// Copyright 2015-2019 Parity Technologies (UK) Ltd.
// This file is part of Parity Ethereum.

// Parity Ethereum 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.

// Parity Ethereum 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 Parity Ethereum.  If not, see <http://www.gnu.org/licenses/>.

//! Request types, verification, and verification errors.

use std::{cmp, sync::Arc};

use bytes::Bytes;
use common_types::{
    basic_account::BasicAccount, encoded, receipt::Receipt, transaction::SignedTransaction,
};
use ethcore::{
    engines::{EthEngine, StateDependentProof},
    machine::EthereumMachine,
    state::{self, ProvedExecution},
};
use ethereum_types::{Address, H256, U256};
use ethtrie::{TrieDB, TrieError};
use hash::{keccak, KECCAK_EMPTY, KECCAK_EMPTY_LIST_RLP, KECCAK_NULL_RLP};
use hash_db::HashDB;
use kvdb::DBValue;
use parking_lot::Mutex;
use request::{self as net_request, CompleteRequest, Field, IncompleteRequest, Output, OutputKind};
use rlp::{Rlp, RlpStream};
use trie::Trie;
use vm::EnvInfo;

const SUPPLIED_MATCHES: &str =
    "supplied responses always match produced requests; enforced by `check_response`; qed";

/// Core unit of the API: submit batches of these to be answered with `Response`s.
#[derive(Clone)]
pub enum Request {
    /// A request for a header proof.
    HeaderProof(HeaderProof),
    /// A request for a header by hash.
    HeaderByHash(HeaderByHash),
    /// A request for a header by hash with a range of its ancestors.
    HeaderWithAncestors(HeaderWithAncestors),
    /// A request for the index of a transaction.
    TransactionIndex(TransactionIndex),
    /// A request for block receipts.
    Receipts(BlockReceipts),
    /// A request for a block body.
    Body(Body),
    /// A request for an account.
    Account(Account),
    /// A request for a contract's code.
    Code(Code),
    /// A request for proof of execution.
    Execution(TransactionProof),
    /// A request for epoch change signal.
    Signal(Signal),
}

/// A request argument.
pub trait RequestArg {
    /// the response type.
    type Out;

    /// Create the request type.
    /// `extract` must not fail when presented with the corresponding
    /// `Response`.
    fn make(self) -> Request;

    /// May not panic if the response corresponds with the request
    /// from `make`.
    /// Is free to panic otherwise.
    fn extract(r: Response) -> Self::Out;
}

/// An adapter can be thought of as a grouping of request argument types.
/// This is implemented for various tuples and convenient types.
pub trait RequestAdapter {
    /// The output type.
    type Out;

    /// Infallibly produce requests. When `extract_from` is presented
    /// with the corresponding response vector, it may not fail.
    fn make_requests(self) -> Vec<Request>;

    /// Extract the output type from the given responses.
    /// If they are the corresponding responses to the requests
    /// made by `make_requests`, do not panic.
    fn extract_from(Vec<Response>) -> Self::Out;
}

impl<T: RequestArg> RequestAdapter for Vec<T> {
    type Out = Vec<T::Out>;

    fn make_requests(self) -> Vec<Request> {
        self.into_iter().map(RequestArg::make).collect()
    }

    fn extract_from(r: Vec<Response>) -> Self::Out {
        r.into_iter().map(T::extract).collect()
    }
}

// helper to implement `RequestArg` and `From` for a single request kind.
macro_rules! impl_single {
    ($variant: ident, $me: ty, $out: ty) => {
        impl RequestArg for $me {
            type Out = $out;

            fn make(self) -> Request {
                Request::$variant(self)
            }

            fn extract(r: Response) -> $out {
                match r {
                    Response::$variant(x) => x,
                    _ => panic!(SUPPLIED_MATCHES),
                }
            }
        }

        impl From<$me> for Request {
            fn from(me: $me) -> Request {
                Request::$variant(me)
            }
        }
    };
}

// implement traits for each kind of request.
impl_single!(HeaderProof, HeaderProof, (H256, U256));
impl_single!(HeaderByHash, HeaderByHash, encoded::Header);
impl_single!(
    HeaderWithAncestors,
    HeaderWithAncestors,
    Vec<encoded::Header>
);
impl_single!(
    TransactionIndex,
    TransactionIndex,
    net_request::TransactionIndexResponse
);
impl_single!(Receipts, BlockReceipts, Vec<Receipt>);
impl_single!(Body, Body, encoded::Block);
impl_single!(Account, Account, Option<BasicAccount>);
impl_single!(Code, Code, Bytes);
impl_single!(Execution, TransactionProof, super::ExecutionResult);
impl_single!(Signal, Signal, Vec<u8>);

macro_rules! impl_args {
	() => {
		impl<T: RequestArg> RequestAdapter for T {
			type Out = T::Out;

			fn make_requests(self) -> Vec<Request> {
				vec![self.make()]
			}

			fn extract_from(mut responses: Vec<Response>) -> Self::Out {
				T::extract(responses.pop().expect(SUPPLIED_MATCHES))
			}
		}
	};
	($first: ident, $($next: ident,)*) => {
		impl<
			$first: RequestArg,
			$($next: RequestArg,)*
		>
		RequestAdapter for ($first, $($next,)*) {
			type Out = ($first::Out, $($next::Out,)*);

			fn make_requests(self) -> Vec<Request> {
				let ($first, $($next,)*) = self;

				vec![
					$first.make(),
					$($next.make(),)*
				]
			}

			fn extract_from(responses: Vec<Response>) -> Self::Out {
				let mut iter = responses.into_iter();
				(
					$first::extract(iter.next().expect(SUPPLIED_MATCHES)),
					$($next::extract(iter.next().expect(SUPPLIED_MATCHES)),)*
				)
			}
		}
		impl_args!($($next,)*);
	}
}

mod impls {
    #![allow(non_snake_case)]

    use super::{Request, RequestAdapter, RequestArg, Response, SUPPLIED_MATCHES};

    impl_args!(A, B, C, D, E, F, G, H, I, J, K, L,);
}

/// A block header to be used for verification.
/// May be stored or an unresolved output of a prior request.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum HeaderRef {
    /// A stored header.
    Stored(encoded::Header),
    /// An unresolved header. The first item here is the index of the request which
    /// will return the header. The second is a back-reference pointing to a block hash
    /// which can be used to make requests until that header is resolved.
    Unresolved(usize, Field<H256>),
}

impl HeaderRef {
    /// Attempt to inspect the header.
    pub fn as_ref(&self) -> Result<&encoded::Header, Error> {
        match *self {
            HeaderRef::Stored(ref hdr) => Ok(hdr),
            HeaderRef::Unresolved(idx, _) => Err(Error::UnresolvedHeader(idx)),
        }
    }

    // get the blockhash field to be used in requests.
    fn field(&self) -> Field<H256> {
        match *self {
            HeaderRef::Stored(ref hdr) => Field::Scalar(hdr.hash()),
            HeaderRef::Unresolved(_, field) => field,
        }
    }

    // yield the index of the request which will produce the header.
    fn needs_header(&self) -> Option<(usize, Field<H256>)> {
        match *self {
            HeaderRef::Stored(_) => None,
            HeaderRef::Unresolved(idx, field) => Some((idx, field)),
        }
    }
}

impl From<encoded::Header> for HeaderRef {
    fn from(header: encoded::Header) -> Self {
        HeaderRef::Stored(header)
    }
}

/// Requests coupled with their required data for verification.
/// This is used internally but not part of the public API.
#[derive(Clone)]
#[allow(missing_docs)]
pub enum CheckedRequest {
    HeaderProof(HeaderProof, net_request::IncompleteHeaderProofRequest),
    HeaderByHash(HeaderByHash, net_request::IncompleteHeadersRequest),
    HeaderWithAncestors(HeaderWithAncestors, net_request::IncompleteHeadersRequest),
    TransactionIndex(
        TransactionIndex,
        net_request::IncompleteTransactionIndexRequest,
    ),
    Receipts(BlockReceipts, net_request::IncompleteReceiptsRequest),
    Body(Body, net_request::IncompleteBodyRequest),
    Account(Account, net_request::IncompleteAccountRequest),
    Code(Code, net_request::IncompleteCodeRequest),
    Execution(TransactionProof, net_request::IncompleteExecutionRequest),
    Signal(Signal, net_request::IncompleteSignalRequest),
}

impl From<Request> for CheckedRequest {
    fn from(req: Request) -> Self {
        match req {
            Request::HeaderByHash(req) => {
                let net_req = net_request::IncompleteHeadersRequest {
                    start: req.0.map(Into::into),
                    skip: 0,
                    max: 1,
                    reverse: false,
                };
                trace!(target: "on_demand", "HeaderByHash Request, {:?}", net_req);
                CheckedRequest::HeaderByHash(req, net_req)
            }
            Request::HeaderWithAncestors(req) => {
                let net_req = net_request::IncompleteHeadersRequest {
                    start: req.block_hash.map(Into::into),
                    skip: 0,
                    max: req.ancestor_count + 1,
                    reverse: true,
                };
                trace!(target: "on_demand", "HeaderWithAncestors Request, {:?}", net_req);
                CheckedRequest::HeaderWithAncestors(req, net_req)
            }
            Request::HeaderProof(req) => {
                let net_req = net_request::IncompleteHeaderProofRequest {
                    num: req.num().into(),
                };
                trace!(target: "on_demand", "HeaderProof Request, {:?}", net_req);
                CheckedRequest::HeaderProof(req, net_req)
            }
            Request::TransactionIndex(req) => {
                let net_req = net_request::IncompleteTransactionIndexRequest { hash: req.0 };
                trace!(target: "on_demand", "TransactionIndex Request, {:?}", net_req);
                CheckedRequest::TransactionIndex(req, net_req)
            }
            Request::Body(req) => {
                let net_req = net_request::IncompleteBodyRequest {
                    hash: req.0.field(),
                };
                trace!(target: "on_demand", "Body Request, {:?}", net_req);
                CheckedRequest::Body(req, net_req)
            }
            Request::Receipts(req) => {
                let net_req = net_request::IncompleteReceiptsRequest {
                    hash: req.0.field(),
                };
                trace!(target: "on_demand", "Receipt Request, {:?}", net_req);
                CheckedRequest::Receipts(req, net_req)
            }
            Request::Account(req) => {
                let net_req = net_request::IncompleteAccountRequest {
                    block_hash: req.header.field(),
                    address_hash: ::hash::keccak(&req.address).into(),
                };
                trace!(target: "on_demand", "Account Request, {:?}", net_req);
                CheckedRequest::Account(req, net_req)
            }
            Request::Code(req) => {
                let net_req = net_request::IncompleteCodeRequest {
                    block_hash: req.header.field(),
                    code_hash: req.code_hash,
                };
                trace!(target: "on_demand", "Code Request, {:?}", net_req);
                CheckedRequest::Code(req, net_req)
            }
            Request::Execution(req) => {
                let net_req = net_request::IncompleteExecutionRequest {
                    block_hash: req.header.field(),
                    from: req.tx.sender(),
                    gas: req.tx.gas,
                    gas_price: req.tx.gas_price,
                    action: req.tx.action.clone(),
                    value: req.tx.value,
                    data: req.tx.data.clone(),
                };
                trace!(target: "on_demand", "Execution request, {:?}", net_req);
                CheckedRequest::Execution(req, net_req)
            }
            Request::Signal(req) => {
                let net_req = net_request::IncompleteSignalRequest {
                    block_hash: req.hash.into(),
                };
                trace!(target: "on_demand", "Signal Request, {:?}", net_req);
                CheckedRequest::Signal(req, net_req)
            }
        }
    }
}

impl CheckedRequest {
    /// Convert this into a network request.
    pub fn into_net_request(self) -> net_request::Request {
        use request::Request as NetRequest;

        match self {
            CheckedRequest::HeaderProof(_, req) => NetRequest::HeaderProof(req),
            CheckedRequest::HeaderByHash(_, req) => NetRequest::Headers(req),
            CheckedRequest::HeaderWithAncestors(_, req) => NetRequest::Headers(req),
            CheckedRequest::TransactionIndex(_, req) => NetRequest::TransactionIndex(req),
            CheckedRequest::Receipts(_, req) => NetRequest::Receipts(req),
            CheckedRequest::Body(_, req) => NetRequest::Body(req),
            CheckedRequest::Account(_, req) => NetRequest::Account(req),
            CheckedRequest::Code(_, req) => NetRequest::Code(req),
            CheckedRequest::Execution(_, req) => NetRequest::Execution(req),
            CheckedRequest::Signal(_, req) => NetRequest::Signal(req),
        }
    }

    /// Whether this needs a header from a prior request.
    /// Returns `Some` with the index of the request returning the header
    /// and the field giving the hash
    /// if so, `None` otherwise.
    pub fn needs_header(&self) -> Option<(usize, Field<H256>)> {
        match *self {
            CheckedRequest::Receipts(ref x, _) => x.0.needs_header(),
            CheckedRequest::Body(ref x, _) => x.0.needs_header(),
            CheckedRequest::Account(ref x, _) => x.header.needs_header(),
            CheckedRequest::Code(ref x, _) => x.header.needs_header(),
            CheckedRequest::Execution(ref x, _) => x.header.needs_header(),
            _ => None,
        }
    }

    /// Provide a header where one was needed. Should only be called if `needs_header`
    /// returns `Some`, and for correctness, only use the header yielded by the correct
    /// request.
    pub fn provide_header(&mut self, header: encoded::Header) {
        match *self {
            CheckedRequest::Receipts(ref mut x, _) => x.0 = HeaderRef::Stored(header),
            CheckedRequest::Body(ref mut x, _) => x.0 = HeaderRef::Stored(header),
            CheckedRequest::Account(ref mut x, _) => x.header = HeaderRef::Stored(header),
            CheckedRequest::Code(ref mut x, _) => x.header = HeaderRef::Stored(header),
            CheckedRequest::Execution(ref mut x, _) => x.header = HeaderRef::Stored(header),
            _ => {}
        }
    }

    /// Attempt to complete the request based on data in the cache.
    pub fn respond_local(&self, cache: &Mutex<::cache::Cache>) -> Option<Response> {
        match *self {
            CheckedRequest::HeaderProof(ref check, _) => {
                let mut cache = cache.lock();
                cache
                    .block_hash(check.num)
                    .and_then(|h| cache.chain_score(&h).map(|s| (h, s)))
                    .map(|(h, s)| Response::HeaderProof((h, s)))
            }
            CheckedRequest::HeaderByHash(_, ref req) => {
                if let Some(&net_request::HashOrNumber::Hash(ref h)) = req.start.as_ref() {
                    return cache.lock().block_header(h).map(Response::HeaderByHash);
                }

                None
            }
            CheckedRequest::HeaderWithAncestors(_, ref req) => {
                if req.skip != 1 || !req.reverse {
                    return None;
                }

                if let Some(&net_request::HashOrNumber::Hash(start)) = req.start.as_ref() {
                    let mut result = Vec::with_capacity(req.max as usize);
                    let mut hash = start;
                    let mut cache = cache.lock();
                    for _ in 0..req.max {
                        match cache.block_header(&hash) {
                            Some(header) => {
                                hash = header.parent_hash();
                                result.push(header);
                            }
                            None => return None,
                        }
                    }
                    Some(Response::HeaderWithAncestors(result))
                } else {
                    None
                }
            }
            CheckedRequest::Receipts(ref check, ref req) => {
                // empty transactions -> no receipts
                if check
                    .0
                    .as_ref()
                    .ok()
                    .map_or(false, |hdr| hdr.receipts_root() == KECCAK_NULL_RLP)
                {
                    return Some(Response::Receipts(Vec::new()));
                }

                req.hash
                    .as_ref()
                    .and_then(|hash| cache.lock().block_receipts(hash))
                    .map(Response::Receipts)
            }
            CheckedRequest::Body(ref check, ref req) => {
                // check for empty body.
                if let Ok(hdr) = check.0.as_ref() {
                    if hdr.transactions_root() == KECCAK_NULL_RLP
                        && hdr.uncles_hash() == KECCAK_EMPTY_LIST_RLP
                    {
                        let mut stream = RlpStream::new_list(3);
                        stream.append_raw(hdr.rlp().as_raw(), 1);
                        stream.begin_list(0);
                        stream.begin_list(0);

                        return Some(Response::Body(encoded::Block::new(stream.out())));
                    }
                }

                // otherwise, check for cached body and header.
                let block_hash = req
                    .hash
                    .as_ref()
                    .cloned()
                    .or_else(|| check.0.as_ref().ok().map(|hdr| hdr.hash()));
                let block_hash = match block_hash {
                    Some(hash) => hash,
                    None => return None,
                };

                let mut cache = cache.lock();
                let cached_header;

                // can't use as_ref here although it seems like you would be able to:
                // it complains about uninitialized `cached_header`.
                let block_header = match check.0.as_ref().ok() {
                    Some(hdr) => Some(hdr),
                    None => {
                        cached_header = cache.block_header(&block_hash);
                        cached_header.as_ref()
                    }
                };

                block_header
                    .and_then(|hdr| cache.block_body(&block_hash).map(|b| (hdr, b)))
                    .map(|(hdr, body)| {
                        Response::Body(encoded::Block::new_from_header_and_body(
                            &hdr.view(),
                            &body.view(),
                        ))
                    })
            }
            CheckedRequest::Code(_, ref req) => {
                if req.code_hash.as_ref().map_or(false, |&h| h == KECCAK_EMPTY) {
                    Some(Response::Code(Vec::new()))
                } else {
                    None
                }
            }
            _ => None,
        }
    }
}

macro_rules! match_me {
    ($me: expr, ($check: pat, $req: pat) => $e: expr) => {
        match $me {
            CheckedRequest::HeaderProof($check, $req) => $e,
            CheckedRequest::HeaderByHash($check, $req) => $e,
            CheckedRequest::HeaderWithAncestors($check, $req) => $e,
            CheckedRequest::TransactionIndex($check, $req) => $e,
            CheckedRequest::Receipts($check, $req) => $e,
            CheckedRequest::Body($check, $req) => $e,
            CheckedRequest::Account($check, $req) => $e,
            CheckedRequest::Code($check, $req) => $e,
            CheckedRequest::Execution($check, $req) => $e,
            CheckedRequest::Signal($check, $req) => $e,
        }
    };
}

impl IncompleteRequest for CheckedRequest {
    type Complete = CompleteRequest;
    type Response = net_request::Response;

    fn check_outputs<F>(&self, mut f: F) -> Result<(), net_request::NoSuchOutput>
    where
        F: FnMut(usize, usize, OutputKind) -> Result<(), net_request::NoSuchOutput>,
    {
        match *self {
            CheckedRequest::HeaderProof(_, ref req) => req.check_outputs(f),
            CheckedRequest::HeaderByHash(ref check, ref req) => {
                req.check_outputs(&mut f)?;

                // make sure the output given is definitively a hash.
                match check.0 {
                    Field::BackReference(r, idx) => f(r, idx, OutputKind::Hash),
                    _ => Ok(()),
                }
            }
            CheckedRequest::HeaderWithAncestors(ref check, ref req) => {
                req.check_outputs(&mut f)?;

                // make sure the output given is definitively a hash.
                match check.block_hash {
                    Field::BackReference(r, idx) => f(r, idx, OutputKind::Hash),
                    _ => Ok(()),
                }
            }
            CheckedRequest::TransactionIndex(_, ref req) => req.check_outputs(f),
            CheckedRequest::Receipts(_, ref req) => req.check_outputs(f),
            CheckedRequest::Body(_, ref req) => req.check_outputs(f),
            CheckedRequest::Account(_, ref req) => req.check_outputs(f),
            CheckedRequest::Code(_, ref req) => req.check_outputs(f),
            CheckedRequest::Execution(_, ref req) => req.check_outputs(f),
            CheckedRequest::Signal(_, ref req) => req.check_outputs(f),
        }
    }

    fn note_outputs<F>(&self, f: F)
    where
        F: FnMut(usize, OutputKind),
    {
        match_me!(*self, (_, ref req) => req.note_outputs(f))
    }

    fn fill<F>(&mut self, f: F)
    where
        F: Fn(usize, usize) -> Result<Output, net_request::NoSuchOutput>,
    {
        match_me!(*self, (_, ref mut req) => req.fill(f))
    }

    fn complete(self) -> Result<Self::Complete, net_request::NoSuchOutput> {
        match self {
            CheckedRequest::HeaderProof(_, req) => {
                trace!(target: "on_demand", "HeaderProof request completed {:?}", req);
                req.complete().map(CompleteRequest::HeaderProof)
            }
            CheckedRequest::HeaderByHash(_, req) => {
                trace!(target: "on_demand", "HeaderByHash request completed {:?}", req);
                req.complete().map(CompleteRequest::Headers)
            }
            CheckedRequest::HeaderWithAncestors(_, req) => {
                trace!(target: "on_demand", "HeaderWithAncestors request completed {:?}", req);
                req.complete().map(CompleteRequest::Headers)
            }
            CheckedRequest::TransactionIndex(_, req) => {
                trace!(target: "on_demand", "TransactionIndex request completed {:?}", req);
                req.complete().map(CompleteRequest::TransactionIndex)
            }
            CheckedRequest::Receipts(_, req) => {
                trace!(target: "on_demand", "Receipt request completed {:?}", req);
                req.complete().map(CompleteRequest::Receipts)
            }
            CheckedRequest::Body(_, req) => {
                trace!(target: "on_demand", "Block request completed {:?}", req);
                req.complete().map(CompleteRequest::Body)
            }
            CheckedRequest::Account(_, req) => {
                trace!(target: "on_demand", "Account request completed {:?}", req);
                req.complete().map(CompleteRequest::Account)
            }
            CheckedRequest::Code(_, req) => {
                trace!(target: "on_demand", "Code request completed {:?}", req);
                req.complete().map(CompleteRequest::Code)
            }
            CheckedRequest::Execution(_, req) => {
                trace!(target: "on_demand", "Execution request completed {:?}", req);
                req.complete().map(CompleteRequest::Execution)
            }
            CheckedRequest::Signal(_, req) => {
                trace!(target: "on_demand", "Signal request completed {:?}", req);
                req.complete().map(CompleteRequest::Signal)
            }
        }
    }

    fn adjust_refs<F>(&mut self, mapping: F)
    where
        F: FnMut(usize) -> usize,
    {
        match_me!(*self, (_, ref mut req) => req.adjust_refs(mapping))
    }
}

impl net_request::CheckedRequest for CheckedRequest {
    type Extract = Response;
    type Error = Error;
    type Environment = Mutex<::cache::Cache>;

    /// Check whether the response matches (beyond the type).
    fn check_response(
        &self,
        complete: &Self::Complete,
        cache: &Mutex<::cache::Cache>,
        response: &Self::Response,
    ) -> Result<Response, Error> {
        use request::Response as NetResponse;

        // helper for expecting a specific response for a given request.
        macro_rules! expect {
            ($res: pat => $e: expr) => {{
                match (response, complete) {
                    $res => $e,
                    _ => Err(Error::WrongKind),
                }
            }};
        }

        // check response against contained prover.
        match *self {
            CheckedRequest::HeaderProof(ref prover, _) => {
                expect!((&NetResponse::HeaderProof(ref res), _) =>
					prover.check_response(cache, &res.proof).map(Response::HeaderProof))
            }
            CheckedRequest::HeaderByHash(ref prover, _) => {
                expect!((&NetResponse::Headers(ref res), &CompleteRequest::Headers(ref req)) =>
					prover.check_response(cache, &req.start, &res.headers).map(Response::HeaderByHash))
            }
            CheckedRequest::HeaderWithAncestors(ref prover, _) => {
                expect!((&NetResponse::Headers(ref res), &CompleteRequest::Headers(ref req)) =>
					prover.check_response(cache, &req.start, &res.headers).map(Response::HeaderWithAncestors))
            }
            CheckedRequest::TransactionIndex(ref prover, _) => {
                expect!((&NetResponse::TransactionIndex(ref res), _) =>
					prover.check_response(cache, res).map(Response::TransactionIndex))
            }
            CheckedRequest::Receipts(ref prover, _) => {
                expect!((&NetResponse::Receipts(ref res), _) =>
					prover.check_response(cache, &res.receipts).map(Response::Receipts))
            }
            CheckedRequest::Body(ref prover, _) => expect!((&NetResponse::Body(ref res), _) =>
					prover.check_response(cache, &res.body).map(Response::Body)),
            CheckedRequest::Account(ref prover, _) => {
                expect!((&NetResponse::Account(ref res), _) =>
					prover.check_response(cache, &res.proof).map(Response::Account))
            }
            CheckedRequest::Code(ref prover, _) => {
                expect!((&NetResponse::Code(ref res), &CompleteRequest::Code(ref req)) =>
					prover.check_response(cache, &req.code_hash, &res.code).map(Response::Code))
            }
            CheckedRequest::Execution(ref prover, _) => {
                expect!((&NetResponse::Execution(ref res), _) =>
					prover.check_response(cache, &res.items).map(Response::Execution))
            }
            CheckedRequest::Signal(ref prover, _) => expect!((&NetResponse::Signal(ref res), _) =>
					prover.check_response(cache, &res.signal).map(Response::Signal)),
        }
    }
}

/// Responses to on-demand requests.
/// All of these are checked.
pub enum Response {
    /// Response to a header proof request.
    /// Returns the hash and chain score.
    HeaderProof((H256, U256)),
    /// Response to a header-by-hash request.
    HeaderByHash(encoded::Header),
    /// Response to a header-by-hash with ancestors request.
    HeaderWithAncestors(Vec<encoded::Header>),
    /// Response to a transaction-index request.
    TransactionIndex(net_request::TransactionIndexResponse),
    /// Response to a receipts request.
    Receipts(Vec<Receipt>),
    /// Response to a block body request.
    Body(encoded::Block),
    /// Response to an Account request.
    // TODO: `unwrap_or(engine_defaults)`
    Account(Option<BasicAccount>),
    /// Response to a request for code.
    Code(Vec<u8>),
    /// Response to a request for proved execution.
    Execution(super::ExecutionResult),
    /// Response to a request for epoch change signal.
    Signal(Vec<u8>),
}

impl net_request::ResponseLike for Response {
    fn fill_outputs<F>(&self, mut f: F)
    where
        F: FnMut(usize, Output),
    {
        match *self {
            Response::HeaderProof((ref hash, _)) => f(0, Output::Hash(*hash)),
            Response::Account(None) => {
                f(0, Output::Hash(KECCAK_EMPTY)); // code hash
                f(1, Output::Hash(KECCAK_NULL_RLP)); // storage root.
            }
            Response::Account(Some(ref acc)) => {
                f(0, Output::Hash(acc.code_hash));
                f(1, Output::Hash(acc.storage_root));
            }
            _ => {}
        }
    }
}

/// Errors in verification.
#[derive(Debug, PartialEq)]
pub enum Error {
    /// RLP decoder error.
    Decoder(::rlp::DecoderError),
    /// Empty response.
    Empty,
    /// Response data length exceeds request max.
    TooManyResults(u64, u64),
    /// Response data is incomplete.
    TooFewResults(u64, u64),
    /// Trie lookup error (result of bad proof)
    Trie(TrieError),
    /// Bad inclusion proof
    BadProof,
    /// Header by number instead of hash.
    HeaderByNumber,
    /// Unresolved header reference.
    UnresolvedHeader(usize),
    /// Wrong header number.
    WrongNumber(u64, u64),
    /// Wrong hash.
    WrongHash(H256, H256),
    /// Wrong trie root.
    WrongTrieRoot(H256, H256),
    /// Wrong response kind.
    WrongKind,
    /// Wrong sequence of headers.
    WrongHeaderSequence,
}

impl From<::rlp::DecoderError> for Error {
    fn from(err: ::rlp::DecoderError) -> Self {
        Error::Decoder(err)
    }
}

impl From<Box<TrieError>> for Error {
    fn from(err: Box<TrieError>) -> Self {
        Error::Trie(*err)
    }
}

/// Request for header proof by number
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct HeaderProof {
    /// The header's number.
    num: u64,
    /// The cht number for the given block number.
    cht_num: u64,
    /// The root of the CHT containing this header.
    cht_root: H256,
}

impl HeaderProof {
    /// Construct a new header-by-number request. Fails if the given number is 0.
    /// Provide the expected CHT root to compare against.
    pub fn new(num: u64, cht_root: H256) -> Option<Self> {
        ::cht::block_to_cht_number(num).map(|cht_num| HeaderProof {
            num,
            cht_num,
            cht_root,
        })
    }

    /// Access the requested block number.
    pub fn num(&self) -> u64 {
        self.num
    }

    /// Access the CHT number.
    pub fn cht_num(&self) -> u64 {
        self.cht_num
    }

    /// Access the expected CHT root.
    pub fn cht_root(&self) -> H256 {
        self.cht_root
    }

    /// Check a response with a CHT proof, get a hash and total difficulty back.
    pub fn check_response(
        &self,
        cache: &Mutex<::cache::Cache>,
        proof: &[Bytes],
    ) -> Result<(H256, U256), Error> {
        match ::cht::check_proof(proof, self.num, self.cht_root) {
            Some((expected_hash, td)) => {
                let mut cache = cache.lock();
                cache.insert_block_hash(self.num, expected_hash);
                cache.insert_chain_score(expected_hash, td);

                Ok((expected_hash, td))
            }
            None => Err(Error::BadProof),
        }
    }
}

/// Request for a header by hash with a range of ancestors.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct HeaderWithAncestors {
    /// Hash of the last block in the range to fetch.
    pub block_hash: Field<H256>,
    /// Number of headers before the last block to fetch in addition.
    pub ancestor_count: u64,
}

impl HeaderWithAncestors {
    /// Check a response for the headers.
    pub fn check_response(
        &self,
        cache: &Mutex<::cache::Cache>,
        start: &net_request::HashOrNumber,
        headers: &[encoded::Header],
    ) -> Result<Vec<encoded::Header>, Error> {
        let expected_hash = match (self.block_hash, start) {
            (Field::Scalar(h), &net_request::HashOrNumber::Hash(h2)) => {
                if h != h2 {
                    return Err(Error::WrongHash(h, h2));
                }
                h
            }
            (_, &net_request::HashOrNumber::Hash(h2)) => h2,
            _ => return Err(Error::HeaderByNumber),
        };

        let start_header = headers.first().ok_or(Error::Empty)?;
        let start_hash = start_header.hash();
        if start_hash != expected_hash {
            return Err(Error::WrongHash(expected_hash, start_hash));
        }

        let expected_len = 1 + cmp::min(self.ancestor_count, start_header.number());
        let actual_len = headers.len() as u64;
        match actual_len.cmp(&expected_len) {
            cmp::Ordering::Less => return Err(Error::TooFewResults(expected_len, actual_len)),
            cmp::Ordering::Greater => return Err(Error::TooManyResults(expected_len, actual_len)),
            cmp::Ordering::Equal => (),
        };

        for (header, prev_header) in headers.iter().zip(headers[1..].iter()) {
            if header.number() != prev_header.number() + 1
                || header.parent_hash() != prev_header.hash()
            {
                return Err(Error::WrongHeaderSequence);
            }
        }

        let mut cache = cache.lock();
        for header in headers {
            cache.insert_block_header(header.hash(), header.clone());
        }

        Ok(headers.to_vec())
    }
}

/// Request for a header by hash.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct HeaderByHash(pub Field<H256>);

impl HeaderByHash {
    /// Check a response for the header.
    pub fn check_response(
        &self,
        cache: &Mutex<::cache::Cache>,
        start: &net_request::HashOrNumber,
        headers: &[encoded::Header],
    ) -> Result<encoded::Header, Error> {
        let expected_hash = match (self.0, start) {
            (Field::Scalar(h), &net_request::HashOrNumber::Hash(h2)) => {
                if h != h2 {
                    return Err(Error::WrongHash(h, h2));
                }
                h
            }
            (_, &net_request::HashOrNumber::Hash(h2)) => h2,
            _ => return Err(Error::HeaderByNumber),
        };

        let header = headers.get(0).ok_or(Error::Empty)?;
        let hash = header.hash();
        if hash == expected_hash {
            cache.lock().insert_block_header(hash, header.clone());
            Ok(header.clone())
        } else {
            Err(Error::WrongHash(expected_hash, hash))
        }
    }
}

/// Request for a transaction index.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct TransactionIndex(pub Field<H256>);

impl TransactionIndex {
    /// Check a response for the transaction index.
    //
    // TODO: proper checking involves looking at canonicality of the
    // hash w.r.t. the current best block header.
    //
    // unlike all other forms of request, we don't know the header to check
    // until we make this request.
    //
    // This would require lookups in the database or perhaps CHT requests,
    // which aren't currently possible.
    //
    // Also, returning a result that is not locally canonical doesn't necessarily
    // indicate misbehavior, so the punishment scheme would need to be revised.
    pub fn check_response(
        &self,
        _cache: &Mutex<::cache::Cache>,
        res: &net_request::TransactionIndexResponse,
    ) -> Result<net_request::TransactionIndexResponse, Error> {
        Ok(res.clone())
    }
}

/// Request for a block, with header for verification.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Body(pub HeaderRef);

impl Body {
    /// Check a response for this block body.
    pub fn check_response(
        &self,
        cache: &Mutex<::cache::Cache>,
        body: &encoded::Body,
    ) -> Result<encoded::Block, Error> {
        // check the integrity of the the body against the header
        let header = self.0.as_ref()?;
        let tx_root =
            ::triehash::ordered_trie_root(body.transactions_rlp().iter().map(|r| r.as_raw()));
        if tx_root != header.transactions_root() {
            trace!(target: "on_demand", "Body Response: \"WrongTrieRoot\" tx_root: {:?} header_root: {:?}", tx_root, header.transactions_root());
            return Err(Error::WrongTrieRoot(header.transactions_root(), tx_root));
        }

        let uncles_hash = keccak(body.uncles_rlp().as_raw());
        if uncles_hash != header.uncles_hash() {
            trace!(target: "on_demand", "Body Response: \"WrongHash\" tx_root: {:?} header_root: {:?}", uncles_hash, header.uncles_hash());
            return Err(Error::WrongHash(header.uncles_hash(), uncles_hash));
        }

        // concatenate the header and the body.
        let block = encoded::Block::new_from_header_and_body(&header.view(), &body.view());

        cache.lock().insert_block_body(header.hash(), body.clone());
        Ok(block)
    }
}

/// Request for a block's receipts with header for verification.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct BlockReceipts(pub HeaderRef);

impl BlockReceipts {
    /// Check a response with receipts against the stored header.
    pub fn check_response(
        &self,
        cache: &Mutex<::cache::Cache>,
        receipts: &[Receipt],
    ) -> Result<Vec<Receipt>, Error> {
        let receipts_root = self.0.as_ref()?.receipts_root();
        let found_root = ::triehash::ordered_trie_root(receipts.iter().map(|r| ::rlp::encode(r)));

        if receipts_root == found_root {
            cache
                .lock()
                .insert_block_receipts(receipts_root, receipts.to_vec());
            Ok(receipts.to_vec())
        } else {
            trace!(target: "on_demand", "Receipt Reponse: \"WrongTrieRoot\" receipts_root: {:?} found_root: {:?}", receipts_root, found_root);
            Err(Error::WrongTrieRoot(receipts_root, found_root))
        }
    }
}

/// Request for an account structure.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Account {
    /// Header for verification.
    pub header: HeaderRef,
    /// Address requested.
    pub address: Address,
}

impl Account {
    /// Check a response with an account against the stored header.
    pub fn check_response(
        &self,
        _: &Mutex<::cache::Cache>,
        proof: &[Bytes],
    ) -> Result<Option<BasicAccount>, Error> {
        let header = self.header.as_ref()?;
        let state_root = header.state_root();

        let mut db = journaldb::new_memory_db();
        for node in proof {
            db.insert(&node[..]);
        }

        match TrieDB::new(&db, &state_root).and_then(|t| t.get(&keccak(&self.address)))? {
            Some(val) => {
                let rlp = Rlp::new(&val);
                Ok(Some(BasicAccount {
                    nonce: rlp.val_at(0)?,
                    balance: rlp.val_at(1)?,
                    storage_root: rlp.val_at(2)?,
                    code_hash: rlp.val_at(3)?,
                }))
            }
            None => {
                trace!(target: "on_demand", "Account {:?} not found", self.address);
                Ok(None)
            }
        }
    }
}

/// Request for account code.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Code {
    /// Header reference.
    pub header: HeaderRef,
    /// Account's code hash.
    pub code_hash: Field<H256>,
}

impl Code {
    /// Check a response with code against the code hash.
    pub fn check_response(
        &self,
        _: &Mutex<::cache::Cache>,
        code_hash: &H256,
        code: &[u8],
    ) -> Result<Vec<u8>, Error> {
        let found_hash = keccak(code);
        if &found_hash == code_hash {
            Ok(code.to_vec())
        } else {
            Err(Error::WrongHash(*code_hash, found_hash))
        }
    }
}

/// Request for transaction execution, along with the parts necessary to verify the proof.
#[derive(Clone)]
pub struct TransactionProof {
    /// The transaction to request proof of.
    pub tx: SignedTransaction,
    /// Block header.
    pub header: HeaderRef,
    /// Transaction environment info.
    // TODO: it's not really possible to provide this if the header is unknown.
    pub env_info: EnvInfo,
    /// Consensus engine.
    pub engine: Arc<dyn EthEngine>,
}

impl TransactionProof {
    /// Check the proof, returning the proved execution or indicate that the proof was bad.
    pub fn check_response(
        &self,
        _: &Mutex<::cache::Cache>,
        state_items: &[DBValue],
    ) -> Result<super::ExecutionResult, Error> {
        let root = self.header.as_ref()?.state_root();

        let mut env_info = self.env_info.clone();
        env_info.gas_limit = self.tx.gas;

        let proved_execution = state::check_proof(
            state_items,
            root,
            &self.tx,
            self.engine.machine(),
            &self.env_info,
        );

        match proved_execution {
            ProvedExecution::BadProof => {
                trace!(target: "on_demand", "BadExecution Proof");
                Err(Error::BadProof)
            }
            ProvedExecution::Failed(e) => {
                trace!(target: "on_demand", "Execution Proof failed: {:?}", e);
                Ok(Err(e))
            }
            ProvedExecution::Complete(e) => {
                trace!(target: "on_demand", "Execution successful: {:?}", e);
                Ok(Ok(e))
            }
        }
    }
}

/// Request for epoch signal.
/// Provide engine and state-dependent proof checker.
#[derive(Clone)]
pub struct Signal {
    /// Block hash and number to fetch proof for.
    pub hash: H256,
    /// Consensus engine, used to check the proof.
    pub engine: Arc<dyn EthEngine>,
    /// Special checker for the proof.
    pub proof_check: Arc<dyn StateDependentProof<EthereumMachine>>,
}

impl Signal {
    /// Check the signal, returning the signal or indicate that it's bad.
    pub fn check_response(
        &self,
        _: &Mutex<::cache::Cache>,
        signal: &[u8],
    ) -> Result<Vec<u8>, Error> {
        self.proof_check
            .check_proof(self.engine.machine(), signal)
            .map(|_| signal.to_owned())
            .map_err(|_| Error::BadProof)
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use ethereum_types::{Address, H256};
    use ethtrie::{SecTrieDB, SecTrieDBMut};
    use hash::keccak;
    use parking_lot::Mutex;
    use std::time::Duration;
    use trie::{Recorder, Trie, TrieMut};

    use common_types::{
        encoded,
        header::Header,
        receipt::{Receipt, TransactionOutcome},
    };
    use ethcore::client::{BlockChainClient, BlockInfo, EachBlockWith, TestBlockChainClient};

    fn make_cache() -> ::cache::Cache {
        ::cache::Cache::new(Default::default(), Duration::from_secs(1))
    }

    #[test]
    fn no_invalid_header_by_number() {
        assert!(HeaderProof::new(0, Default::default()).is_none())
    }

    #[test]
    fn check_header_proof() {
        use cht;

        let test_client = TestBlockChainClient::new();
        test_client.add_blocks(10500, EachBlockWith::Nothing);

        let cht = {
            let fetcher = |id| {
                let hdr = test_client.block_header(id).unwrap();
                let td = test_client.block_total_difficulty(id).unwrap();
                Some(cht::BlockInfo {
                    hash: hdr.hash(),
                    parent_hash: hdr.parent_hash(),
                    total_difficulty: td,
                })
            };

            cht::build(cht::block_to_cht_number(10_000).unwrap(), fetcher).unwrap()
        };

        let proof = cht.prove(10_000, 0).unwrap().unwrap();
        let req = HeaderProof::new(10_000, cht.root()).unwrap();

        let cache = Mutex::new(make_cache());
        assert!(req.check_response(&cache, &proof[..]).is_ok());
    }

    #[test]
    fn check_header_by_hash() {
        let mut header = Header::new();
        header.set_number(10_000);
        header.set_extra_data(b"test_header".to_vec());
        let hash = header.hash();
        let raw_header = encoded::Header::new(::rlp::encode(&header));

        let cache = Mutex::new(make_cache());
        assert!(HeaderByHash(hash.into())
            .check_response(&cache, &hash.into(), &[raw_header])
            .is_ok())
    }

    #[test]
    fn check_header_with_ancestors() {
        let mut last_header_hash = H256::default();
        let mut headers = (0..11)
            .map(|num| {
                let mut header = Header::new();
                header.set_number(num);
                header.set_parent_hash(last_header_hash);

                last_header_hash = header.hash();
                header
            })
            .collect::<Vec<_>>();

        headers.reverse(); // because responses are in reverse order

        let raw_headers = headers
            .iter()
            .map(|hdr| encoded::Header::new(::rlp::encode(hdr)))
            .collect::<Vec<_>>();

        let mut invalid_successor = Header::new();
        invalid_successor.set_number(11);
        invalid_successor.set_parent_hash(headers[1].hash());

        let raw_invalid_successor = encoded::Header::new(::rlp::encode(&invalid_successor));

        let cache = Mutex::new(make_cache());

        let header_with_ancestors = |hash, count| HeaderWithAncestors {
            block_hash: hash,
            ancestor_count: count,
        };

        // Correct responses
        assert!(header_with_ancestors(headers[0].hash().into(), 0)
            .check_response(&cache, &headers[0].hash().into(), &raw_headers[0..1])
            .is_ok());
        assert!(header_with_ancestors(headers[0].hash().into(), 2)
            .check_response(&cache, &headers[0].hash().into(), &raw_headers[0..3])
            .is_ok());
        assert!(header_with_ancestors(headers[0].hash().into(), 10)
            .check_response(&cache, &headers[0].hash().into(), &raw_headers[0..11])
            .is_ok());
        assert!(header_with_ancestors(headers[2].hash().into(), 2)
            .check_response(&cache, &headers[2].hash().into(), &raw_headers[2..5])
            .is_ok());
        assert!(header_with_ancestors(headers[2].hash().into(), 10)
            .check_response(&cache, &headers[2].hash().into(), &raw_headers[2..11])
            .is_ok());
        assert!(header_with_ancestors(invalid_successor.hash().into(), 0)
            .check_response(
                &cache,
                &invalid_successor.hash().into(),
                &[raw_invalid_successor.clone()]
            )
            .is_ok());

        // Incorrect responses
        assert_eq!(
            header_with_ancestors(invalid_successor.hash().into(), 0).check_response(
                &cache,
                &headers[0].hash().into(),
                &raw_headers[0..1]
            ),
            Err(Error::WrongHash(
                invalid_successor.hash(),
                headers[0].hash()
            ))
        );
        assert_eq!(
            header_with_ancestors(headers[0].hash().into(), 0).check_response(
                &cache,
                &headers[0].hash().into(),
                &[]
            ),
            Err(Error::Empty)
        );
        assert_eq!(
            header_with_ancestors(headers[0].hash().into(), 10).check_response(
                &cache,
                &headers[0].hash().into(),
                &raw_headers[0..10]
            ),
            Err(Error::TooFewResults(11, 10))
        );
        assert_eq!(
            header_with_ancestors(headers[0].hash().into(), 9).check_response(
                &cache,
                &headers[0].hash().into(),
                &raw_headers[0..11]
            ),
            Err(Error::TooManyResults(10, 11))
        );

        let response = &[raw_headers[0].clone(), raw_headers[2].clone()];
        assert_eq!(
            header_with_ancestors(headers[0].hash().into(), 1).check_response(
                &cache,
                &headers[0].hash().into(),
                response
            ),
            Err(Error::WrongHeaderSequence)
        );

        let response = &[raw_invalid_successor.clone(), raw_headers[0].clone()];
        assert_eq!(
            header_with_ancestors(invalid_successor.hash().into(), 1).check_response(
                &cache,
                &invalid_successor.hash().into(),
                response
            ),
            Err(Error::WrongHeaderSequence)
        );

        let response = &[raw_invalid_successor.clone(), raw_headers[1].clone()];
        assert_eq!(
            header_with_ancestors(invalid_successor.hash().into(), 1).check_response(
                &cache,
                &invalid_successor.hash().into(),
                response
            ),
            Err(Error::WrongHeaderSequence)
        );
    }

    #[test]
    fn check_body() {
        use rlp::RlpStream;

        let header = Header::new();
        let mut body_stream = RlpStream::new_list(2);
        body_stream.begin_list(0).begin_list(0);

        let req = Body(encoded::Header::new(::rlp::encode(&header)).into());

        let cache = Mutex::new(make_cache());
        let response = encoded::Body::new(body_stream.drain());
        assert!(req.check_response(&cache, &response).is_ok())
    }

    #[test]
    fn check_receipts() {
        let receipts = (0..5)
            .map(|_| Receipt {
                outcome: TransactionOutcome::StateRoot(H256::random()),
                gas_used: 21_000u64.into(),
                log_bloom: Default::default(),
                logs: Vec::new(),
            })
            .collect::<Vec<_>>();

        let mut header = Header::new();
        let receipts_root =
            ::triehash::ordered_trie_root(receipts.iter().map(|x| ::rlp::encode(x)));

        header.set_receipts_root(receipts_root);

        let req = BlockReceipts(encoded::Header::new(::rlp::encode(&header)).into());

        let cache = Mutex::new(make_cache());
        assert!(req.check_response(&cache, &receipts).is_ok())
    }

    #[test]
    fn check_state_proof() {
        use rlp::RlpStream;

        let mut root = H256::default();
        let mut db = journaldb::new_memory_db();
        let mut header = Header::new();
        header.set_number(123_456);
        header.set_extra_data(b"test_header".to_vec());

        let addr = Address::random();
        let rand_acc = || {
            let mut stream = RlpStream::new_list(4);
            stream
                .append(&2u64)
                .append(&100_000_000u64)
                .append(&H256::random())
                .append(&H256::random());

            stream.out()
        };
        {
            let mut trie = SecTrieDBMut::new(&mut db, &mut root);
            for _ in 0..100 {
                let address = Address::random();
                trie.insert(&*address, &rand_acc()).unwrap();
            }

            trie.insert(&*addr, &rand_acc()).unwrap();
        }

        let proof = {
            let trie = SecTrieDB::new(&db, &root).unwrap();
            let mut recorder = Recorder::new();

            trie.get_with(&*addr, &mut recorder).unwrap().unwrap();

            recorder
                .drain()
                .into_iter()
                .map(|r| r.data)
                .collect::<Vec<_>>()
        };

        header.set_state_root(root.clone());

        let req = Account {
            header: encoded::Header::new(::rlp::encode(&header)).into(),
            address: addr,
        };

        let cache = Mutex::new(make_cache());
        assert!(req.check_response(&cache, &proof[..]).is_ok());
    }

    #[test]
    fn check_code() {
        let code = vec![1u8; 256];
        let code_hash = keccak(&code);
        let header = Header::new();
        let req = Code {
            header: encoded::Header::new(::rlp::encode(&header)).into(),
            code_hash: code_hash.into(),
        };

        let cache = Mutex::new(make_cache());
        assert!(req.check_response(&cache, &code_hash, &code).is_ok());
        assert!(req.check_response(&cache, &code_hash, &[]).is_err());
    }
}