Add benchmark of snapshot merkle root calculation

benches/against_snapshot.rs

@@ -0,0 +1,67 @@
+use std::rc::Rc;
+
+use criterion::{black_box, criterion_group, criterion_main, Criterion};
+use kairos_trie::{
+    stored::{
+        memory_db::MemoryDb,
+        merkle::{Snapshot, SnapshotBuilder},
+    },
+    DigestHasher, KeyHash, PortableHash, PortableHasher, Transaction, TrieRoot,
+};
+use sha2::Sha256;
+
+fn txn_over_fraction_of_trie(size: u64, denominator: u64) -> Snapshot<u64> {
+    let db = Rc::new(MemoryDb::<u64>::empty());
+
+    let mut txn =
+        Transaction::from_snapshot_builder(SnapshotBuilder::new(db.clone(), TrieRoot::Empty));
+
+    let hasher = &mut DigestHasher::<Sha256>::default();
+
+    for k in 0..size {
+        k.portable_hash(hasher);
+        let key_hash = KeyHash::from_bytes(&hasher.finalize_reset());
+
+        txn.insert(&key_hash, k).unwrap();
+    }
+
+    let root = txn.commit(hasher).unwrap();
+
+    let mut txn = Transaction::from_snapshot_builder(SnapshotBuilder::new(db, root));
+
+    for k in 0..size / denominator {
+        k.portable_hash(hasher);
+        let key_hash = KeyHash::from_bytes(&hasher.finalize_reset());
+
+        *txn.entry(&key_hash).unwrap().get_mut().unwrap() += size;
+    }
+
+    txn.build_initial_snapshot()
+}
+
+fn against_snapshot(c: &mut Criterion) {
+    // A snapshot for a txn that increments 10% of the values in the trie
+    let snapshot = txn_over_fraction_of_trie(100_000, 10);
+
+    let hasher = &mut DigestHasher::<Sha256>::default();
+
+    let root = snapshot
+        .calc_root_hash(&mut DigestHasher::<Sha256>::default())
+        .unwrap();
+
+    c.bench_function("verify snapshot", |b| {
+        b.iter(|| {
+            assert_eq!(
+                root,
+                black_box(
+                    black_box(&snapshot)
+                        .calc_root_hash(black_box(hasher))
+                        .unwrap()
+                )
+            )
+        })
+    });
+}
+
+criterion_group!(benches, against_snapshot);
+criterion_main!(benches);