miden-crypto/src/merkle/smt/mod.rs
src/merkle/smt/mod.rsRaw
1use alloc::{collections::BTreeMap, vec::Vec};
2
3use super::{EmptySubtreeRoots, InnerNodeInfo, MerkleError, MerklePath, NodeIndex};
4use crate::{
5 hash::rpo::{Rpo256, RpoDigest},
6 Felt, Word, EMPTY_WORD,
7};
8
9mod full;
10pub use full::{Smt, SmtLeaf, SmtLeafError, SmtProof, SmtProofError, SMT_DEPTH};
11
12mod simple;
13pub use simple::SimpleSmt;
14
15// CONSTANTS
16// ================================================================================================
17
18/// Minimum supported depth.
19pub const SMT_MIN_DEPTH: u8 = 1;
20
21/// Maximum supported depth.
22pub const SMT_MAX_DEPTH: u8 = 64;
23
24// SPARSE MERKLE TREE
25// ================================================================================================
26
27/// An abstract description of a sparse Merkle tree.
28///
29/// A sparse Merkle tree is a key-value map which also supports proving that a given value is indeed
30/// stored at a given key in the tree. It is viewed as always being fully populated. If a leaf's
31/// value was not explicitly set, then its value is the default value. Typically, the vast majority
32/// of leaves will store the default value (hence it is "sparse"), and therefore the internal
33/// representation of the tree will only keep track of the leaves that have a different value from
34/// the default.
35///
36/// All leaves sit at the same depth. The deeper the tree, the more leaves it has; but also the
37/// longer its proofs are - of exactly `log(depth)` size. A tree cannot have depth 0, since such a
38/// tree is just a single value, and is probably a programming mistake.
39///
40/// Every key maps to one leaf. If there are as many keys as there are leaves, then
41/// [Self::Leaf] should be the same type as [Self::Value], as is the case with
42/// [crate::merkle::SimpleSmt]. However, if there are more keys than leaves, then [`Self::Leaf`]
43/// must accomodate all keys that map to the same leaf.
44///
45/// [SparseMerkleTree] currently doesn't support optimizations that compress Merkle proofs.
46pub(crate) trait SparseMerkleTree<const DEPTH: u8> {
47 /// The type for a key
48 type Key: Clone + Ord;
49 /// The type for a value
50 type Value: Clone + PartialEq;
51 /// The type for a leaf
52 type Leaf: Clone;
53 /// The type for an opening (i.e. a "proof") of a leaf
54 type Opening;
55
56 /// The default value used to compute the hash of empty leaves
57 const EMPTY_VALUE: Self::Value;
58
59 // PROVIDED METHODS
60 // ---------------------------------------------------------------------------------------------
61
62 /// Returns an opening of the leaf associated with `key`. Conceptually, an opening is a Merkle
63 /// path to the leaf, as well as the leaf itself.
64 fn open(&self, key: &Self::Key) -> Self::Opening {
65 let leaf = self.get_leaf(key);
66
67 let mut index: NodeIndex = {
68 let leaf_index: LeafIndex<DEPTH> = Self::key_to_leaf_index(key);
69 leaf_index.into()
70 };
71
72 let merkle_path = {
73 let mut path = Vec::with_capacity(index.depth() as usize);
74 for _ in 0..index.depth() {
75 let is_right = index.is_value_odd();
76 index.move_up();
77 let InnerNode { left, right } = self.get_inner_node(index);
78 let value = if is_right { left } else { right };
79 path.push(value);
80 }
81
82 MerklePath::new(path)
83 };
84
85 Self::path_and_leaf_to_opening(merkle_path, leaf)
86 }
87
88 /// Inserts a value at the specified key, returning the previous value associated with that key.
89 /// Recall that by definition, any key that hasn't been updated is associated with
90 /// [`Self::EMPTY_VALUE`].
91 ///
92 /// This also recomputes all hashes between the leaf (associated with the key) and the root,
93 /// updating the root itself.
94 fn insert(&mut self, key: Self::Key, value: Self::Value) -> Self::Value {
95 let old_value = self.insert_value(key.clone(), value.clone()).unwrap_or(Self::EMPTY_VALUE);
96
97 // if the old value and new value are the same, there is nothing to update
98 if value == old_value {
99 return value;
100 }
101
102 let leaf = self.get_leaf(&key);
103 let node_index = {
104 let leaf_index: LeafIndex<DEPTH> = Self::key_to_leaf_index(&key);
105 leaf_index.into()
106 };
107
108 self.recompute_nodes_from_index_to_root(node_index, Self::hash_leaf(&leaf));
109
110 old_value
111 }
112
113 /// Recomputes the branch nodes (including the root) from `index` all the way to the root.
114 /// `node_hash_at_index` is the hash of the node stored at index.
115 fn recompute_nodes_from_index_to_root(
116 &mut self,
117 mut index: NodeIndex,
118 node_hash_at_index: RpoDigest,
119 ) {
120 let mut node_hash = node_hash_at_index;
121 for node_depth in (0..index.depth()).rev() {
122 let is_right = index.is_value_odd();
123 index.move_up();
124 let InnerNode { left, right } = self.get_inner_node(index);
125 let (left, right) = if is_right {
126 (left, node_hash)
127 } else {
128 (node_hash, right)
129 };
130 node_hash = Rpo256::merge(&[left, right]);
131
132 if node_hash == *EmptySubtreeRoots::entry(DEPTH, node_depth) {
133 // If a subtree is empty, when can remove the inner node, since it's equal to the
134 // default value
135 self.remove_inner_node(index)
136 } else {
137 self.insert_inner_node(index, InnerNode { left, right });
138 }
139 }
140 self.set_root(node_hash);
141 }
142
143 /// Computes what changes are necessary to insert the specified key-value pairs into this Merkle
144 /// tree, allowing for validation before applying those changes.
145 ///
146 /// This method returns a [`MutationSet`], which contains all the information for inserting
147 /// `kv_pairs` into this Merkle tree already calculated, including the new root hash, which can
148 /// be queried with [`MutationSet::root()`]. Once a mutation set is returned,
149 /// [`SparseMerkleTree::apply_mutations()`] can be called in order to commit these changes to
150 /// the Merkle tree, or [`drop()`] to discard them.
151 fn compute_mutations(
152 &self,
153 kv_pairs: impl IntoIterator<Item = (Self::Key, Self::Value)>,
154 ) -> MutationSet<DEPTH, Self::Key, Self::Value> {
155 use NodeMutation::*;
156
157 let mut new_root = self.root();
158 let mut new_pairs: BTreeMap<Self::Key, Self::Value> = Default::default();
159 let mut node_mutations: BTreeMap<NodeIndex, NodeMutation> = Default::default();
160
161 for (key, value) in kv_pairs {
162 // If the old value and the new value are the same, there is nothing to update.
163 // For the unusual case that kv_pairs has multiple values at the same key, we'll have
164 // to check the key-value pairs we've already seen to get the "effective" old value.
165 let old_value = new_pairs.get(&key).cloned().unwrap_or_else(|| self.get_value(&key));
166 if value == old_value {
167 continue;
168 }
169
170 let leaf_index = Self::key_to_leaf_index(&key);
171 let mut node_index = NodeIndex::from(leaf_index);
172
173 // We need the current leaf's hash to calculate the new leaf, but in the rare case that
174 // `kv_pairs` has multiple pairs that go into the same leaf, then those pairs are also
175 // part of the "current leaf".
176 let old_leaf = {
177 let pairs_at_index = new_pairs
178 .iter()
179 .filter(|&(new_key, _)| Self::key_to_leaf_index(new_key) == leaf_index);
180
181 pairs_at_index.fold(self.get_leaf(&key), |acc, (k, v)| {
182 // Most of the time `pairs_at_index` should only contain a single entry (or
183 // none at all), as multi-leaves should be really rare.
184 let existing_leaf = acc.clone();
185 self.construct_prospective_leaf(existing_leaf, k, v)
186 })
187 };
188
189 let new_leaf = self.construct_prospective_leaf(old_leaf, &key, &value);
190
191 let mut new_child_hash = Self::hash_leaf(&new_leaf);
192
193 for node_depth in (0..node_index.depth()).rev() {
194 // Whether the node we're replacing is the right child or the left child.
195 let is_right = node_index.is_value_odd();
196 node_index.move_up();
197
198 let old_node = node_mutations
199 .get(&node_index)
200 .map(|mutation| match mutation {
201 Addition(node) => node.clone(),
202 Removal => EmptySubtreeRoots::get_inner_node(DEPTH, node_depth),
203 })
204 .unwrap_or_else(|| self.get_inner_node(node_index));
205
206 let new_node = if is_right {
207 InnerNode {
208 left: old_node.left,
209 right: new_child_hash,
210 }
211 } else {
212 InnerNode {
213 left: new_child_hash,
214 right: old_node.right,
215 }
216 };
217
218 // The next iteration will operate on this new node's hash.
219 new_child_hash = new_node.hash();
220
221 let &equivalent_empty_hash = EmptySubtreeRoots::entry(DEPTH, node_depth);
222 let is_removal = new_child_hash == equivalent_empty_hash;
223 let new_entry = if is_removal { Removal } else { Addition(new_node) };
224 node_mutations.insert(node_index, new_entry);
225 }
226
227 // Once we're at depth 0, the last node we made is the new root.
228 new_root = new_child_hash;
229 // And then we're done with this pair; on to the next one.
230 new_pairs.insert(key, value);
231 }
232
233 MutationSet {
234 old_root: self.root(),
235 new_root,
236 node_mutations,
237 new_pairs,
238 }
239 }
240
241 /// Apply the prospective mutations computed with [`SparseMerkleTree::compute_mutations()`] to
242 /// this tree.
243 ///
244 /// # Errors
245 /// If `mutations` was computed on a tree with a different root than this one, returns
246 /// [`MerkleError::ConflictingRoots`] with a two-item [`Vec`]. The first item is the root hash
247 /// the `mutations` were computed against, and the second item is the actual current root of
248 /// this tree.
249 fn apply_mutations(
250 &mut self,
251 mutations: MutationSet<DEPTH, Self::Key, Self::Value>,
252 ) -> Result<(), MerkleError>
253 where
254 Self: Sized,
255 {
256 use NodeMutation::*;
257 let MutationSet {
258 old_root,
259 node_mutations,
260 new_pairs,
261 new_root,
262 } = mutations;
263
264 // Guard against accidentally trying to apply mutations that were computed against a
265 // different tree, including a stale version of this tree.
266 if old_root != self.root() {
267 return Err(MerkleError::ConflictingRoots(vec![old_root, self.root()]));
268 }
269
270 for (index, mutation) in node_mutations {
271 match mutation {
272 Removal => self.remove_inner_node(index),
273 Addition(node) => self.insert_inner_node(index, node),
274 }
275 }
276
277 for (key, value) in new_pairs {
278 self.insert_value(key, value);
279 }
280
281 self.set_root(new_root);
282
283 Ok(())
284 }
285
286 // REQUIRED METHODS
287 // ---------------------------------------------------------------------------------------------
288
289 /// The root of the tree
290 fn root(&self) -> RpoDigest;
291
292 /// Sets the root of the tree
293 fn set_root(&mut self, root: RpoDigest);
294
295 /// Retrieves an inner node at the given index
296 fn get_inner_node(&self, index: NodeIndex) -> InnerNode;
297
298 /// Inserts an inner node at the given index
299 fn insert_inner_node(&mut self, index: NodeIndex, inner_node: InnerNode);
300
301 /// Removes an inner node at the given index
302 fn remove_inner_node(&mut self, index: NodeIndex);
303
304 /// Inserts a leaf node, and returns the value at the key if already exists
305 fn insert_value(&mut self, key: Self::Key, value: Self::Value) -> Option<Self::Value>;
306
307 /// Returns the value at the specified key. Recall that by definition, any key that hasn't been
308 /// updated is associated with [`Self::EMPTY_VALUE`].
309 fn get_value(&self, key: &Self::Key) -> Self::Value;
310
311 /// Returns the leaf at the specified index.
312 fn get_leaf(&self, key: &Self::Key) -> Self::Leaf;
313
314 /// Returns the hash of a leaf
315 fn hash_leaf(leaf: &Self::Leaf) -> RpoDigest;
316
317 /// Returns what a leaf would look like if a key-value pair were inserted into the tree, without
318 /// mutating the tree itself. The existing leaf can be empty.
319 ///
320 /// To get a prospective leaf based on the current state of the tree, use `self.get_leaf(key)`
321 /// as the argument for `existing_leaf`. The return value from this function can be chained back
322 /// into this function as the first argument to continue making prospective changes.
323 ///
324 /// # Invariants
325 /// Because this method is for a prospective key-value insertion into a specific leaf,
326 /// `existing_leaf` must have the same leaf index as `key` (as determined by
327 /// [`SparseMerkleTree::key_to_leaf_index()`]), or the result will be meaningless.
328 fn construct_prospective_leaf(
329 &self,
330 existing_leaf: Self::Leaf,
331 key: &Self::Key,
332 value: &Self::Value,
333 ) -> Self::Leaf;
334
335 /// Maps a key to a leaf index
336 fn key_to_leaf_index(key: &Self::Key) -> LeafIndex<DEPTH>;
337
338 /// Maps a (MerklePath, Self::Leaf) to an opening.
339 ///
340 /// The length `path` is guaranteed to be equal to `DEPTH`
341 fn path_and_leaf_to_opening(path: MerklePath, leaf: Self::Leaf) -> Self::Opening;
342}
343
344// INNER NODE
345// ================================================================================================
346
347#[derive(Debug, Default, Clone, PartialEq, Eq)]
348#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
349pub(crate) struct InnerNode {
350 pub left: RpoDigest,
351 pub right: RpoDigest,
352}
353
354impl InnerNode {
355 pub fn hash(&self) -> RpoDigest {
356 Rpo256::merge(&[self.left, self.right])
357 }
358}
359
360// LEAF INDEX
361// ================================================================================================
362
363/// The index of a leaf, at a depth known at compile-time.
364#[derive(Debug, Default, Copy, Clone, Eq, PartialEq, PartialOrd, Ord, Hash)]
365#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
366pub struct LeafIndex<const DEPTH: u8> {
367 index: NodeIndex,
368}
369
370impl<const DEPTH: u8> LeafIndex<DEPTH> {
371 pub fn new(value: u64) -> Result<Self, MerkleError> {
372 if DEPTH < SMT_MIN_DEPTH {
373 return Err(MerkleError::DepthTooSmall(DEPTH));
374 }
375
376 Ok(LeafIndex { index: NodeIndex::new(DEPTH, value)? })
377 }
378
379 pub fn value(&self) -> u64 {
380 self.index.value()
381 }
382}
383
384impl LeafIndex<SMT_MAX_DEPTH> {
385 pub const fn new_max_depth(value: u64) -> Self {
386 LeafIndex {
387 index: NodeIndex::new_unchecked(SMT_MAX_DEPTH, value),
388 }
389 }
390}
391
392impl<const DEPTH: u8> From<LeafIndex<DEPTH>> for NodeIndex {
393 fn from(value: LeafIndex<DEPTH>) -> Self {
394 value.index
395 }
396}
397
398impl<const DEPTH: u8> TryFrom<NodeIndex> for LeafIndex<DEPTH> {
399 type Error = MerkleError;
400
401 fn try_from(node_index: NodeIndex) -> Result<Self, Self::Error> {
402 if node_index.depth() != DEPTH {
403 return Err(MerkleError::InvalidDepth {
404 expected: DEPTH,
405 provided: node_index.depth(),
406 });
407 }
408
409 Self::new(node_index.value())
410 }
411}
412
413// MUTATIONS
414// ================================================================================================
415
416/// A change to an inner node of a [`SparseMerkleTree`] that hasn't yet been applied.
417/// [`MutationSet`] stores this type in relation to a [`NodeIndex`] to keep track of what changes
418/// need to occur at which node indices.
419#[derive(Debug, Clone, PartialEq, Eq)]
420pub(crate) enum NodeMutation {
421 /// Corresponds to [`SparseMerkleTree::remove_inner_node()`].
422 Removal,
423 /// Corresponds to [`SparseMerkleTree::insert_inner_node()`].
424 Addition(InnerNode),
425}
426
427/// Represents a group of prospective mutations to a `SparseMerkleTree`, created by
428/// `SparseMerkleTree::compute_mutations()`, and that can be applied with
429/// `SparseMerkleTree::apply_mutations()`.
430#[derive(Debug, Clone, PartialEq, Eq, Default)]
431pub struct MutationSet<const DEPTH: u8, K, V> {
432 /// The root of the Merkle tree this MutationSet is for, recorded at the time
433 /// [`SparseMerkleTree::compute_mutations()`] was called. Exists to guard against applying
434 /// mutations to the wrong tree or applying stale mutations to a tree that has since changed.
435 old_root: RpoDigest,
436 /// The set of nodes that need to be removed or added. The "effective" node at an index is the
437 /// Merkle tree's existing node at that index, with the [`NodeMutation`] in this map at that
438 /// index overlayed, if any. Each [`NodeMutation::Addition`] corresponds to a
439 /// [`SparseMerkleTree::insert_inner_node()`] call, and each [`NodeMutation::Removal`]
440 /// corresponds to a [`SparseMerkleTree::remove_inner_node()`] call.
441 node_mutations: BTreeMap<NodeIndex, NodeMutation>,
442 /// The set of top-level key-value pairs we're prospectively adding to the tree, including
443 /// adding empty values. The "effective" value for a key is the value in this BTreeMap, falling
444 /// back to the existing value in the Merkle tree. Each entry corresponds to a
445 /// [`SparseMerkleTree::insert_value()`] call.
446 new_pairs: BTreeMap<K, V>,
447 /// The calculated root for the Merkle tree, given these mutations. Publicly retrievable with
448 /// [`MutationSet::root()`]. Corresponds to a [`SparseMerkleTree::set_root()`]. call.
449 new_root: RpoDigest,
450}
451
452impl<const DEPTH: u8, K, V> MutationSet<DEPTH, K, V> {
453 /// Queries the root that was calculated during `SparseMerkleTree::compute_mutations()`. See
454 /// that method for more information.
455 pub fn root(&self) -> RpoDigest {
456 self.new_root
457 }
458}