[storage/index] Add insert_and_prune benchmark

Exercises the full Cursor lifecycle (next, find, delete, insert, drop)
on every key, revealing the ~59% speedup from the Cursor simplification.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: roberto-bayardo

storage/src/index/benches/bench.rs

@@ -9,6 +9,7 @@ mod hashmap_insert;
 mod hashmap_insert_fixed;
 mod hashmap_iteration;
 mod insert;
+mod insert_and_prune;
 mod lookup;
 mod lookup_miss;
 
@@ -51,6 +52,7 @@ criterion_main!(
     hashmap_insert_fixed::benches,
     hashmap_insert::benches,
     insert::benches,
+    insert_and_prune::benches,
     lookup::benches,
     lookup_miss::benches,
 );

storage/src/index/benches/insert_and_prune.rs

@@ -0,0 +1,60 @@
+use super::DummyMetrics;
+use commonware_cryptography::{Hasher, Sha256};
+use commonware_storage::{
+    index::{unordered, Unordered},
+    translator::FourCap,
+};
+use criterion::{criterion_group, Criterion};
+use rand::{rngs::StdRng, seq::SliceRandom, SeedableRng};
+use std::time::{Duration, Instant};
+
+#[cfg(not(full_bench))]
+const N_ITEMS: [usize; 2] = [10_000, 50_000];
+#[cfg(full_bench)]
+const N_ITEMS: [usize; 4] = [10_000, 50_000, 100_000, 500_000];
+
+fn bench_insert_and_prune(c: &mut Criterion) {
+    for items in N_ITEMS {
+        let mut rng = StdRng::seed_from_u64(0);
+        let mut kvs = Vec::with_capacity(items);
+        for i in 0..items {
+            kvs.push((Sha256::hash(&i.to_be_bytes()), i as u64));
+        }
+        kvs.shuffle(&mut rng);
+
+        c.bench_function(&format!("{}/items={items}", module_path!()), |b| {
+            let kvs_data = kvs.clone();
+            b.iter_custom(move |iters| {
+                let mut total = Duration::ZERO;
+                for _ in 0..iters {
+                    let mut index = unordered::Index::new(DummyMetrics, FourCap);
+                    total += run_benchmark(&mut index, &kvs_data);
+                }
+                total
+            });
+        });
+    }
+}
+
+fn run_benchmark<I: Unordered<Value = u64>>(
+    index: &mut I,
+    kvs: &[(<Sha256 as Hasher>::Digest, u64)],
+) -> Duration {
+    // Seed the index with initial values.
+    for (k, v) in kvs {
+        index.insert(k, *v);
+    }
+
+    // Overwrite every key using insert_and_prune: prune the old value, insert the new one.
+    let start = Instant::now();
+    for (k, v) in kvs {
+        index.insert_and_prune(k, *v + 1, |old| *old == *v);
+    }
+    start.elapsed()
+}
+
+criterion_group! {
+    name = benches;
+    config = Criterion::default().sample_size(10);
+    targets = bench_insert_and_prune
+}