WIP: remove a *bunch* of allocations and clones in hash_prospective_leaf

src/merkle/smt/full/mod.rs

@@ -3,6 +3,7 @@ use alloc::{
     string::ToString,
     vec::Vec,
 };
+use core::iter;
 
 use super::{
     EmptySubtreeRoots, Felt, InnerNode, InnerNodeInfo, LeafIndex, MerkleError, MerklePath,
@@ -286,25 +287,88 @@ impl SparseMerkleTree<SMT_DEPTH> for Smt {
     }
 
     fn hash_prospective_leaf(&self, key: &RpoDigest, value: &Word) -> RpoDigest {
-        // If this key already has a value, then the hash will be based off a
-        // prospective mutation on the leaf.
-        let leaf_index: LeafIndex<SMT_DEPTH> = Self::key_to_leaf_index(&key);
+        // This function combines logic from SmtLeaf::insert() and SmtLeaf::hash() to determine what
+        // the hash of a leaf would be with the `(key, value)` pair inserted into it, without simply
+        // cloning the leaf which could be expensive for some leaves, and is easily avoidable when
+        // we can combine the insertion and hashing operations.
+        let new_pair = (*key, *value);
+        let is_removal: bool = *value == EMPTY_WORD;
+
+        let leaf_index: LeafIndex<SMT_DEPTH> = Self::key_to_leaf_index(key);
         match self.leaves.get(&leaf_index.value()) {
-            Some(existing_leaf) => {
-                if value == &Self::EMPTY_VALUE {
-                    // A leaf with an empty value is conceptually a removal the
-                    // value in that leaf with this key.
-                    // TODO: avoid cloning the leaf.
-                    let mut cloned = existing_leaf.clone();
-                    cloned.remove(*key);
-                    return cloned.hash();
-                }
-                // TODO: avoid cloning the leaf.
-                let mut cloned = existing_leaf.clone();
-                cloned.insert(*key, *value);
-                cloned.hash()
+            // If this key doesn't have a value, our job is very simple.
+            None => SmtLeaf::Single(new_pair).hash(),
+
+            // If this key already has a value, then the hash will be based off a prospective
+            // mutation on the leaf.
+            Some(existing_leaf) => match existing_leaf {
+                // Inserting an empty value into an empty leaf or a single leaf both do the same
+                // thing.
+                SmtLeaf::Empty(_) | SmtLeaf::Single(_) if is_removal => {
+                    SmtLeaf::new_empty(key.into()).hash()
+                },
+
+                SmtLeaf::Empty(_) => SmtLeaf::Single(new_pair).hash(),
+
+                SmtLeaf::Single(pair) => {
+                    if pair.0 == *key {
+                        SmtLeaf::Single(new_pair).hash()
+                    } else {
+                        // Inserting a non-empty value into a new key would change this to a
+                        // multi-leaf.
+                        // TODO: mini-optimization: use an array with each key's and value's Felts
+                        // flattened inline to avoid the Vec allocation.
+                        let elements: Vec<Felt> =
+                            [*pair, new_pair].into_iter().flat_map(leaf::kv_to_elements).collect();
+
+                        Rpo256::hash_elements(&elements)
+                    }
+                },
+
+                SmtLeaf::Multiple(pairs) => {
+                    match pairs.binary_search_by(|&(cur_key, _)| leaf::cmp_keys(cur_key, *key)) {
+                        Ok(pos) => {
+                            if is_removal && pairs.len() == 2 {
+                                // This removal would convert this Multi into a Single, so we can
+                                // just stop here.
+                                return SmtLeaf::Single(pairs[0]).hash();
+                            }
+
+                            let (before_pos, rest) = pairs.split_at(pos);
+                            let with_pos_removed = rest.iter().copied().skip(1);
+                            let middle = iter::once(new_pair).filter(|_| !is_removal);
+                            let elements: Vec<Felt> = before_pos
+                                .iter()
+                                .copied()
+                                .chain(middle)
+                                .chain(with_pos_removed)
+                                .flat_map(leaf::kv_to_elements)
+                                .collect();
+
+                            Rpo256::hash_elements(&elements)
+                        },
+                        Err(pos_for_insert) => {
+                            if is_removal {
+                                // The only values are at other keys, so we just hash the leaf
+                                // as-is.
+                                return existing_leaf.hash();
+                            }
+
+                            let (before_pos, rest) = pairs.split_at(pos_for_insert);
+                            let middle = iter::once(new_pair);
+                            let elements: Vec<Felt> = before_pos
+                                .iter()
+                                .copied()
+                                .chain(middle)
+                                .chain(rest.iter().copied())
+                                .flat_map(leaf::kv_to_elements)
+                                .collect();
+
+                            Rpo256::hash_elements(&elements)
+                        },
+                    }
+                },
             },
-            None => SmtLeaf::new_single(*key, *value).hash(),
         }
     }