bench(diskann): add hot search benchmark

Fixes #677

Co-Authored-By: Leo <noreply@khive.ai>

crates/ruvector-diskann/examples/bench_search_hot.rs

@@ -0,0 +1,83 @@
+use rand::rngs::StdRng;
+use rand::{Rng, SeedableRng};
+use ruvector_diskann::{DiskAnnConfig, DiskAnnIndex};
+use std::hint::black_box;
+use std::time::Instant;
+
+const DIM: usize = 128;
+const QUERY_COUNT: usize = 2_000;
+const WARMUP_COUNT: usize = 100;
+const K: usize = 10;
+
+fn random_unit_vector(rng: &mut StdRng) -> Vec<f32> {
+    let mut vector: Vec<f32> = (0..DIM).map(|_| rng.gen_range(-1.0..1.0)).collect();
+    let norm = vector.iter().map(|value| value * value).sum::<f32>().sqrt();
+    for value in &mut vector {
+        *value /= norm;
+    }
+    vector
+}
+
+fn parse_n() -> usize {
+    let mut args = std::env::args().skip(1);
+    match args.next() {
+        Some(flag) if flag == "--n" => args
+            .next()
+            .expect("--n requires a value")
+            .parse()
+            .expect("n must be a positive integer"),
+        Some(value) => value.parse().expect("n must be a positive integer"),
+        None => 100_000,
+    }
+}
+
+fn main() {
+    let n = parse_n();
+    assert!(n > 1, "n must be greater than one");
+
+    let mut data_rng = StdRng::seed_from_u64(0x677D_15CA_77);
+    let mut index = DiskAnnIndex::new(DiskAnnConfig {
+        dim: DIM,
+        max_degree: 4,
+        build_beam: 8,
+        search_beam: 64,
+        alpha: 1.0,
+        ..Default::default()
+    });
+
+    let build_started = Instant::now();
+    for id in 0..n {
+        index
+            .insert(id.to_string(), random_unit_vector(&mut data_rng))
+            .expect("benchmark insert failed");
+    }
+    index.build().expect("benchmark build failed");
+    let build_seconds = build_started.elapsed().as_secs_f64();
+
+    let mut query_rng = StdRng::seed_from_u64(0x6770_0A11_CE);
+    let queries: Vec<Vec<f32>> = (0..QUERY_COUNT)
+        .map(|_| random_unit_vector(&mut query_rng))
+        .collect();
+
+    for query in queries.iter().take(WARMUP_COUNT) {
+        black_box(index.search(black_box(query), K).expect("warmup failed"));
+    }
+
+    let mut latencies_ns = Vec::with_capacity(QUERY_COUNT);
+    let run_started = Instant::now();
+    for query in &queries {
+        let query_started = Instant::now();
+        black_box(index.search(black_box(query), K).expect("search failed"));
+        latencies_ns.push(query_started.elapsed().as_nanos() as u64);
+    }
+    let elapsed = run_started.elapsed();
+
+    latencies_ns.sort_unstable();
+    let p50_us = latencies_ns[QUERY_COUNT / 2] as f64 / 1_000.0;
+    let p99_us = latencies_ns[(QUERY_COUNT * 99 / 100).min(QUERY_COUNT - 1)] as f64 / 1_000.0;
+    let qps = QUERY_COUNT as f64 / elapsed.as_secs_f64();
+
+    println!(
+        "RESULT n={n} d={DIM} k={K} queries={QUERY_COUNT} build_s={build_seconds:.3} p50_us={p50_us:.3} p99_us={p99_us:.3} qps={qps:.3}"
+    );
+}