557 lines
19 KiB
Rust
557 lines
19 KiB
Rust
//! Note Commitment Trees.
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//!
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//! A note commitment tree is an incremental Merkle tree of fixed depth
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//! used to store note commitments that JoinSplit transfers or Spend
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//! transfers produce. Just as the unspent transaction output set (UTXO
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//! set) used in Bitcoin, it is used to express the existence of value and
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//! the capability to spend it. However, unlike the UTXO set, it is not
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//! the job of this tree to protect against double-spending, as it is
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//! append-only.
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//!
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//! A root of a note commitment tree is associated with each treestate.
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#![allow(dead_code)]
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use std::{
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fmt,
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hash::{Hash, Hasher},
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io,
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ops::Deref,
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sync::Arc,
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};
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use bitvec::prelude::*;
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use incrementalmerkletree::{
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bridgetree::{self, Leaf},
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Frontier,
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};
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use lazy_static::lazy_static;
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use thiserror::Error;
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use zcash_encoding::{Optional, Vector};
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use zcash_primitives::merkle_tree::{self, Hashable};
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use super::commitment::pedersen_hashes::pedersen_hash;
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use crate::serialization::{
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serde_helpers, ReadZcashExt, SerializationError, ZcashDeserialize, ZcashSerialize,
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};
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pub(super) const MERKLE_DEPTH: usize = 32;
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/// MerkleCRH^Sapling Hash Function
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///
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/// Used to hash incremental Merkle tree hash values for Sapling.
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///
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/// MerkleCRH^Sapling(layer, left, right) := PedersenHash("Zcash_PH", l || left || right)
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/// where l = I2LEBSP_6(MerkleDepth^Sapling − 1 − layer) and
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/// left, right, and the output are all technically 255 bits (l_MerkleSapling), not 256.
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///
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/// <https://zips.z.cash/protocol/protocol.pdf#merklecrh>
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fn merkle_crh_sapling(layer: u8, left: [u8; 32], right: [u8; 32]) -> [u8; 32] {
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let mut s = bitvec![u8, Lsb0;];
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// Prefix: l = I2LEBSP_6(MerkleDepth^Sapling − 1 − layer)
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let l = (MERKLE_DEPTH - 1) as u8 - layer;
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s.extend_from_bitslice(&BitSlice::<_, Lsb0>::from_element(&l)[0..6]);
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s.extend_from_bitslice(&BitArray::<_, Lsb0>::from(left)[0..255]);
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s.extend_from_bitslice(&BitArray::<_, Lsb0>::from(right)[0..255]);
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pedersen_hash(*b"Zcash_PH", &s).to_bytes()
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}
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lazy_static! {
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/// List of "empty" Sapling note commitment nodes, one for each layer.
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///
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/// The list is indexed by the layer number (0: root; MERKLE_DEPTH: leaf).
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///
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/// <https://zips.z.cash/protocol/protocol.pdf#constants>
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pub(super) static ref EMPTY_ROOTS: Vec<[u8; 32]> = {
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// The empty leaf node. This is layer 32.
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let mut v = vec![NoteCommitmentTree::uncommitted()];
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// Starting with layer 31 (the first internal layer, after the leaves),
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// generate the empty roots up to layer 0, the root.
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for layer in (0..MERKLE_DEPTH).rev() {
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// The vector is generated from the end, pushing new nodes to its beginning.
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// For this reason, the layer below is v[0].
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let next = merkle_crh_sapling(layer as u8, v[0], v[0]);
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v.insert(0, next);
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}
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v
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};
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}
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/// The index of a note's commitment at the leafmost layer of its Note
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/// Commitment Tree.
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///
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/// <https://zips.z.cash/protocol/protocol.pdf#merkletree>
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pub struct Position(pub(crate) u64);
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/// Sapling note commitment tree root node hash.
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///
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/// The root hash in LEBS2OSP256(rt) encoding of the Sapling note
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/// commitment tree corresponding to the final Sapling treestate of
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/// this block. A root of a note commitment tree is associated with
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/// each treestate.
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#[derive(Clone, Copy, Default, Eq, Serialize, Deserialize)]
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pub struct Root(#[serde(with = "serde_helpers::Fq")] pub(crate) jubjub::Base);
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impl fmt::Debug for Root {
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fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
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f.debug_tuple("Root")
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.field(&hex::encode(&self.0.to_bytes()))
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.finish()
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}
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}
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impl From<Root> for [u8; 32] {
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fn from(root: Root) -> Self {
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root.0.to_bytes()
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}
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}
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impl From<&Root> for [u8; 32] {
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fn from(root: &Root) -> Self {
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(*root).into()
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}
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}
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impl PartialEq for Root {
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fn eq(&self, other: &Self) -> bool {
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self.0 == other.0
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}
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}
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impl Hash for Root {
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fn hash<H: Hasher>(&self, state: &mut H) {
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self.0.to_bytes().hash(state)
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}
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}
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impl TryFrom<[u8; 32]> for Root {
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type Error = SerializationError;
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fn try_from(bytes: [u8; 32]) -> Result<Self, Self::Error> {
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let possible_point = jubjub::Base::from_bytes(&bytes);
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if possible_point.is_some().into() {
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Ok(Self(possible_point.unwrap()))
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} else {
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Err(SerializationError::Parse(
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"Invalid jubjub::Base value for Sapling note commitment tree root",
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))
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}
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}
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}
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impl ZcashSerialize for Root {
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fn zcash_serialize<W: io::Write>(&self, mut writer: W) -> Result<(), io::Error> {
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writer.write_all(&<[u8; 32]>::from(*self)[..])?;
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Ok(())
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}
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}
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impl ZcashDeserialize for Root {
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fn zcash_deserialize<R: io::Read>(mut reader: R) -> Result<Self, SerializationError> {
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Self::try_from(reader.read_32_bytes()?)
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}
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}
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/// A node of the Sapling Incremental Note Commitment Tree.
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///
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/// Note that it's handled as a byte buffer and not a point coordinate (jubjub::Fq)
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/// because that's how the spec handles the MerkleCRH^Sapling function inputs and outputs.
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#[derive(Copy, Clone, Debug, Eq, PartialEq)]
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struct Node([u8; 32]);
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/// Required to convert [`NoteCommitmentTree`] into [`SerializedTree`].
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///
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/// Zebra stores Sapling note commitment trees as [`Frontier`][1]s while the
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/// [`z_gettreestate`][2] RPC requires [`CommitmentTree`][3]s. Implementing
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/// [`merkle_tree::Hashable`] for [`Node`]s allows the conversion.
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///
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/// [1]: bridgetree::Frontier
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/// [2]: https://zcash.github.io/rpc/z_gettreestate.html
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/// [3]: merkle_tree::CommitmentTree
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impl merkle_tree::Hashable for Node {
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fn read<R: io::Read>(mut reader: R) -> io::Result<Self> {
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let mut node = [0u8; 32];
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reader.read_exact(&mut node)?;
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Ok(Self(node))
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}
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fn write<W: io::Write>(&self, mut writer: W) -> io::Result<()> {
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writer.write_all(self.0.as_ref())
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}
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fn combine(level: usize, a: &Self, b: &Self) -> Self {
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let level = u8::try_from(level).expect("level must fit into u8");
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let layer = (MERKLE_DEPTH - 1) as u8 - level;
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Self(merkle_crh_sapling(layer, a.0, b.0))
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}
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fn blank() -> Self {
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Self(NoteCommitmentTree::uncommitted())
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}
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fn empty_root(level: usize) -> Self {
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let layer_below = MERKLE_DEPTH - level;
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Self(EMPTY_ROOTS[layer_below])
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}
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}
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impl incrementalmerkletree::Hashable for Node {
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fn empty_leaf() -> Self {
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Self(NoteCommitmentTree::uncommitted())
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}
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/// Combine two nodes to generate a new node in the given level.
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/// Level 0 is the layer above the leaves (layer 31).
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/// Level 31 is the root (layer 0).
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fn combine(level: incrementalmerkletree::Altitude, a: &Self, b: &Self) -> Self {
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let layer = (MERKLE_DEPTH - 1) as u8 - u8::from(level);
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Self(merkle_crh_sapling(layer, a.0, b.0))
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}
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/// Return the node for the level below the given level. (A quirk of the API)
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fn empty_root(level: incrementalmerkletree::Altitude) -> Self {
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let layer_below: usize = MERKLE_DEPTH - usize::from(level);
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Self(EMPTY_ROOTS[layer_below])
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}
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}
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impl From<jubjub::Fq> for Node {
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fn from(x: jubjub::Fq) -> Self {
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Node(x.into())
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}
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}
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impl serde::Serialize for Node {
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fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
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where
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S: serde::Serializer,
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{
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self.0.serialize(serializer)
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}
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}
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impl<'de> serde::Deserialize<'de> for Node {
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fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
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where
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D: serde::Deserializer<'de>,
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{
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let bytes = <[u8; 32]>::deserialize(deserializer)?;
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Option::<jubjub::Fq>::from(jubjub::Fq::from_bytes(&bytes))
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.map(Node::from)
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.ok_or_else(|| serde::de::Error::custom("invalid JubJub field element"))
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}
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}
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#[allow(dead_code, missing_docs)]
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#[derive(Error, Debug, Clone, PartialEq, Eq)]
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pub enum NoteCommitmentTreeError {
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#[error("The note commitment tree is full")]
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FullTree,
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}
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/// Sapling Incremental Note Commitment Tree.
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#[derive(Debug, Serialize, Deserialize)]
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pub struct NoteCommitmentTree {
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/// The tree represented as a [`Frontier`](bridgetree::Frontier).
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///
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/// A Frontier is a subset of the tree that allows to fully specify it.
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/// It consists of nodes along the rightmost (newer) branch of the tree that
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/// has non-empty nodes. Upper (near root) empty nodes of the branch are not
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/// stored.
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///
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/// # Consensus
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///
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/// > [Sapling onward] A block MUST NOT add Sapling note commitments that
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/// > would result in the Sapling note commitment tree exceeding its capacity
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/// > of 2^(MerkleDepth^Sapling) leaf nodes.
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///
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/// <https://zips.z.cash/protocol/protocol.pdf#merkletree>
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///
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/// Note: MerkleDepth^Sapling = MERKLE_DEPTH = 32.
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inner: bridgetree::Frontier<Node, { MERKLE_DEPTH as u8 }>,
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/// A cached root of the tree.
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///
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/// Every time the root is computed by [`Self::root`] it is cached here, and
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/// the cached value will be returned by [`Self::root`] until the tree is
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/// changed by [`Self::append`]. This greatly increases performance because
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/// it avoids recomputing the root when the tree does not change between
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/// blocks. In the finalized state, the tree is read from disk for every
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/// block processed, which would also require recomputing the root even if
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/// it has not changed (note that the cached root is serialized with the
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/// tree). This is particularly important since we decided to instantiate
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/// the trees from the genesis block, for simplicity.
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///
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/// We use a [`RwLock`](std::sync::RwLock) for this cache, because it is only written once per
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/// tree update. Each tree has its own cached root, a new lock is created
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/// for each clone.
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cached_root: std::sync::RwLock<Option<Root>>,
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}
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impl NoteCommitmentTree {
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/// Adds a note commitment u-coordinate to the tree.
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///
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/// The leaves of the tree are actually a base field element, the
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/// u-coordinate of the commitment, the data that is actually stored on the
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/// chain and input into the proof.
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///
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/// Returns an error if the tree is full.
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#[allow(clippy::unwrap_in_result)]
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pub fn append(&mut self, cm_u: jubjub::Fq) -> Result<(), NoteCommitmentTreeError> {
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if self.inner.append(&cm_u.into()) {
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// Invalidate cached root
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let cached_root = self
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.cached_root
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.get_mut()
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.expect("a thread that previously held exclusive lock access panicked");
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*cached_root = None;
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Ok(())
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} else {
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Err(NoteCommitmentTreeError::FullTree)
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}
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}
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/// Returns the current root of the tree, used as an anchor in Sapling
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/// shielded transactions.
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pub fn root(&self) -> Root {
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if let Some(root) = self
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.cached_root
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.read()
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.expect("a thread that previously held exclusive lock access panicked")
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.deref()
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{
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// Return cached root.
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return *root;
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}
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// Get exclusive access, compute the root, and cache it.
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let mut write_root = self
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.cached_root
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.write()
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.expect("a thread that previously held exclusive lock access panicked");
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let read_root = write_root.as_ref().cloned();
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match read_root {
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// Another thread got write access first, return cached root.
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Some(root) => root,
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None => {
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// Compute root and cache it.
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let root = Root::try_from(self.inner.root().0).unwrap();
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*write_root = Some(root);
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root
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}
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}
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}
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/// Gets the Jubjub-based Pedersen hash of root node of this merkle tree of
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/// note commitments.
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pub fn hash(&self) -> [u8; 32] {
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self.root().into()
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}
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/// An as-yet unused Sapling note commitment tree leaf node.
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///
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/// Distinct for Sapling, a distinguished hash value of:
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///
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/// Uncommitted^Sapling = I2LEBSP_l_MerkleSapling(1)
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pub fn uncommitted() -> [u8; 32] {
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jubjub::Fq::one().to_bytes()
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}
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/// Counts of note commitments added to the tree.
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///
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/// For Sapling, the tree is capped at 2^32.
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pub fn count(&self) -> u64 {
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self.inner.position().map_or(0, |pos| u64::from(pos) + 1)
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}
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}
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impl Clone for NoteCommitmentTree {
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/// Clones the inner tree, and creates a new [`RwLock`](std::sync::RwLock)
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/// with the cloned root data.
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fn clone(&self) -> Self {
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let cached_root = *self
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.cached_root
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.read()
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.expect("a thread that previously held exclusive lock access panicked");
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Self {
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inner: self.inner.clone(),
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cached_root: std::sync::RwLock::new(cached_root),
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}
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}
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}
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impl Default for NoteCommitmentTree {
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fn default() -> Self {
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Self {
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inner: bridgetree::Frontier::empty(),
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cached_root: Default::default(),
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}
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}
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}
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impl Eq for NoteCommitmentTree {}
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impl PartialEq for NoteCommitmentTree {
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fn eq(&self, other: &Self) -> bool {
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self.hash() == other.hash()
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}
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}
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impl From<Vec<jubjub::Fq>> for NoteCommitmentTree {
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/// Computes the tree from a whole bunch of note commitments at once.
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fn from(values: Vec<jubjub::Fq>) -> Self {
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let mut tree = Self::default();
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if values.is_empty() {
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return tree;
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}
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for cm_u in values {
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let _ = tree.append(cm_u);
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}
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tree
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}
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}
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/// A serialized Sapling note commitment tree.
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///
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/// The format of the serialized data is compatible with
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/// [`CommitmentTree`](merkle_tree::CommitmentTree) from `librustzcash` and not
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/// with [`Frontier`](bridgetree::Frontier) from the crate
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/// [`incrementalmerkletree`]. Zebra follows the former format in order to stay
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/// consistent with `zcashd` in RPCs. Note that [`NoteCommitmentTree`] itself is
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/// represented as [`Frontier`](bridgetree::Frontier).
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///
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/// The formats are semantically equivalent. The primary difference between them
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/// is that in [`Frontier`](bridgetree::Frontier), the vector of parents is
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/// dense (we know where the gaps are from the position of the leaf in the
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/// overall tree); whereas in [`CommitmentTree`](merkle_tree::CommitmentTree),
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/// the vector of parent hashes is sparse with [`None`] values in the gaps.
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///
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/// The sparse format, used in this implementation, allows representing invalid
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/// commitment trees while the dense format allows representing only valid
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/// commitment trees.
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///
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/// It is likely that the dense format will be used in future RPCs, in which
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/// case the current implementation will have to change and use the format
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/// compatible with [`Frontier`](bridgetree::Frontier) instead.
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#[derive(Clone, Debug, Eq, PartialEq, serde::Serialize)]
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pub struct SerializedTree(Vec<u8>);
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impl From<&NoteCommitmentTree> for SerializedTree {
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fn from(tree: &NoteCommitmentTree) -> Self {
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let mut serialized_tree = vec![];
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// Convert the note commitment tree represented as a frontier into the
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// format compatible with `zcashd`.
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//
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// `librustzcash` has a function [`from_frontier()`][1], which returns a
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// commitment tree in the sparse format. However, the returned tree
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// always contains [`MERKLE_DEPTH`] parent nodes, even though some
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// trailing parents are empty. Such trees are incompatible with Sapling
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// commitment trees returned by `zcashd` because `zcashd` returns
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// Sapling commitment trees without empty trailing parents. For this
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// reason, Zebra implements its own conversion between the dense and
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// sparse formats for Sapling.
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//
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// [1]: <https://github.com/zcash/librustzcash/blob/a63a37a/zcash_primitives/src/merkle_tree.rs#L125>
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if let Some(frontier) = tree.inner.value() {
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let (left_leaf, right_leaf) = match frontier.leaf() {
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Leaf::Left(left_value) => (Some(left_value), None),
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Leaf::Right(left_value, right_value) => (Some(left_value), Some(right_value)),
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};
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// Ommers are siblings of parent nodes along the branch from the
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// most recent leaf to the root of the tree.
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let mut ommers_iter = frontier.ommers().iter();
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// Set bits in the binary representation of the position indicate
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// the presence of ommers along the branch from the most recent leaf
|
||
// node to the root of the tree, except for the lowest bit.
|
||
let mut position: usize = frontier.position().into();
|
||
|
||
// The lowest bit does not indicate the presence of any ommers. We
|
||
// clear it so that we can test if there are no set bits left in
|
||
// [`position`].
|
||
position &= !1;
|
||
|
||
// Run through the bits of [`position`], and push an ommer for each
|
||
// set bit, or `None` otherwise. In contrast to the 'zcashd' code
|
||
// linked above, we want to skip any trailing `None` parents at the
|
||
// top of the tree. To do that, we clear the bits as we go through
|
||
// them, and break early if the remaining bits are all zero (i.e.
|
||
// [`position`] is zero).
|
||
let mut parents = vec![];
|
||
for i in 1..MERKLE_DEPTH {
|
||
// Test each bit in [`position`] individually. Don't test the
|
||
// lowest bit since it doesn't actually indicate the position of
|
||
// any ommer.
|
||
let bit_mask = 1 << i;
|
||
|
||
if position & bit_mask == 0 {
|
||
parents.push(None);
|
||
} else {
|
||
parents.push(ommers_iter.next());
|
||
// Clear the set bit so that we can test if there are no set
|
||
// bits left.
|
||
position &= !bit_mask;
|
||
// If there are no set bits left, exit early so that there
|
||
// are no empty trailing parent nodes in the serialized
|
||
// tree.
|
||
if position == 0 {
|
||
break;
|
||
}
|
||
}
|
||
}
|
||
|
||
// Serialize the converted note commitment tree.
|
||
|
||
Optional::write(&mut serialized_tree, left_leaf, |tree, leaf| {
|
||
leaf.write(tree)
|
||
})
|
||
.expect("A leaf in a note commitment tree should be serializable");
|
||
|
||
Optional::write(&mut serialized_tree, right_leaf, |tree, leaf| {
|
||
leaf.write(tree)
|
||
})
|
||
.expect("A leaf in a note commitment tree should be serializable");
|
||
|
||
Vector::write(&mut serialized_tree, &parents, |tree, parent| {
|
||
Optional::write(tree, *parent, |tree, parent| parent.write(tree))
|
||
})
|
||
.expect("Parent nodes in a note commitment tree should be serializable");
|
||
}
|
||
|
||
Self(serialized_tree)
|
||
}
|
||
}
|
||
|
||
impl From<Option<Arc<NoteCommitmentTree>>> for SerializedTree {
|
||
fn from(maybe_tree: Option<Arc<NoteCommitmentTree>>) -> Self {
|
||
match maybe_tree {
|
||
Some(tree) => tree.as_ref().into(),
|
||
None => Self(vec![]),
|
||
}
|
||
}
|
||
}
|
||
|
||
impl AsRef<[u8]> for SerializedTree {
|
||
fn as_ref(&self) -> &[u8] {
|
||
&self.0
|
||
}
|
||
}
|