247 lines
8.2 KiB
Rust
247 lines
8.2 KiB
Rust
//! History tree (Merkle mountain range) structure that contains information about
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//! the block history as specified in ZIP-221.
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use std::{
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collections::{BTreeMap, HashSet},
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io,
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sync::Arc,
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};
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use thiserror::Error;
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use crate::{
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block::{Block, ChainHistoryMmrRootHash, Height},
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orchard,
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parameters::{Network, NetworkUpgrade},
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primitives::zcash_history::{Entry, Tree as InnerHistoryTree},
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sapling,
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};
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/// An error describing why a history tree operation failed.
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#[derive(Debug, Error)]
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#[non_exhaustive]
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#[allow(missing_docs)]
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pub enum HistoryTreeError {
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#[error("zcash_history error: {inner:?}")]
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#[non_exhaustive]
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InnerError { inner: zcash_history::Error },
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#[error("I/O error")]
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IOError(#[from] io::Error),
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}
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/// History tree (Merkle mountain range) structure that contains information about
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// the block history, as specified in [ZIP-221][https://zips.z.cash/zip-0221].
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pub struct HistoryTree {
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network: Network,
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network_upgrade: NetworkUpgrade,
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/// Merkle mountain range tree from `zcash_history`.
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/// This is a "runtime" structure used to add / remove nodes, and it's not
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/// persistent.
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inner: InnerHistoryTree,
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/// The number of nodes in the tree.
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size: u32,
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/// The peaks of the tree, indexed by their position in the array representation
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/// of the tree. This can be persisted to save the tree.
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peaks: BTreeMap<u32, Entry>,
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/// The height of the most recent block added to the tree.
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current_height: Height,
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}
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impl HistoryTree {
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/// Create a new history tree with a single block.
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pub fn from_block(
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network: Network,
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block: Arc<Block>,
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sapling_root: &sapling::tree::Root,
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_orchard_root: Option<&orchard::tree::Root>,
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) -> Result<Self, io::Error> {
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let height = block
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.coinbase_height()
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.expect("block must have coinbase height during contextual verification");
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let network_upgrade = NetworkUpgrade::current(network, height);
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// TODO: handle Orchard root, see https://github.com/ZcashFoundation/zebra/issues/2283
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let (tree, entry) = InnerHistoryTree::new_from_block(network, block, sapling_root)?;
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let mut peaks = BTreeMap::new();
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peaks.insert(0u32, entry);
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Ok(HistoryTree {
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network,
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network_upgrade,
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inner: tree,
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size: 1,
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peaks,
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current_height: height,
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})
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}
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/// Add block data to the tree.
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///
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/// # Panics
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///
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/// If the block height is not one more than the previously pushed block.
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pub fn push(
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&mut self,
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block: Arc<Block>,
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sapling_root: &sapling::tree::Root,
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_orchard_root: Option<&orchard::tree::Root>,
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) -> Result<(), HistoryTreeError> {
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// Check if the block has the expected height.
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// librustzcash assumes the heights are correct and corrupts the tree if they are wrong,
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// resulting in a confusing error, which we prevent here.
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let height = block
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.coinbase_height()
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.expect("block must have coinbase height during contextual verification");
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if height - self.current_height != 1 {
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panic!(
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"added block with height {:?} but it must be {:?}+1",
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height, self.current_height
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);
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}
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// TODO: handle orchard root
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let new_entries = self
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.inner
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.append_leaf(block, sapling_root)
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.map_err(|e| HistoryTreeError::InnerError { inner: e })?;
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for entry in new_entries {
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// Not every entry is a peak; those will be trimmed later
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self.peaks.insert(self.size, entry);
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self.size += 1;
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}
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self.prune()?;
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// TODO: implement network upgrade logic: drop previous history, start new history
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self.current_height = height;
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Ok(())
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}
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/// Extend the history tree with the given blocks.
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pub fn try_extend<
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'a,
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T: IntoIterator<
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Item = (
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Arc<Block>,
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&'a sapling::tree::Root,
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Option<&'a orchard::tree::Root>,
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),
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>,
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>(
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&mut self,
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iter: T,
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) -> Result<(), HistoryTreeError> {
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for (block, sapling_root, orchard_root) in iter {
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self.push(block, sapling_root, orchard_root)?;
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}
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Ok(())
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}
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/// Prune tree, removing all non-peak entries.
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fn prune(&mut self) -> Result<(), io::Error> {
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// Go through all the peaks of the tree.
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// This code is based on a librustzcash example:
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// https://github.com/zcash/librustzcash/blob/02052526925fba9389f1428d6df254d4dec967e6/zcash_history/examples/long.rs
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// The explanation of how it works is from zcashd:
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// https://github.com/zcash/zcash/blob/0247c0c682d59184a717a6536edb0d18834be9a7/src/coins.cpp#L351
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let mut peak_pos_set = HashSet::new();
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// Assume the following example peak layout with 14 leaves, and 25 stored nodes in
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// total (the "tree length"):
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//
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// P
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// /\
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// / \
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// / \ \
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// / \ \ Altitude
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// _A_ \ \ 3
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// _/ \_ B \ 2
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// / \ / \ / \ C 1
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// /\ /\ /\ /\ /\ /\ /\ 0
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//
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// We start by determining the altitude of the highest peak (A).
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let mut alt = (32 - ((self.size + 1) as u32).leading_zeros() - 1) - 1;
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// We determine the position of the highest peak (A) by pretending it is the right
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// sibling in a tree, and its left-most leaf has position 0. Then the left sibling
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// of (A) has position -1, and so we can "jump" to the peak's position by computing
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// -1 + 2^(alt + 1) - 1.
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let mut peak_pos = (1 << (alt + 1)) - 2;
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// Now that we have the position and altitude of the highest peak (A), we collect
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// the remaining peaks (B, C). We navigate the peaks as if they were nodes in this
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// Merkle tree (with additional imaginary nodes 1 and 2, that have positions beyond
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// the MMR's length):
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//
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// / \
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// / \
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// / \
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// / \
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// A ==========> 1
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// / \ // \
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// _/ \_ B ==> 2
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// /\ /\ /\ //
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// / \ / \ / \ C
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// /\ /\ /\ /\ /\ /\ /\
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//
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loop {
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// If peak_pos is out of bounds of the tree, we compute the position of its left
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// child, and drop down one level in the tree.
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if peak_pos >= self.size {
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// left child, -2^alt
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peak_pos -= 1 << alt;
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alt -= 1;
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}
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// If the peak exists, we take it and then continue with its right sibling.
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if peak_pos < self.size {
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// There is a peak at index `peak_pos`
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peak_pos_set.insert(peak_pos);
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// right sibling
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peak_pos = peak_pos + (1 << (alt + 1)) - 1;
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}
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if alt == 0 {
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break;
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}
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}
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// Remove all non-peak entries
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self.peaks.retain(|k, _| peak_pos_set.contains(k));
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// Rebuild tree
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self.inner = InnerHistoryTree::new_from_cache(
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self.network,
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self.network_upgrade,
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self.size,
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&self.peaks,
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&Default::default(),
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)?;
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Ok(())
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}
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/// Return the hash of the tree root.
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pub fn hash(&self) -> ChainHistoryMmrRootHash {
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self.inner.hash()
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}
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}
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impl Clone for HistoryTree {
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fn clone(&self) -> Self {
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let tree = InnerHistoryTree::new_from_cache(
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self.network,
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self.network_upgrade,
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self.size,
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&self.peaks,
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&Default::default(),
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)
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.expect("rebuilding an existing tree should always work");
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HistoryTree {
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network: self.network,
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network_upgrade: self.network_upgrade,
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inner: tree,
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size: self.size,
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peaks: self.peaks.clone(),
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current_height: self.current_height,
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}
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}
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}
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