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// Copyright (c) The Diem Core Contributors
// SPDX-License-Identifier: Apache-2.0
//! This module implements an in-memory Merkle Accumulator that is similar to what we use in
//! storage. This accumulator will only store a small portion of the tree -- for any subtree that
//! is full, we store only the root. Also we only store the frozen nodes, therefore this structure
//! will always store up to `Log(n)` number of nodes, where `n` is the total number of leaves in
//! the tree.
//!
//! This accumulator is immutable once constructed. If we append new leaves to the tree we will
//! obtain a new accumulator instance and the old one remains unchanged.
#[cfg(test)]
mod accumulator_test;
#[cfg(any(test, feature = "fuzzing"))]
pub mod mock;
use super::MerkleTreeInternalNode;
use crate::proof::definition::{LeafCount, MAX_ACCUMULATOR_LEAVES};
use anyhow::{ensure, format_err, Result};
use diem_crypto::{
hash::{CryptoHash, CryptoHasher, ACCUMULATOR_PLACEHOLDER_HASH},
HashValue,
};
use serde::{Deserialize, Serialize};
use std::marker::PhantomData;
/// The Accumulator implementation.
#[derive(Clone, Debug, Deserialize, Serialize)]
pub struct InMemoryAccumulator<H> {
/// Represents the roots of all the full subtrees from left to right in this accumulator. For
/// example, if we have the following accumulator, this vector will have two hashes that
/// correspond to `X` and `e`.
/// ```text
/// root
/// / \
/// / \
/// / \
/// X o
/// / \ / \
/// / \ / \
/// o o o placeholder
/// / \ / \ / \
/// a b c d e placeholder
/// ```
frozen_subtree_roots: Vec<HashValue>,
/// The total number of leaves in this accumulator.
num_leaves: LeafCount,
/// The root hash of this accumulator.
root_hash: HashValue,
phantom: PhantomData<H>,
}
impl<H> InMemoryAccumulator<H>
where
H: CryptoHasher,
{
/// Constructs a new accumulator with roots of existing frozen subtrees. Returns error if the
/// number of frozen subtree roots does not match the number of leaves.
pub fn new(frozen_subtree_roots: Vec<HashValue>, num_leaves: LeafCount) -> Result<Self> {
ensure!(
frozen_subtree_roots.len() == num_leaves.count_ones() as usize,
"The number of frozen subtrees does not match the number of leaves. \
frozen_subtree_roots.len(): {}. num_leaves: {}.",
frozen_subtree_roots.len(),
num_leaves,
);
let root_hash = Self::compute_root_hash(&frozen_subtree_roots, num_leaves);
Ok(Self {
frozen_subtree_roots,
num_leaves,
root_hash,
phantom: PhantomData,
})
}
/// Constructs a new accumulator with given leaves.
pub fn from_leaves(leaves: &[HashValue]) -> Self {
Self::default().append(leaves)
}
/// Appends a list of new leaves to an existing accumulator. Since the accumulator is
/// immutable, the existing one remains unchanged and a new one representing the result is
/// returned.
pub fn append(&self, leaves: &[HashValue]) -> Self {
let mut frozen_subtree_roots = self.frozen_subtree_roots.clone();
let mut num_leaves = self.num_leaves;
for leaf in leaves {
Self::append_one(&mut frozen_subtree_roots, num_leaves, *leaf);
num_leaves += 1;
}
Self::new(frozen_subtree_roots, num_leaves).expect(
"Appending leaves to a valid accumulator should create another valid accumulator.",
)
}
/// Appends one leaf. This will update `frozen_subtree_roots` to store new frozen root nodes
/// and remove old nodes if they are now part of a larger frozen subtree.
fn append_one(
frozen_subtree_roots: &mut Vec<HashValue>,
num_existing_leaves: LeafCount,
leaf: HashValue,
) {
// For example, this accumulator originally had N = 7 leaves. Appending a leaf is like
// adding one to this number N: 0b0111 + 1 = 0b1000. Every time we carry a bit to the left
// we merge the rightmost two subtrees and compute their parent.
// ```text
// A
// / \
// / \
// o o B
// / \ / \ / \
// o o o o o o o
// ```
// First just append the leaf.
frozen_subtree_roots.push(leaf);
// Next, merge the last two subtrees into one. If `num_existing_leaves` has N trailing
// ones, the carry will happen N times.
let num_trailing_ones = (!num_existing_leaves).trailing_zeros();
for _i in 0..num_trailing_ones {
let right_hash = frozen_subtree_roots.pop().expect("Invalid accumulator.");
let left_hash = frozen_subtree_roots.pop().expect("Invalid accumulator.");
let parent_hash = MerkleTreeInternalNode::<H>::new(left_hash, right_hash).hash();
frozen_subtree_roots.push(parent_hash);
}
}
/// Appends a list of new subtrees to the existing accumulator. This is similar to
/// [`append`](Accumulator::append) except that the new leaves themselves are not known and
/// they are represented by `subtrees`. As an example, given the following accumulator that
/// currently has 10 leaves, the frozen subtree roots and the new subtrees are annotated below.
/// Note that in this case `subtrees[0]` represents two new leaves `A` and `B`, `subtrees[1]`
/// represents four new leaves `C`, `D`, `E` and `F`, `subtrees[2]` represents four new leaves
/// `G`, `H`, `I` and `J`, and the last `subtrees[3]` represents one new leaf `K`.
///
/// ```text
/// new_root
/// / \
/// / \
/// / \
/// / \
/// / \
/// / \
/// / \
/// / \
/// / \
/// / \
/// / \
/// / \
/// / \
/// old_root o
/// / \ / \
/// / \ / placeholder
/// / \ /
/// / \ /
/// / \ /
/// / \ o
/// / \ / \
/// / \ / \
/// / o / \
/// frozen_subtree_roots[0] / \ / \
/// / \ / \ / \
/// / \ / \ / \
/// o o o subtrees[1] subtrees[2] o
/// / \ / \ / \ / \ / \ / \
/// o o o o frozen_subtree_roots[1] subtrees[0] o o o o o placeholder
/// / \ / \ / \ / \ / \ / \ / \ / \ / \ / \ / \
/// o o o o o o o o o o A B C D E F G H I J K (subtrees[3]) placeholder
/// ```
pub fn append_subtrees(
&self,
subtrees: &[HashValue],
num_new_leaves: LeafCount,
) -> Result<Self> {
ensure!(
num_new_leaves <= MAX_ACCUMULATOR_LEAVES - self.num_leaves,
"Too many new leaves. self.num_leaves: {}. num_new_leaves: {}.",
self.num_leaves,
num_new_leaves,
);
if self.num_leaves == 0 {
return Self::new(subtrees.to_vec(), num_new_leaves);
}
let mut current_subtree_roots = self.frozen_subtree_roots.clone();
let mut current_num_leaves = self.num_leaves;
let mut remaining_new_leaves = num_new_leaves;
let mut subtree_iter = subtrees.iter();
// Check if we want to combine a new subtree with the rightmost frozen subtree. To do that
// this new subtree needs to represent `rightmost_frozen_subtree_size` leaves, so we need
// to have at least this many new leaves remaining.
let mut rightmost_frozen_subtree_size = 1 << current_num_leaves.trailing_zeros();
while remaining_new_leaves >= rightmost_frozen_subtree_size {
// Note that after combining the rightmost frozen subtree of size X with a new subtree,
// we obtain a subtree of size 2X. If there was already a frozen subtree of size 2X, we
// need to carry this process further.
let mut mask = rightmost_frozen_subtree_size;
let mut current_hash = *subtree_iter
.next()
.ok_or_else(|| format_err!("Too few subtrees."))?;
while current_num_leaves & mask != 0 {
let left_hash = current_subtree_roots
.pop()
.expect("This frozen subtree must exist.");
current_hash = MerkleTreeInternalNode::<H>::new(left_hash, current_hash).hash();
mask <<= 1;
}
current_subtree_roots.push(current_hash);
current_num_leaves += rightmost_frozen_subtree_size;
remaining_new_leaves -= rightmost_frozen_subtree_size;
rightmost_frozen_subtree_size = mask;
}
// Now all the new subtrees are smaller than the rightmost frozen subtree. We just append
// all of them. Note that if the number of new subtrees does not actually match the number
// of new leaves, `Self::new` below will raise an error.
current_num_leaves += remaining_new_leaves;
current_subtree_roots.extend(subtree_iter);
Self::new(current_subtree_roots, current_num_leaves)
}
/// Returns the root hash of the accumulator.
pub fn root_hash(&self) -> HashValue {
self.root_hash
}
pub fn version(&self) -> u64 {
if self.num_leaves() == 0 {
0
} else {
self.num_leaves() - 1
}
}
/// Computes the root hash of an accumulator given the frozen subtree roots and the number of
/// leaves in this accumulator.
fn compute_root_hash(frozen_subtree_roots: &[HashValue], num_leaves: LeafCount) -> HashValue {
match frozen_subtree_roots.len() {
0 => return *ACCUMULATOR_PLACEHOLDER_HASH,
1 => return frozen_subtree_roots[0],
_ => (),
}
// The trailing zeros do not matter since anything below the lowest frozen subtree is
// already represented by the subtree roots.
let mut bitmap = num_leaves >> num_leaves.trailing_zeros();
let mut current_hash = *ACCUMULATOR_PLACEHOLDER_HASH;
let mut frozen_subtree_iter = frozen_subtree_roots.iter().rev();
while bitmap > 0 {
current_hash = if bitmap & 1 != 0 {
MerkleTreeInternalNode::<H>::new(
*frozen_subtree_iter
.next()
.expect("This frozen subtree should exist."),
current_hash,
)
} else {
MerkleTreeInternalNode::<H>::new(current_hash, *ACCUMULATOR_PLACEHOLDER_HASH)
}
.hash();
bitmap >>= 1;
}
current_hash
}
/// Returns the set of frozen subtree roots in this accumulator
pub fn frozen_subtree_roots(&self) -> &Vec<HashValue> {
&self.frozen_subtree_roots
}
/// Returns the total number of leaves in this accumulator.
pub fn num_leaves(&self) -> LeafCount {
self.num_leaves
}
/// Returns true if this accumulator is empty and has no leaves.
pub fn is_empty(&self) -> bool {
self.num_leaves == 0
}
}
// #[derive(..)] doesn't seem to work b/c of the PhantomData :(
impl<H> std::cmp::Eq for InMemoryAccumulator<H> {}
impl<H> std::cmp::PartialEq for InMemoryAccumulator<H> {
fn eq(&self, other: &Self) -> bool {
self.num_leaves == other.num_leaves
&& self.root_hash == other.root_hash
&& self.frozen_subtree_roots == other.frozen_subtree_roots
}
}
impl<H> Default for InMemoryAccumulator<H>
where
H: CryptoHasher,
{
fn default() -> Self {
Self::new(vec![], 0).expect("Constructing empty accumulator should work.")
}
}