From 85940dd56f8cb11e53221dbc82d9ed25ed306692 Mon Sep 17 00:00:00 2001 From: Qyriad Date: Thu, 27 Feb 2025 17:26:28 +0100 Subject: [PATCH 1/3] WIP: smt: add SparseMerklePath --- src/merkle/mod.rs | 3 + src/merkle/sparse_path.rs | 113 ++++++++++++++++++++++++++++++++++++++ 2 files changed, 116 insertions(+) create mode 100644 src/merkle/sparse_path.rs diff --git a/src/merkle/mod.rs b/src/merkle/mod.rs index 39c58c5..ac305c3 100644 --- a/src/merkle/mod.rs +++ b/src/merkle/mod.rs @@ -20,6 +20,9 @@ pub use merkle_tree::{path_to_text, tree_to_text, MerkleTree}; mod path; pub use path::{MerklePath, RootPath, ValuePath}; +mod sparse_path; +pub use sparse_path::SparseMerklePath; + mod smt; #[cfg(feature = "internal")] pub use smt::{build_subtree_for_bench, SubtreeLeaf}; diff --git a/src/merkle/sparse_path.rs b/src/merkle/sparse_path.rs new file mode 100644 index 0000000..bc14217 --- /dev/null +++ b/src/merkle/sparse_path.rs @@ -0,0 +1,113 @@ +use alloc::vec::Vec; +use core::iter; + +use super::{EmptySubtreeRoots, MerklePath, RpoDigest, SMT_MAX_DEPTH}; + +/// A different representation of [`MerklePath`] designed for memory efficiency for Merkle paths +/// with empty nodes. +/// +/// Empty nodes in the path are stored only as their position, represented with a bitmask. A +/// maximum of 64 nodes in the path can be empty. The number of empty nodes has no effect on memory +/// usage by this struct, but will incur overhead during iteration or conversion to a +/// [`MerklePath`], for each empty node. +#[derive(Clone, Debug, Default, PartialEq, Eq)] +#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))] +pub struct SparseMerklePath { + /// A bitmask representing empty nodes. The set bit corresponds to the depth of an empty node. + empty_nodes: u64, + /// The non-empty nodes, stored in depth-order, but not contiguous across depth. + nodes: Vec, +} + +impl SparseMerklePath { + /// Converts a Merkle path to a sparse representation. + pub fn from_path(tree_depth: u8, path: MerklePath) -> Self { + // Note that the path does not include the node itself that it is a path to. + // That is to say, the path is not inclusive of its ending. + assert!( + path.len() <= SMT_MAX_DEPTH.into(), + "SparseMerklePath may have at most 256 items", + ); + let path_depth: u8 = path.len().try_into().unwrap(); + + let mut nodes: Vec = Default::default(); + let mut empty_nodes: u64 = 0; + + for (depth, node) in iter::zip(0..path_depth, path) { + let &equivalent_empty_node = EmptySubtreeRoots::entry(tree_depth, depth); + if node == equivalent_empty_node { + // FIXME: should we just fallback to the Vec if we're out of bits? + assert!(depth < 64, "SparseMerklePath may have at most 64 empty nodes"); + empty_nodes |= u64::checked_shl(1, depth.into()).unwrap(); + } else { + nodes.push(node); + } + } + + Self { empty_nodes, nodes } + } + + /// Converts this sparse representation back to a normal [`MerklePath`]. + pub fn into_path(mut self, tree_depth: u8) -> MerklePath { + let path_depth = self.depth(); + let mut nodes: Vec = Default::default(); + let mut sparse_nodes = self.nodes.drain(..); + + for depth in 0..path_depth { + let empty_bit = u64::checked_shl(1, depth.into()).unwrap(); + let is_empty = (self.empty_nodes & empty_bit) != 0; + if is_empty { + let &equivalent_empty_node = EmptySubtreeRoots::entry(tree_depth, depth); + nodes.push(equivalent_empty_node); + } else { + nodes.push(sparse_nodes.next().unwrap()); + } + } + + debug_assert_eq!(sparse_nodes.next(), None); + drop(sparse_nodes); + + debug_assert!(self.nodes.is_empty()); + + MerklePath::from(nodes) + } + + pub fn depth(&self) -> u8 { + (self.nodes.len() + self.empty_nodes.count_ones() as usize) as u8 + } +} + +#[cfg(test)] +mod tests { + use alloc::vec::Vec; + + use super::SparseMerklePath; + use crate::{ + hash::rpo::RpoDigest, + merkle::{smt::SparseMerkleTree, Smt, SMT_DEPTH}, + Felt, Word, ONE, + }; + + #[test] + fn roundtrip() { + let pair_count: u64 = 8192; + let entries: Vec<(RpoDigest, Word)> = (0..pair_count) + .map(|n| { + let leaf_index = ((n as f64 / pair_count as f64) * 255.0) as u64; + let key = RpoDigest::new([ONE, ONE, Felt::new(n), Felt::new(leaf_index)]); + let value = [ONE, ONE, ONE, ONE]; + (key, value) + }) + .collect(); + let tree = Smt::with_entries(entries).unwrap(); + + for (key, _value) in tree.entries() { + let control_path = tree.path(key); + let sparse_path = SparseMerklePath::from_path(SMT_DEPTH, control_path.clone()); + assert_eq!(control_path.depth(), sparse_path.depth()); + let test_path = sparse_path.into_path(SMT_DEPTH); + + assert_eq!(control_path, test_path); + } + } +} From 76c30a5c3733fb733641f9ccafa412c25301430d Mon Sep 17 00:00:00 2001 From: Qyriad Date: Fri, 21 Mar 2025 13:43:43 +0100 Subject: [PATCH 2/3] SparseMerklePath: implement random access --- src/merkle/sparse_path.rs | 67 +++++++++++++++++++++++++++++++++++++++ 1 file changed, 67 insertions(+) diff --git a/src/merkle/sparse_path.rs b/src/merkle/sparse_path.rs index bc14217..03f1571 100644 --- a/src/merkle/sparse_path.rs +++ b/src/merkle/sparse_path.rs @@ -75,6 +75,44 @@ impl SparseMerklePath { pub fn depth(&self) -> u8 { (self.nodes.len() + self.empty_nodes.count_ones() as usize) as u8 } + + /// Get a specific node in this path at a given depth. + /// + /// # Panics + /// With debug assertions enabled, this function panics if `node_depth` is greater than + /// `tree_depth` (as it is impossible to have a node of greater depth than the tree it is + /// contained in). + pub fn get(&self, tree_depth: u8, node_depth: u8) -> Option { + if node_depth == tree_depth || node_depth > self.depth() { + return None; + } + + debug_assert!( + tree_depth >= node_depth, + "tree depth {tree_depth} must be greater than node depth {node_depth}", + ); + + let empty_bit = u64::checked_shl(1, node_depth.into()).unwrap(); + let is_empty = (self.empty_nodes & empty_bit) != 0; + + if is_empty { + return Some(*EmptySubtreeRoots::entry(tree_depth, node_depth)); + } + + // Our index needs to account for all the empty nodes that aren't in `self.nodes`. + let nonempty_index: usize = { + // TODO: this could also be u64::unbounded_shl(1, node_depth + 1).wrapping_sub(1). + // We should check if that has any performance benefits over using 128-bit integers. + let mask: u64 = ((1u128 << (node_depth + 1)) - 1u128).try_into().unwrap(); + + let empty_before = u64::count_ones(self.empty_nodes & mask); + u64::checked_sub(node_depth as u64, empty_before as u64) + .unwrap() + .try_into() + .unwrap() + }; + Some(self.nodes[nonempty_index]) + } } #[cfg(test)] @@ -88,6 +126,19 @@ mod tests { Felt, Word, ONE, }; + fn make_smt(pair_count: u64) -> Smt { + let entries: Vec<(RpoDigest, Word)> = (0..pair_count) + .map(|n| { + let leaf_index = ((n as f64 / pair_count as f64) * 255.0) as u64; + let key = RpoDigest::new([ONE, ONE, Felt::new(n), Felt::new(leaf_index)]); + let value = [ONE, ONE, ONE, ONE]; + (key, value) + }) + .collect(); + + Smt::with_entries(entries).unwrap() + } + #[test] fn roundtrip() { let pair_count: u64 = 8192; @@ -110,4 +161,20 @@ mod tests { assert_eq!(control_path, test_path); } } + + #[test] + fn random_access() { + let tree = make_smt(8192); + + for (i, (key, _value)) in tree.entries().enumerate() { + let control_path = tree.path(key); + let sparse_path = SparseMerklePath::from_path(SMT_DEPTH, control_path.clone()); + assert_eq!(control_path.depth(), sparse_path.depth()); + + for (depth, control_node) in control_path.iter().enumerate() { + let test_node = sparse_path.get(SMT_DEPTH, depth as u8).unwrap(); + assert_eq!(*control_node, test_node, "at depth {depth} for entry {i}"); + } + } + } } From bff0a4f443cd3da3a8a5af0f52ee953d4df2eba7 Mon Sep 17 00:00:00 2001 From: Qyriad Date: Fri, 21 Mar 2025 13:45:19 +0100 Subject: [PATCH 3/3] SparseMerklePath: implement iterators --- src/merkle/sparse_path.rs | 106 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 106 insertions(+) diff --git a/src/merkle/sparse_path.rs b/src/merkle/sparse_path.rs index 03f1571..7890d89 100644 --- a/src/merkle/sparse_path.rs +++ b/src/merkle/sparse_path.rs @@ -115,6 +115,87 @@ impl SparseMerklePath { } } +// ITERATORS +// ================================================================================================ + +impl IntoIterator for SparseMerklePath { + type Item = ::Item; + type IntoIter = SparseMerkleIter; + + fn into_iter(self) -> SparseMerkleIter { + let tree_depth = self.depth(); + SparseMerkleIter { + path: self, + next_depth: Some(0), + tree_depth, + } + } +} + +/// Owning iterator for [`SparseMerklePath`]. +// TODO: add a non-owning iterator too. +pub struct SparseMerkleIter { + /// The "inner" value we're iterating over. + path: SparseMerklePath, + + /// The depth a `next()` call will get. It will only be None if someone calls `next_back()` at + /// depth 0, to indicate that all further `next_back()` calls must also return `None`. + next_depth: Option, + + /// "Cached" value of `path.depth()`. + tree_depth: u8, +} + +impl Iterator for SparseMerkleIter { + type Item = RpoDigest; + + fn next(&mut self) -> Option { + // If `next_depth` is None, then someone called `next_back()` at depth 0. + let next_depth = self.next_depth.unwrap_or(0); + if next_depth > self.tree_depth { + return None; + } + + match self.path.get(self.tree_depth, next_depth) { + Some(node) => { + self.next_depth = Some(next_depth + 1); + Some(node) + }, + None => None, + } + } + + /// SparseMerkleIter always knows its exact size. + fn size_hint(&self) -> (usize, Option) { + let next_depth = self.next_depth.unwrap_or(0); + let len: usize = self.path.depth().into(); + let remaining = len - next_depth as usize; + (remaining, Some(remaining)) + } +} + +impl ExactSizeIterator for SparseMerkleIter { + fn len(&self) -> usize { + let next_depth = self.next_depth.unwrap_or(0); + (self.path.depth() - next_depth) as usize + } +} + +impl DoubleEndedIterator for SparseMerkleIter { + fn next_back(&mut self) -> Option { + // While `next_depth` is None, all calls to `next_back()` also return `None`. + let next_depth = self.next_depth?; + + match self.path.get(self.tree_depth, next_depth) { + Some(node) => { + self.next_depth = if next_depth == 0 { None } else { Some(next_depth - 1) }; + Some(node) + }, + None => None, + } + } +} + #[cfg(test)] mod tests { use alloc::vec::Vec; @@ -177,4 +258,29 @@ mod tests { } } } + + #[test] + fn iterator() { + let tree = make_smt(8192); + + for (i, (key, _value)) in tree.entries().enumerate() { + let path = tree.path(key); + let sparse_path = SparseMerklePath::from_path(SMT_DEPTH, path.clone()); + assert_eq!(path.depth(), sparse_path.depth()); + assert_eq!(sparse_path.depth(), SMT_DEPTH); + for (depth, iter_node) in sparse_path.clone().into_iter().enumerate() { + let control_node = sparse_path.get(SMT_DEPTH, depth as u8).unwrap(); + assert_eq!(control_node, iter_node, "at depth {depth} for entry {i}"); + } + + let iter = sparse_path.clone().into_iter().enumerate().rev().skip(1); + for (depth, iter_node) in iter { + let control_node = sparse_path.get(SMT_DEPTH, depth as u8).unwrap(); + assert_eq!( + control_node, iter_node, + "at depth {depth} for entry {i} during reverse-iteration", + ); + } + } + } }