CHT builder and prover

This commit is contained in:
Robert Habermeier 2017-02-03 18:38:28 +01:00
parent 8e82b2f631
commit fc8e9e39ec

View File

@ -12,10 +12,110 @@
// GNU General Public License for more details. // GNU General Public License for more details.
//! Canonical hash trie definitions and helper functions. //! Canonical hash trie definitions and helper functions.
//!
//! Each CHT is a trie mapping block numbers to canonical hashes and total difficulty.
//! One is generated for every `SIZE` blocks, allowing us to discard those blocks in
//! favor the the trie root. When the "ancient" blocks need to be accessed, we simply
//! request an inclusion proof of a specific block number against the trie with the
//! root has. A correct proof implies that the claimed block is identical to the one
//! we discarded.
use ethcore::ids::BlockId;
use util::{Bytes, H256, U256, HashDB, MemoryDB};
use util::trie::{self, TrieMut, TrieDBMut, Trie, TrieDB, Recorder};
use rlp::{Stream, RlpStream};
// encode a key.
macro_rules! key {
($num: expr) => { ::rlp::encode(&$num) }
}
macro_rules! val {
($hash: expr, $td: expr) => {{
let mut stream = RlpStream::new_list(2);
stream.append(&$hash).append(&$td);
stream.out()
}}
}
/// The size of each CHT. /// The size of each CHT.
pub const SIZE: u64 = 2048; pub const SIZE: u64 = 2048;
/// A canonical hash trie. This is generic over any database it can query.
/// See module docs for more details.
#[derive(Debug, Clone)]
pub struct CHT<DB: HashDB> {
db: DB,
root: H256, // the root of this CHT.
number: u64,
}
impl<DB: HashDB> CHT<DB> {
/// Query the root of the CHT.
pub fn root(&self) -> H256 { self.root }
/// Query the number of the CHT.
pub fn number(&self) -> u64 { self.number }
/// Generate an inclusion proof for the entry at a specific block.
/// Nodes before level `from_level` will be omitted.
/// Returns an error on an incomplete trie, and `Ok(None)` on an unprovable request.
pub fn prove(&self, num: u64, from_level: u32) -> trie::Result<Option<Vec<Bytes>>> {
if block_to_cht_number(num) != Some(self.number) { return Ok(None) }
let mut recorder = Recorder::with_depth(from_level);
let t = TrieDB::new(&self.db, &self.root)?;
t.get_with(&key!(num), &mut recorder)?;
Ok(Some(recorder.drain().into_iter().map(|x| x.data).collect()))
}
}
/// Block information necessary to build a CHT.
pub struct BlockInfo {
/// The block's hash.
pub hash: H256,
/// The block's parent's hash.
pub parent_hash: H256,
/// The block's total difficulty.
pub total_difficulty: U256,
}
/// Build an in-memory CHT from a closure which provides necessary information
/// about blocks. If the fetcher ever fails to provide the info, the CHT
/// will not be generated.
pub fn build<F>(cht_num: u64, mut fetcher: F) -> Option<CHT<MemoryDB>>
where F: FnMut(BlockId) -> Option<BlockInfo>
{
let mut db = MemoryDB::new();
// start from the last block by number and work backwards.
let last_num = start_number(cht_num + 1) - 1;
let mut id = BlockId::Number(last_num);
let mut root = H256::default();
{
let mut t = TrieDBMut::new(&mut db, &mut root);
for blk_num in (0..SIZE).map(|n| last_num - n) {
let info = match fetcher(id) {
Some(info) => info,
None => return None,
};
id = BlockId::Hash(info.parent_hash);
t.insert(&key!(blk_num), &val!(info.hash, info.total_difficulty))
.expect("fresh in-memory database is infallible; qed");
}
}
Some(CHT {
db: db,
root: root,
number: cht_num,
})
}
/// Convert a block number to a CHT number. /// Convert a block number to a CHT number.
/// Returns `None` for `block_num` == 0, `Some` otherwise. /// Returns `None` for `block_num` == 0, `Some` otherwise.
pub fn block_to_cht_number(block_num: u64) -> Option<u64> { pub fn block_to_cht_number(block_num: u64) -> Option<u64> {
@ -37,6 +137,12 @@ pub fn start_number(cht_num: u64) -> u64 {
#[cfg(test)] #[cfg(test)]
mod tests { mod tests {
#[test]
fn size_is_lt_usize() {
// to ensure safe casting on the target platform.
assert!(::cht::SIZE < usize::max_value() as u64)
}
#[test] #[test]
fn block_to_cht_number() { fn block_to_cht_number() {
assert!(::cht::block_to_cht_number(0).is_none()); assert!(::cht::block_to_cht_number(0).is_none());