bench(spann): add a Criterion harness for the distance kernels

crates/ruvector-spann/Cargo.toml

@@ -13,3 +13,9 @@ path = "src/bin/benchmark.rs"
 [dependencies]
 
 [dev-dependencies]
+criterion = { workspace = true }
+rand = { workspace = true }
+
+[[bench]]
+name = "distance"
+harness = false

crates/ruvector-spann/benches/distance.rs

@@ -0,0 +1,42 @@
+use criterion::{black_box, criterion_group, criterion_main, BenchmarkId, Criterion};
+use rand::rngs::StdRng;
+use rand::{Rng, SeedableRng};
+use ruvector_spann::distance::{cosine_distance, l2_squared};
+
+const DIMENSIONS: [usize; 4] = [128, 384, 768, 1536];
+const SEED: u64 = 0x5eed_5eed_5eed_5eed;
+
+fn random_vector(rng: &mut StdRng, dim: usize) -> Vec<f32> {
+    (0..dim).map(|_| rng.gen_range(-1.0f32..1.0)).collect()
+}
+
+fn bench_l2_squared(c: &mut Criterion) {
+    let mut group = c.benchmark_group("l2_squared");
+    for &dim in DIMENSIONS.iter() {
+        let mut rng = StdRng::seed_from_u64(SEED);
+        let a = random_vector(&mut rng, dim);
+        let b = random_vector(&mut rng, dim);
+
+        group.bench_with_input(BenchmarkId::from_parameter(dim), &dim, |bench, _| {
+            bench.iter(|| black_box(l2_squared(black_box(&a), black_box(&b))));
+        });
+    }
+    group.finish();
+}
+
+fn bench_cosine_distance(c: &mut Criterion) {
+    let mut group = c.benchmark_group("cosine_distance");
+    for &dim in DIMENSIONS.iter() {
+        let mut rng = StdRng::seed_from_u64(SEED);
+        let a = random_vector(&mut rng, dim);
+        let b = random_vector(&mut rng, dim);
+
+        group.bench_with_input(BenchmarkId::from_parameter(dim), &dim, |bench, _| {
+            bench.iter(|| black_box(cosine_distance(black_box(&a), black_box(&b))));
+        });
+    }
+    group.finish();
+}
+
+criterion_group!(benches, bench_l2_squared, bench_cosine_distance);
+criterion_main!(benches);