miden_crypto/merkle/smt/mod.rs
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use super::{EmptySubtreeRoots, InnerNodeInfo, MerkleError, MerklePath, NodeIndex};
use crate::{
hash::rpo::{Rpo256, RpoDigest},
Felt, Word, EMPTY_WORD,
};
use alloc::vec::Vec;
mod full;
pub use full::{Smt, SmtLeaf, SmtLeafError, SmtProof, SmtProofError, SMT_DEPTH};
mod simple;
pub use simple::SimpleSmt;
// CONSTANTS
// ================================================================================================
/// Minimum supported depth.
pub const SMT_MIN_DEPTH: u8 = 1;
/// Maximum supported depth.
pub const SMT_MAX_DEPTH: u8 = 64;
// SPARSE MERKLE TREE
// ================================================================================================
/// An abstract description of a sparse Merkle tree.
///
/// A sparse Merkle tree is a key-value map which also supports proving that a given value is indeed
/// stored at a given key in the tree. It is viewed as always being fully populated. If a leaf's
/// value was not explicitly set, then its value is the default value. Typically, the vast majority
/// of leaves will store the default value (hence it is "sparse"), and therefore the internal
/// representation of the tree will only keep track of the leaves that have a different value from
/// the default.
///
/// All leaves sit at the same depth. The deeper the tree, the more leaves it has; but also the
/// longer its proofs are - of exactly `log(depth)` size. A tree cannot have depth 0, since such a
/// tree is just a single value, and is probably a programming mistake.
///
/// Every key maps to one leaf. If there are as many keys as there are leaves, then
/// [Self::Leaf] should be the same type as [Self::Value], as is the case with
/// [crate::merkle::SimpleSmt]. However, if there are more keys than leaves, then [`Self::Leaf`]
/// must accomodate all keys that map to the same leaf.
///
/// [SparseMerkleTree] currently doesn't support optimizations that compress Merkle proofs.
pub(crate) trait SparseMerkleTree<const DEPTH: u8> {
/// The type for a key
type Key: Clone;
/// The type for a value
type Value: Clone + PartialEq;
/// The type for a leaf
type Leaf;
/// The type for an opening (i.e. a "proof") of a leaf
type Opening;
/// The default value used to compute the hash of empty leaves
const EMPTY_VALUE: Self::Value;
// PROVIDED METHODS
// ---------------------------------------------------------------------------------------------
/// Returns an opening of the leaf associated with `key`. Conceptually, an opening is a Merkle
/// path to the leaf, as well as the leaf itself.
fn open(&self, key: &Self::Key) -> Self::Opening {
let leaf = self.get_leaf(key);
let mut index: NodeIndex = {
let leaf_index: LeafIndex<DEPTH> = Self::key_to_leaf_index(key);
leaf_index.into()
};
let merkle_path = {
let mut path = Vec::with_capacity(index.depth() as usize);
for _ in 0..index.depth() {
let is_right = index.is_value_odd();
index.move_up();
let InnerNode { left, right } = self.get_inner_node(index);
let value = if is_right { left } else { right };
path.push(value);
}
MerklePath::new(path)
};
Self::path_and_leaf_to_opening(merkle_path, leaf)
}
/// Inserts a value at the specified key, returning the previous value associated with that key.
/// Recall that by definition, any key that hasn't been updated is associated with
/// [`Self::EMPTY_VALUE`].
///
/// This also recomputes all hashes between the leaf (associated with the key) and the root,
/// updating the root itself.
fn insert(&mut self, key: Self::Key, value: Self::Value) -> Self::Value {
let old_value = self.insert_value(key.clone(), value.clone()).unwrap_or(Self::EMPTY_VALUE);
// if the old value and new value are the same, there is nothing to update
if value == old_value {
return value;
}
let leaf = self.get_leaf(&key);
let node_index = {
let leaf_index: LeafIndex<DEPTH> = Self::key_to_leaf_index(&key);
leaf_index.into()
};
self.recompute_nodes_from_index_to_root(node_index, Self::hash_leaf(&leaf));
old_value
}
/// Recomputes the branch nodes (including the root) from `index` all the way to the root.
/// `node_hash_at_index` is the hash of the node stored at index.
fn recompute_nodes_from_index_to_root(
&mut self,
mut index: NodeIndex,
node_hash_at_index: RpoDigest,
) {
let mut node_hash = node_hash_at_index;
for node_depth in (0..index.depth()).rev() {
let is_right = index.is_value_odd();
index.move_up();
let InnerNode { left, right } = self.get_inner_node(index);
let (left, right) = if is_right {
(left, node_hash)
} else {
(node_hash, right)
};
node_hash = Rpo256::merge(&[left, right]);
if node_hash == *EmptySubtreeRoots::entry(DEPTH, node_depth) {
// If a subtree is empty, when can remove the inner node, since it's equal to the
// default value
self.remove_inner_node(index)
} else {
self.insert_inner_node(index, InnerNode { left, right });
}
}
self.set_root(node_hash);
}
// REQUIRED METHODS
// ---------------------------------------------------------------------------------------------
/// The root of the tree
fn root(&self) -> RpoDigest;
/// Sets the root of the tree
fn set_root(&mut self, root: RpoDigest);
/// Retrieves an inner node at the given index
fn get_inner_node(&self, index: NodeIndex) -> InnerNode;
/// Inserts an inner node at the given index
fn insert_inner_node(&mut self, index: NodeIndex, inner_node: InnerNode);
/// Removes an inner node at the given index
fn remove_inner_node(&mut self, index: NodeIndex);
/// Inserts a leaf node, and returns the value at the key if already exists
fn insert_value(&mut self, key: Self::Key, value: Self::Value) -> Option<Self::Value>;
/// Returns the leaf at the specified index.
fn get_leaf(&self, key: &Self::Key) -> Self::Leaf;
/// Returns the hash of a leaf
fn hash_leaf(leaf: &Self::Leaf) -> RpoDigest;
/// Maps a key to a leaf index
fn key_to_leaf_index(key: &Self::Key) -> LeafIndex<DEPTH>;
/// Maps a (MerklePath, Self::Leaf) to an opening.
///
/// The length `path` is guaranteed to be equal to `DEPTH`
fn path_and_leaf_to_opening(path: MerklePath, leaf: Self::Leaf) -> Self::Opening;
}
// INNER NODE
// ================================================================================================
#[derive(Debug, Default, Clone, PartialEq, Eq)]
#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
pub(crate) struct InnerNode {
pub left: RpoDigest,
pub right: RpoDigest,
}
impl InnerNode {
pub fn hash(&self) -> RpoDigest {
Rpo256::merge(&[self.left, self.right])
}
}
// LEAF INDEX
// ================================================================================================
/// The index of a leaf, at a depth known at compile-time.
#[derive(Debug, Default, Copy, Clone, Eq, PartialEq, PartialOrd, Ord, Hash)]
#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
pub struct LeafIndex<const DEPTH: u8> {
index: NodeIndex,
}
impl<const DEPTH: u8> LeafIndex<DEPTH> {
pub fn new(value: u64) -> Result<Self, MerkleError> {
if DEPTH < SMT_MIN_DEPTH {
return Err(MerkleError::DepthTooSmall(DEPTH));
}
Ok(LeafIndex { index: NodeIndex::new(DEPTH, value)? })
}
pub fn value(&self) -> u64 {
self.index.value()
}
}
impl LeafIndex<SMT_MAX_DEPTH> {
pub const fn new_max_depth(value: u64) -> Self {
LeafIndex {
index: NodeIndex::new_unchecked(SMT_MAX_DEPTH, value),
}
}
}
impl<const DEPTH: u8> From<LeafIndex<DEPTH>> for NodeIndex {
fn from(value: LeafIndex<DEPTH>) -> Self {
value.index
}
}
impl<const DEPTH: u8> TryFrom<NodeIndex> for LeafIndex<DEPTH> {
type Error = MerkleError;
fn try_from(node_index: NodeIndex) -> Result<Self, Self::Error> {
if node_index.depth() != DEPTH {
return Err(MerkleError::InvalidDepth {
expected: DEPTH,
provided: node_index.depth(),
});
}
Self::new(node_index.value())
}
}