tests for hashes

tests/core/t_hash.ml

@@ -16,4 +16,63 @@ t @@ fun () -> string "abc" <> string "abcd";;
 
 q Q.int (fun i ->
     Q.assume (i >= 0);
-    int i = int64 (Int64.of_int i))
+    int i = int64 (Int64.of_int i));;
+
+(* --- stress tests -------------------------------------------------------- *)
+
+(* Chi-squared distribution test over [count] consecutive integers in [buckets] buckets.
+   A uniform hash gives chi2 ~ buckets-1; we allow 4 standard deviations of slack. *)
+t ~name:"int hash distribution chi2" @@ fun () ->
+  let count = 50_000 and buckets = 500 in
+  let counts = Array.make buckets 0 in
+  for i = 0 to count - 1 do
+    let b = CCHash.int i mod buckets in
+    counts.(b) <- counts.(b) + 1
+  done;
+  let expected = float count /. float buckets in
+  let c2 =
+    Array.fold_left
+      (fun acc c -> acc +. ((float c -. expected) ** 2.0 /. expected))
+      0.0 counts
+  in
+  let df = float (buckets - 1) in
+  c2 < df +. 4.0 *. sqrt (2.0 *. df);;
+
+(* Strict avalanche criterion: flip one input bit, expect ~50% output bits to change. *)
+t ~name:"int hash avalanche" @@ fun () ->
+  let bits = Sys.int_size - 1 in
+  let total_flips = ref 0 in
+  let total = ref 0 in
+  let rng = Random.State.make [| 42; 17; 99 |] in
+  for _ = 1 to 300 do
+    let x = Random.State.bits rng in
+    let hx = CCHash.int x in
+    for b = 0 to bits - 1 do
+      let hx' = CCHash.int (x lxor (1 lsl b)) in
+      total_flips := !total_flips + CCInt.popcount (hx lxor hx');
+      total := !total + bits
+    done
+  done;
+  let frac = float !total_flips /. float !total in
+  frac >= 0.45 && frac <= 0.55;;
+
+(* String hash: no collisions among distinct keys. *)
+t ~name:"string hash no collisions" @@ fun () ->
+  let n = 50_000 in
+  let tbl = Hashtbl.create n in
+  let ok = ref true in
+  for i = 0 to n - 1 do
+    let h = CCHash.string (Printf.sprintf "key:%d" i) in
+    if Hashtbl.mem tbl h then ok := false;
+    Hashtbl.replace tbl h ()
+  done;
+  !ok;;
+
+(* CCHash64 pipeline matches CCHash.pair combiner. *)
+q Q.int (fun i ->
+    let j = i lxor 0xdeadbeef in
+    let h_pair = CCHash.pair CCHash.int CCHash.int (i, j) in
+    let h_manual =
+      CCHash64.(finalize (int (int seed (CCHash.int i)) (CCHash.int j)))
+    in
+    h_pair = h_manual)