empty_step_rlp(step, parent_hash);
        let signature = engine.sign(keccak(&empty_step_rlp)).unwrap().into();
        let parent_hash = parent_hash.clone();
        EmptyStep {
            step,
            signature,
            parent_hash,
        }
    }

    fn sealed_empty_step(engine: &dyn EthEngine, step: u64, parent_hash: &H256) -> SealedEmptyStep {
        let empty_step_rlp = super::empty_step_rlp(step, parent_hash);
        let signature = engine.sign(keccak(&empty_step_rlp)).unwrap().into();
        SealedEmptyStep { signature, step }
    }

    fn set_empty_steps_seal(
        header: &mut Header,
        step: u64,
        block_signature: &crypto::publickey::Signature,
        empty_steps: &[SealedEmptyStep],
    ) {
        header.set_seal(vec![
            encode(&(step as usize)),
            encode(&(&**block_signature as &[u8])),
            ::rlp::encode_list(&empty_steps),
        ]);
    }

    fn assert_insufficient_proof<T: std::fmt::Debug>(result: Result<T, Error>, contains: &str) {
        match result {
            Err(Error(ErrorKind::Engine(EngineError::InsufficientProof(ref s)), _)) => {
                assert!(
                    s.contains(contains),
                    "Expected {:?} to contain {:?}",
                    s,
                    contains
                );
            }
            e => assert!(false, "Unexpected result: {:?}", e),
        }
    }

    #[test]
    fn broadcast_empty_step_message() {
        let (spec, tap, accounts) = setup_empty_steps();

        let addr1 = accounts[0];

        let engine = &*spec.engine;
        let genesis_header = spec.genesis_header();
        let db1 = spec
            .ensure_db_good(get_temp_state_db(), &Default::default())
            .unwrap();

        let last_hashes = Arc::new(vec![genesis_header.hash()]);

        let client = generate_dummy_client_with_spec(Spec::new_test_round_empty_steps);
        let notify = Arc::new(TestNotify::default());
        client.add_notify(notify.clone());
        engine.register_client(Arc::downgrade(&client) as _);

        engine.set_signer(Some(Box::new((tap.clone(), addr1, "1".into()))));

        let b1 = OpenBlock::new(
            engine,
            Default::default(),
            false,
            db1,
            &genesis_header,
            last_hashes.clone(),
            addr1,
            (3141562.into(), 31415620.into()),
            vec![],
            false,
            None,
        )
        .unwrap();
        let b1 = b1.close_and_lock().unwrap();

        // the block is empty so we don't seal and instead broadcast an empty step message
        assert_eq!(engine.generate_seal(&b1, &genesis_header), Seal::None);

        // spec starts with step 2
        let empty_step_rlp = encode(&empty_step(engine, 2, &genesis_header.hash()));

        // we've received the message
        assert!(notify.messages.read().contains(&empty_step_rlp));
        let len = notify.messages.read().len();

        // make sure that we don't generate empty step for the second time
        assert_eq!(engine.generate_seal(&b1, &genesis_header), Seal::None);
        assert_eq!(len, notify.messages.read().len());
    }

    #[test]
    fn seal_with_empty_steps() {
        let (spec, tap, accounts) = setup_empty_steps();

        let addr1 = accounts[0];
        let addr2 = accounts[1];

        let engine = &*spec.engine;
        let genesis_header = spec.genesis_header();
        let db1 = spec
            .ensure_db_good(get_temp_state_db(), &Default::default())
            .unwrap();
        let db2 = spec
            .ensure_db_good(get_temp_state_db(), &Default::default())
            .unwrap();

        let last_hashes = Arc::new(vec![genesis_header.hash()]);

        let client = generate_dummy_client_with_spec(Spec::new_test_round_empty_steps);
        let notify = Arc::new(TestNotify::default());
        client.add_notify(notify.clone());
        engine.register_client(Arc::downgrade(&client) as _);

        // step 2
        let b1 = OpenBlock::new(
            engine,
            Default::default(),
            false,
            db1,
            &genesis_header,
            last_hashes.clone(),
            addr1,
            (3141562.into(), 31415620.into()),
            vec![],
            false,
            None,
        )
        .unwrap();
        let b1 = b1.close_and_lock().unwrap();

        // since the block is empty it isn't sealed and we generate empty steps
        engine.set_signer(Some(Box::new((tap.clone(), addr1, "1".into()))));
        assert_eq!(engine.generate_seal(&b1, &genesis_header), Seal::None);
        engine.step();

        // step 3
        let mut b2 = OpenBlock::new(
            engine,
            Default::default(),
            false,
            db2,
            &genesis_header,
            last_hashes.clone(),
            addr2,
            (3141562.into(), 31415620.into()),
            vec![],
            false,
            None,
        )
        .unwrap();
        b2.push_transaction(
            TypedTransaction::Legacy(Transaction {
                action: Action::Create,
                nonce: U256::from(0),
                gas_price: U256::from(3000),
                gas: U256::from(53_000),
                value: U256::from(1),
                data: vec![],
            })
            .fake_sign(addr2),
            None,
        )
        .unwrap();
        let b2 = b2.close_and_lock().unwrap();

        // we will now seal a block with 1tx and include the accumulated empty step message
        engine.set_signer(Some(Box::new((tap.clone(), addr2, "0".into()))));
        if let Seal::Regular(seal) = engine.generate_seal(&b2, &genesis_header) {
            engine.set_signer(Some(Box::new((tap.clone(), addr1, "1".into()))));
            let empty_step2 = sealed_empty_step(engine, 2, &genesis_header.hash());
            let empty_steps = ::rlp::encode_list(&vec![empty_step2]);

            assert_eq!(seal[0], encode(&3usize));
            assert_eq!(seal[2], empty_steps);
        }
    }

    #[test]
    fn seal_empty_block_with_empty_steps() {
        let (spec, tap, accounts) = setup_empty_steps();

        let addr1 = accounts[0];
        let addr2 = accounts[1];

        let engine = &*spec.engine;
        let genesis_header = spec.genesis_header();
        let db1 = spec
            .ensure_db_good(get_temp_state_db(), &Default::default())
            .unwrap();
        let db2 = spec
            .ensure_db_good(get_temp_state_db(), &Default::default())
            .unwrap();
        let db3 = spec
            .ensure_db_good(get_temp_state_db(), &Default::default())
            .unwrap();

        let last_hashes = Arc::new(vec![genesis_header.hash()]);

        let client = generate_dummy_client_with_spec(Spec::new_test_round_empty_steps);
        let notify = Arc::new(TestNotify::default());
        client.add_notify(notify.clone());
        engine.register_client(Arc::downgrade(&client) as _);

        // step 2
        let b1 = OpenBlock::new(
            engine,
            Default::default(),
            false,
            db1,
            &genesis_header,
            last_hashes.clone(),
            addr1,
            (3141562.into(), 31415620.into()),
            vec![],
            false,
            None,
        )
        .unwrap();
        let b1 = b1.close_and_lock().unwrap();

        // since the block is empty it isn't sealed and we generate empty steps
        engine.set_signer(Some(Box::new((tap.clone(), addr1, "1".into()))));
        assert_eq!(engine.generate_seal(&b1, &genesis_header), Seal::None);
        engine.step();

        // step 3
        let b2 = OpenBlock::new(
            engine,
            Default::default(),
            false,
            db2,
            &genesis_header,
            last_hashes.clone(),
            addr2,
            (3141562.into(), 31415620.into()),
            vec![],
            false,
            None,
        )
        .unwrap();
        let b2 = b2.close_and_lock().unwrap();
        engine.set_signer(Some(Box::new((tap.clone(), addr2, "0".into()))));
        assert_eq!(engine.generate_seal(&b2, &genesis_header), Seal::None);
        engine.step();

        // step 4
        // the spec sets the maximum_empty_steps to 2 so we will now seal an empty block and include the empty step messages
        let b3 = OpenBlock::new(
            engine,
            Default::default(),
            false,
            db3,
            &genesis_header,
            last_hashes.clone(),
            addr1,
            (3141562.into(), 31415620.into()),
            vec![],
            false,
            None,
        )
        .unwrap();
        let b3 = b3.close_and_lock().unwrap();

        engine.set_signer(Some(Box::new((tap.clone(), addr1, "1".into()))));
        if let Seal::Regular(seal) = engine.generate_seal(&b3, &genesis_header) {
            let empty_step2 = sealed_empty_step(engine, 2, &genesis_header.hash());
            engine.set_signer(Some(Box::new((tap.clone(), addr2, "0".into()))));
            let empty_step3 = sealed_empty_step(engine, 3, &genesis_header.hash());

            let empty_steps = ::rlp::encode_list(&vec![empty_step2, empty_step3]);

            assert_eq!(seal[0], encode(&4usize));
            assert_eq!(seal[2], empty_steps);
        }
    }

    #[test]
    fn reward_empty_steps() {
        let (spec, tap, accounts) = setup_empty_steps();

        let addr1 = accounts[0];

        let engine = &*spec.engine;
        let genesis_header = spec.genesis_header();
        let db1 = spec
            .ensure_db_good(get_temp_state_db(), &Default::default())
            .unwrap();
        let db2 = spec
            .ensure_db_good(get_temp_state_db(), &Default::default())
            .unwrap();

        let last_hashes = Arc::new(vec![genesis_header.hash()]);

        let client = generate_dummy_client_with_spec(Spec::new_test_round_empty_steps);
        engine.register_client(Arc::downgrade(&client) as _);

        // step 2
        let b1 = OpenBlock::new(
            engine,
            Default::default(),
            false,
            db1,
            &genesis_header,
            last_hashes.clone(),
            addr1,
            (3141562.into(), 31415620.into()),
            vec![],
            false,
            None,
        )
        .unwrap();
        let b1 = b1.close_and_lock().unwrap();

        // since the block is empty it isn't sealed and we generate empty steps
        engine.set_signer(Some(Box::new((tap.clone(), addr1, "1".into()))));
        assert_eq!(engine.generate_seal(&b1, &genesis_header), Seal::None);
        engine.step();

        // step 3
        // the signer of the accumulated empty step message should be rewarded
        let b2 = OpenBlock::new(
            engine,
            Default::default(),
            false,
            db2,
            &genesis_header,
            last_hashes.clone(),
            addr1,
            (3141562.into(), 31415620.into()),
            vec![],
            false,
            None,
        )
        .unwrap();
        let addr1_balance = b2.state.balance(&addr1).unwrap();

        // after closing the block `addr1` should be reward twice, one for the included empty step message and another for block creation
        let b2 = b2.close_and_lock().unwrap();

        // the spec sets the block reward to 10
        assert_eq!(b2.state.balance(&addr1).unwrap(), addr1_balance + (10 * 2))
    }

    #[test]
    fn verify_seal_empty_steps() {
        let (spec, tap, accounts) = setup_empty_steps();
        let addr1 = accounts[0];
        let addr2 = accounts[1];
        let engine = &*spec.engine;

        let mut parent_header: Header = Header::default();
        parent_header.set_seal(vec![encode(&0usize)]);
        parent_header.set_gas_limit("222222".parse::<U256>().unwrap());

        let mut header: Header = Header::default();
        header.set_parent_hash(parent_header.hash());
        header.set_number(1);
        header.set_gas_limit("222222".parse::<U256>().unwrap());
        header.set_author(addr1);

        let signature = tap
            .sign(addr1, Some("1".into()), header.bare_hash())
            .unwrap();

        // empty step with invalid step
        let empty_steps = vec![SealedEmptyStep {
            signature: H520::zero(),
            step: 2,
        }];
        set_empty_steps_seal(&mut header, 2, &signature, &empty_steps);

        assert_insufficient_proof(
            engine.verify_block_family(&header, &parent_header),
            "invalid step",
        );

        // empty step with invalid signature
        let empty_steps = vec![SealedEmptyStep {
            signature: H520::zero(),
            step: 1,
        }];
        set_empty_steps_seal(&mut header, 2, &signature, &empty_steps);

        assert_insufficient_proof(
            engine.verify_block_family(&header, &parent_header),
            "invalid empty step proof",
        );

        // empty step with valid signature from incorrect proposer for step
        engine.set_signer(Some(Box::new((tap.clone(), addr1, "1".into()))));
        let empty_steps = vec![sealed_empty_step(engine, 1, &parent_header.hash())];
        set_empty_steps_seal(&mut header, 2, &signature, &empty_steps);

        assert_insufficient_proof(
            engine.verify_block_family(&header, &parent_header),
            "invalid empty step proof",
        );

        // valid empty steps
        engine.set_signer(Some(Box::new((tap.clone(), addr1, "1".into()))));
        let empty_step2 = sealed_empty_step(engine, 2, &parent_header.hash());
        engine.set_signer(Some(Box::new((tap.clone(), addr2, "0".into()))));
        let empty_step3 = sealed_empty_step(engine, 3, &parent_header.hash());

        let empty_steps = vec![empty_step2, empty_step3];
        header.set_difficulty(calculate_score(0, 4, 2));
        let signature = tap
            .sign(addr1, Some("1".into()), header.bare_hash())
            .unwrap();
        set_empty_steps_seal(&mut header, 4, &signature, &empty_steps);

        assert!(engine.verify_block_family(&header, &parent_header).is_ok());
    }

    #[test]
    fn block_reward_contract() {
        let spec = Spec::new_test_round_block_reward_contract();
        let tap = Arc::new(AccountProvider::transient_provider());

        let addr1 = tap.insert_account(keccak("1").into(), &"1".into()).unwrap();

        let engine = &*spec.engine;
        let genesis_header = spec.genesis_header();
        let db1 = spec
            .ensure_db_good(get_temp_state_db(), &Default::default())
            .unwrap();
        let db2 = spec
            .ensure_db_good(get_temp_state_db(), &Default::default())
            .unwrap();

        let last_hashes = Arc::new(vec![genesis_header.hash()]);

        let client = generate_dummy_client_with_spec(Spec::new_test_round_block_reward_contract);
        engine.register_client(Arc::downgrade(&client) as _);

        // step 2
        let b1 = OpenBlock::new(
            engine,
            Default::default(),
            false,
            db1,
            &genesis_header,
            last_hashes.clone(),
            addr1,
            (3141562.into(), 31415620.into()),
            vec![],
            false,
            None,
        )
        .unwrap();
        let b1 = b1.close_and_lock().unwrap();

        // since the block is empty it isn't sealed and we generate empty steps
        engine.set_signer(Some(Box::new((tap.clone(), addr1, "1".into()))));
        assert_eq!(engine.generate_seal(&b1, &genesis_header), Seal::None);
        engine.step();

        // step 3
        // the signer of the accumulated empty step message should be rewarded
        let b2 = OpenBlock::new(
            engine,
            Default::default(),
            false,
            db2,
            &genesis_header,
            last_hashes.clone(),
            addr1,
            (3141562.into(), 31415620.into()),
            vec![],
            false,
            None,
        )
        .unwrap();
        let addr1_balance = b2.state.balance(&addr1).unwrap();

        // after closing the block `addr1` should be reward twice, one for the included empty step
        // message and another for block creation
        let b2 = b2.close_and_lock().unwrap();

        // the contract rewards (1000 + kind) for each benefactor/reward kind
        assert_eq!(
            b2.state.balance(&addr1).unwrap(),
            addr1_balance + (1000 + 0) + (1000 + 2),
        )
    }

    #[test]
    fn randomness_contract() -> Result<(), super::util::CallError> {
        use_contract!(
            rand_contract,
            "res/contracts/test_authority_round_random.json"
        );

        env_logger::init();

        let contract_addr = Address::from_str("0000000000000000000000000000000000000042").unwrap();
        let client = generate_dummy_client_with_spec_and_data(
            Spec::new_test_round_randomness_contract,
            0,
            0,
            &[],
            true,
        );

        let tap = Arc::new(AccountProvider::transient_provider());

        let addr1 = tap.insert_account(keccak("1").into(), &"1".into()).unwrap();
        // Unlock account so that the engine can decrypt the secret.
        tap.unlock_account_permanently(addr1, "1".into())
            .expect("unlock");

        let signer = Box::new((tap.clone(), addr1, "1".into()));
        client.miner().set_author(Author::Sealer(signer.clone()));
        client
            .miner()
            .set_gas_range_target((U256::from(1000000), U256::from(1000000)));

        let engine = client.engine();
        engine.set_signer(Some(signer));
        engine.register_client(Arc::downgrade(&client) as _);
        let bc = BoundContract::new(&*client, BlockId::Latest, contract_addr);

        // First the contract is in the commit phase, and we haven't committed yet.
        assert!(bc.call_const(rand_contract::functions::is_commit_phase::call())?);
        assert!(!bc.call_const(rand_contract::functions::is_committed::call(0, addr1))?);

        // We produce a block and commit.
        engine.step();
        assert!(bc.call_const(rand_contract::functions::is_committed::call(0, addr1))?);

        // After two more blocks we are in the reveal phase...
        engine.step();
        engine.step();
        assert!(bc.call_const(rand_contract::functions::is_reveal_phase::call())?);
        assert!(!bc.call_const(rand_contract::functions::sent_reveal::call(0, addr1))?);
        assert!(bc
            .call_const(rand_contract::functions::get_value::call())?
            .is_zero());

        // ...so in the next step, we reveal our random value, and the contract's random value is not zero anymore.
        engine.step();
        assert!(bc.call_const(rand_contract::functions::sent_reveal::call(0, addr1))?);
        assert!(!bc
            .call_const(rand_contract::functions::get_value::call())?
            .is_zero());
        Ok(())
    }

    #[test]
    fn extra_info_from_seal() {
        let (spec, tap, accounts) = setup_empty_steps();
        let engine = &*spec.engine;

        let addr1 = accounts[0];
        engine.set_signer(Some(Box::new((tap.clone(), addr1, "1".into()))));

        let mut header: Header = Header::default();
        let empty_step = empty_step(engine, 1, &header.parent_hash());
        let sealed_empty_step = empty_step.sealed();

        header.set_number(2);
        header.set_seal(vec![
            encode(&2usize),
            encode(&H520::default()),
            ::rlp::encode_list(&vec![sealed_empty_step]),
        ]);

        let info = engine.extra_info(&header);

        let mut expected = BTreeMap::default();
        expected.insert("step".into(), "2".into());
        expected.insert(
            "signature".into(),
            Signature::from(H520::default()).to_string(),
        );
        expected.insert("emptySteps".into(), format!("[{}]", empty_step));

        assert_eq!(info, expected);

        header.set_seal(vec![]);

        assert_eq!(engine.extra_info(&header), BTreeMap::default(),);
    }

    #[test]
    fn test_empty_steps() {
        let engine = aura(|p| {
            p.step_durations = [(0, 4)].to_vec().into_iter().collect();
            p.empty_steps_transition = 0;
            p.maximum_empty_steps = 0;
        });

        let parent_hash = H256::from_low_u64_be(1);
        let signature = H520::default();
        let step = |step: u64| EmptyStep {
            step,
            parent_hash,
            signature,
        };

        engine.handle_empty_step_message(step(1));
        engine.handle_empty_step_message(step(3));
        engine.handle_empty_step_message(step(2));
        engine.handle_empty_step_message(step(1));

        assert_eq!(
            engine.empty_steps(0, 4, parent_hash),
            vec![step(1), step(2), step(3)]
        );
        assert_eq!(engine.empty_steps(2, 3, parent_hash), vec![]);
        assert_eq!(engine.empty_steps(2, 4, parent_hash), vec![step(3)]);

        engine.clear_empty_steps(2);

        assert_eq!(engine.empty_steps(0, 3, parent_hash), vec![]);
        assert_eq!(engine.empty_steps(0, 4, parent_hash), vec![step(3)]);
    }

    #[test]
    fn should_reject_duplicate_empty_steps() {
        // given
        let (_spec, tap, accounts) = setup_empty_steps();
        let engine = aura(|p| {
            p.validators = Box::new(SimpleList::new(accounts.clone()));
            p.step_durations = [(0, 4)].to_vec().into_iter().collect();
            p.empty_steps_transition = 0;
            p.maximum_empty_steps = 0;
        });

        let mut parent = Header::default();
        parent.set_seal(vec![encode(&0usize)]);

        let mut header = Header::default();
        header.set_number(parent.number() + 1);
        header.set_parent_hash(parent.hash());
        header.set_author(accounts[0]);

        // when
        engine.set_signer(Some(Box::new((tap.clone(), accounts[1], "0".into()))));
        let empty_steps = vec![
            sealed_empty_step(&*engine, 1, &parent.hash()),
            sealed_empty_step(&*engine, 1, &parent.hash()),
        ];
        let step = 2;
        let signature = tap
            .sign(accounts[0], Some("1".into()), header.bare_hash())
            .unwrap();
        set_empty_steps_seal(&mut header, step, &signature, &empty_steps);
        header.set_difficulty(calculate_score(0, step, empty_steps.len()));

        // then
        assert_insufficient_proof(
            engine.verify_block_family(&header, &parent),
            "duplicate empty step",
        );
    }

    #[test]
    fn should_reject_empty_steps_out_of_order() {
        // given
        let (_spec, tap, accounts) = setup_empty_steps();
        let engine = aura(|p| {
            p.validators = Box::new(SimpleList::new(accounts.clone()));
            p.step_durations = [(0, 4)].to_vec().into_iter().collect();
            p.empty_steps_transition = 0;
            p.maximum_empty_steps = 0;
        });

        let mut parent = Header::default();
        parent.set_seal(vec![encode(&0usize)]);

        let mut header = Header::default();
        header.set_number(parent.number() + 1);
        header.set_parent_hash(parent.hash());
        header.set_author(accounts[0]);

        // when
        engine.set_signer(Some(Box::new((tap.clone(), accounts[1], "0".into()))));
        let es1 = sealed_empty_step(&*engine, 1, &parent.hash());
        engine.set_signer(Some(Box::new((tap.clone(), accounts[0], "1".into()))));
        let es2 = sealed_empty_step(&*engine, 2, &parent.hash());

        let mut empty_steps = vec![es2, es1];

        let step = 3;
        let signature = tap
            .sign(accounts[1], Some("0".into()), header.bare_hash())
            .unwrap();
        set_empty_steps_seal(&mut header, step, &signature, &empty_steps);
        header.set_difficulty(calculate_score(0, step, empty_steps.len()));

        // then make sure it's rejected because of the order
        assert_insufficient_proof(
            engine.verify_block_family(&header, &parent),
            "unordered empty step",
        );

        // now try to fix the order
        empty_steps.reverse();
        set_empty_steps_seal(&mut header, step, &signature, &empty_steps);
        assert_eq!(engine.verify_block_family(&header, &parent).unwrap(), ());
    }

    #[test]
    fn should_collect_block_reward_transitions() {
        let config = r#"{
			"params": {
				"stepDuration": "5",
				"validators": {
					"list" : ["0x1000000000000000000000000000000000000001"]
				},
                "blockRewardContractTransition": "0",
				"blockRewardContractAddress": "0x2000000000000000000000000000000000000002",
				"blockRewardContractTransitions": {
					"7": "0x3000000000000000000000000000000000000003",
					"42": "0x4000000000000000000000000000000000000004"
				}
			}
		}"#;
        let deserialized: ethjson::spec::AuthorityRound = serde_json::from_str(config).unwrap();
        let params = AuthorityRoundParams::from(deserialized.params);
        for ((block_num1, address1), (block_num2, address2)) in
            params.block_reward_contract_transitions.iter().zip(
                [
                    (
                        0u64,
                        BlockRewardContract::new_from_address(
                            Address::from_str("2000000000000000000000000000000000000002").unwrap(),
                        ),
                    ),
                    (
                        7u64,
                        BlockRewardContract::new_from_address(
                            Address::from_str("3000000000000000000000000000000000000003").unwrap(),
                        ),
                    ),
                    (
                        42u64,
                        BlockRewardContract::new_from_address(
                            Address::from_str("4000000000000000000000000000000000000004").unwrap(),
                        ),
                    ),
                ]
                .iter(),
            )
        {
            assert_eq!(block_num1, block_num2);
            assert_eq!(address1, address2);
        }
    }

    #[test]
    #[should_panic(
        expected = "blockRewardContractTransition should be less than any of the keys in blockRewardContractTransitions"
    )]
    fn should_reject_out_of_order_block_reward_transition() {
        let config = r#"{
			"params": {
				"stepDuration": "5",
				"validators": {
					"list" : ["0x1000000000000000000000000000000000000001"]
				},
                "blockRewardContractTransition": "7",
				"blockRewardContractAddress": "0x2000000000000000000000000000000000000002",
				"blockRewardContractTransitions": {
					"0": "0x3000000000000000000000000000000000000003",
					"42": "0x4000000000000000000000000000000000000004"
				}
			}
		}"#;
        let deserialized: ethjson::spec::AuthorityRound = serde_json::from_str(config).unwrap();
        AuthorityRoundParams::from(deserialized.params);
    }
}