487 lines
16 KiB
Rust
487 lines
16 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 Action
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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(clippy::derive_hash_xor_eq)]
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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 halo2::pasta::{group::ff::PrimeField, pallas};
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use incrementalmerkletree::{bridgetree, Frontier};
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use lazy_static::lazy_static;
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use thiserror::Error;
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use zcash_primitives::merkle_tree::{self, CommitmentTree};
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use super::sinsemilla::*;
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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^Orchard Hash Function
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///
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/// Used to hash incremental Merkle tree hash values for Orchard.
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///
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/// MerkleCRH^Orchard: {0..MerkleDepth^Orchard − 1} × P𝑥 × P𝑥 → P𝑥
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///
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/// MerkleCRH^Orchard(layer, left, right) := 0 if hash == ⊥; hash otherwise
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///
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/// where hash = SinsemillaHash("z.cash:Orchard-MerkleCRH", l || left || right),
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/// l = I2LEBSP_10(MerkleDepth^Orchard − 1 − layer), and left, right, and
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/// the output are the x-coordinates of Pallas affine points.
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///
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/// <https://zips.z.cash/protocol/protocol.pdf#orchardmerklecrh>
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/// <https://zips.z.cash/protocol/protocol.pdf#constants>
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fn merkle_crh_orchard(layer: u8, left: pallas::Base, right: pallas::Base) -> pallas::Base {
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let mut s = bitvec![u8, Lsb0;];
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// Prefix: l = I2LEBSP_10(MerkleDepth^Orchard − 1 − layer)
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let l = MERKLE_DEPTH - 1 - layer as usize;
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s.extend_from_bitslice(&BitArray::<_, Lsb0>::from([l, 0])[0..10]);
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s.extend_from_bitslice(&BitArray::<_, Lsb0>::from(left.to_repr())[0..255]);
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s.extend_from_bitslice(&BitArray::<_, Lsb0>::from(right.to_repr())[0..255]);
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match sinsemilla_hash(b"z.cash:Orchard-MerkleCRH", &s) {
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Some(h) => h,
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None => pallas::Base::zero(),
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}
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}
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lazy_static! {
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/// List of "empty" Orchard 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<pallas::Base> = {
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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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{
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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_orchard(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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/// Orchard note commitment tree root node hash.
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///
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/// The root hash in LEBS2OSP256(rt) encoding of the Orchard note commitment
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/// tree corresponding to the final Orchard treestate of this block. A root of a
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/// note commitment tree is associated with each treestate.
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#[derive(Clone, Copy, Default, Eq, PartialEq, Serialize, Deserialize)]
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pub struct Root(#[serde(with = "serde_helpers::Base")] pub(crate) pallas::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_repr()))
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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.into()
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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 Hash for Root {
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fn hash<H: Hasher>(&self, state: &mut H) {
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self.0.to_repr().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 = pallas::Base::from_repr(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 pallas::Base value for Orchard 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 Orchard Incremental Note Commitment Tree.
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#[derive(Copy, Clone, Debug, Eq, PartialEq)]
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struct Node(pallas::Base);
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/// Required to convert [`NoteCommitmentTree`] into [`SerializedTree`].
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///
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/// Zebra stores Orchard 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 repr = [0u8; 32];
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reader.read_exact(&mut repr)?;
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let maybe_node = pallas::Base::from_repr(repr).map(Self);
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<Option<_>>::from(maybe_node).ok_or_else(|| {
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io::Error::new(
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io::ErrorKind::InvalidInput,
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"Non-canonical encoding of Pallas base field value.",
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)
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})
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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.to_repr())
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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_orchard(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: usize = 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_orchard(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<pallas::Base> for Node {
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fn from(x: pallas::Base) -> Self {
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Node(x)
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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.to_repr().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::<pallas::Base>::from(pallas::Base::from_repr(bytes))
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.map(Node)
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.ok_or_else(|| serde::de::Error::custom("invalid Pallas 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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/// Orchard 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.
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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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/// > [NU5 onward] A block MUST NOT add Orchard note commitments that would result in the Orchard note
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/// > commitment tree exceeding its capacity of 2^(MerkleDepth^Orchard) 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^Orchard = 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,
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/// and 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
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/// because it avoids recomputing the root when the tree does not change
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/// between blocks. In the finalized state, the tree is read from
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/// disk for every block processed, which would also require recomputing
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/// the root even if it has not changed (note that the cached root is
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/// serialized with the tree). This is particularly important since we decided
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/// to instantiate 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
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/// only written once per tree update. Each tree has its own cached root, a
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/// new lock is created 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 x-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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/// x-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_x: pallas::Base) -> Result<(), NoteCommitmentTreeError> {
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if self.inner.append(&cm_x.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 Orchard
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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(self.inner.root().0);
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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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/// Get the Pallas-based Sinsemilla hash / 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 Orchard note commitment tree leaf node.
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///
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/// Distinct for Orchard, a distinguished hash value of:
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///
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/// Uncommitted^Orchard = I2LEBSP_l_MerkleOrchard(2)
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pub fn uncommitted() -> pallas::Base {
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pallas::Base::one().double()
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}
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/// Count of note commitments added to the tree.
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///
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/// For Orchard, 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` 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<pallas::Base>> for NoteCommitmentTree {
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/// Compute the tree from a whole bunch of note commitments at once.
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fn from(values: Vec<pallas::Base>) -> 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_x in values {
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let _ = tree.append(cm_x);
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}
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tree
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}
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}
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/// A serialized Orchard 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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// Skip the serialization of empty trees.
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//
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// Note: This ensures compatibility with `zcashd` in the
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// [`z_gettreestate`][1] RPC.
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//
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// [1]: https://zcash.github.io/rpc/z_gettreestate.html
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if tree.inner == bridgetree::Frontier::empty() {
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return Self(serialized_tree);
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}
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// Convert the note commitment tree from
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// [`Frontier`](bridgetree::Frontier) to
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// [`CommitmentTree`](merkle_tree::CommitmentTree).
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let tree = CommitmentTree::from_frontier(&tree.inner);
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tree.write(&mut serialized_tree)
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.expect("note commitment tree should be serializable");
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Self(serialized_tree)
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}
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}
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impl From<Option<Arc<NoteCommitmentTree>>> for SerializedTree {
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fn from(maybe_tree: Option<Arc<NoteCommitmentTree>>) -> Self {
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match maybe_tree {
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Some(tree) => tree.as_ref().into(),
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None => Self(Vec::new()),
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}
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}
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}
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impl AsRef<[u8]> for SerializedTree {
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fn as_ref(&self) -> &[u8] {
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&self.0
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}
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}
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