ocamlformat

Replace the pure-OCaml FNV-1 implementation in CCInt64 with C stubs
that call the same cc_fnv_hash_int64 core as CCInt.hash:

- hash: int64 -> int via caml_cc_hash_int64 ([@unboxed] + [@@noalloc])
- hash_to_int64: int64 -> int64 via new caml_cc_hash_int64_to_int64
  entry points (masks result to non-negative, needs caml_copy_int64
  for bytecode so cannot be [@@noalloc])

Add 18 tests in t_int64.ml covering:
- Non-negativity of hash and hash_to_int64
- Consistency between hash and hash_to_int64
- Different inputs produce different hashes
- Determinism
- QuickCheck: hash is non-negative for random inputs

benchs/run_benchs.ml

@@ -1830,7 +1830,7 @@ module Hash = struct
     let prime = 0x100000001b3L in
     let h = ref offset_basis in
     for k = 0 to 7 do
-      h := Int64.(mul !h prime);
+      (h := Int64.(mul !h prime));
       h := Int64.(logxor !h (of_int ((n lsr (k * 8)) land 0xff)))
     done;
     Int64.to_int !h land max_int
@@ -1846,10 +1846,7 @@ module Hash = struct
       done
     in
     B.throughputN 3 ~repeat
-      [
-        "ocaml_fnv", run_ocaml, ();
-        "c_stub", run_c_stub, ();
-      ]
+      [ "ocaml_fnv", run_ocaml, (); "c_stub", run_c_stub, () ]
 
   let () =
     B.Tree.register ("hash" @>>> [ "int" @>> app_ints bench_hash [ 1_000 ] ])

src/core/CCHash.ml

@@ -7,7 +7,7 @@ type 'a t = 'a -> hash
 type 'a iter = ('a -> unit) -> unit
 type 'a gen = unit -> 'a option
 
-(* FNV hashing
+(* FNV-1 hashing (multiply then XOR)
    https://en.wikipedia.org/wiki/Fowler%E2%80%93Noll%E2%80%93Vo_hash_function
 *)
 let fnv_offset_basis = 0xcbf29ce484222325L

src/core/CCInt.ml

@@ -4,10 +4,11 @@ include Int
 
 type 'a iter = ('a -> unit) -> unit
 
-(* use FNV:
+(* use FNV-1:
    https://en.wikipedia.org/wiki/Fowler%E2%80%93Noll%E2%80%93Vo_hash_function *)
 external hash : (int[@untagged]) -> (int[@untagged])
-  = "caml_cc_hash_int_byte" "caml_cc_hash_int" [@@noalloc]
+  = "caml_cc_hash_int_byte" "caml_cc_hash_int"
+[@@noalloc]
 
 let range i j yield =
   let rec up i j yield =

src/core/CCInt64.ml

@@ -11,21 +11,14 @@ let max : t -> t -> t = Stdlib.max
 
 let sign i = compare i zero
 
-(* use FNV:
+(* use FNV-1:
    https://en.wikipedia.org/wiki/Fowler%E2%80%93Noll%E2%80%93Vo_hash_function *)
-let hash_to_int64 (n : t) =
-  let offset_basis = 0xcbf29ce484222325L in
-  let prime = 0x100000001b3L in
+external hash_to_int64 : (int64[@unboxed]) -> (int64[@unboxed])
+  = "caml_cc_hash_int64_to_int64_byte" "caml_cc_hash_int64_to_int64"
 
-  let h = ref offset_basis in
-  for k = 0 to 7 do
-    h := mul !h prime;
-    (* h := h xor (k-th byte of n) *)
-    h := logxor !h (logand (shift_right n (k * 8)) 0xffL)
-  done;
-  logand !h max_int
-
-let[@inline] hash (n : t) : int = to_int (hash_to_int64 n) land Stdlib.max_int
+external hash : (int64[@unboxed]) -> (int[@untagged])
+  = "caml_cc_hash_int64_byte" "caml_cc_hash_int64"
+[@@noalloc]
 
 (* see {!CCInt.popcount} for more details *)
 let[@inline] popcount (b : t) : int =

src/core/cc_stubs.c

@@ -1,9 +1,15 @@
+#include <caml/alloc.h>
 #include <caml/mlvalues.h>
 #include <stdint.h>
 
-/* FNV-1a hash for a 64-bit integer.
+/* FNV-1 hash for a 64-bit integer.
    https://en.wikipedia.org/wiki/Fowler%E2%80%93Noll%E2%80%93Vo_hash_function
 
+   FNV-1 order: multiply then XOR (as opposed to FNV-1a which XORs first).
+   Uses the standard 64-bit FNV parameters:
+     offset_basis = 0xcbf29ce484222325
+     prime        = 0x00000100000001b3
+
    Core routine: operates on all 8 bytes of an int64_t. */
 
 static inline int64_t cc_fnv_hash_int64(int64_t n) {
@@ -46,3 +52,16 @@ CAMLprim intnat caml_cc_hash_int64(int64_t n) {
 CAMLprim value caml_cc_hash_int64_byte(value v_n) {
   return Val_long(caml_cc_hash_int64(Int64_val(v_n)));
 }
+
+/* --- int64 -> int64 entry points (for hash_to_int64) --- */
+
+/* native: unboxed int64 in, unboxed int64 out.
+   Masks to non-negative int64 (matches OCaml's Int64.max_int). */
+CAMLprim int64_t caml_cc_hash_int64_to_int64(int64_t n) {
+  return cc_fnv_hash_int64(n) & INT64_MAX;
+}
+
+/* bytecode: boxed int64 in, boxed int64 out */
+CAMLprim value caml_cc_hash_int64_to_int64_byte(value v_n) {
+  return caml_copy_int64(cc_fnv_hash_int64(Int64_val(v_n)) & INT64_MAX);
+}

tests/core/t_int64.ml

@@ -101,3 +101,42 @@ eq' 63 (popcount max_int);;
 eq' 1 (popcount min_int);;
 eq' 10 (popcount 0b1110010110110001010L);;
 eq' 5 (popcount 0b1101110000000000L)
+
+(* hash tests *)
+let ( >= ) = Stdlib.( >= )
+let ( = ) = Stdlib.( = )
+let ( <> ) = Stdlib.( <> );;
+
+(* hash is non-negative *)
+t @@ fun () -> hash 0L >= 0;;
+t @@ fun () -> hash 1L >= 0;;
+t @@ fun () -> hash (-1L) >= 0;;
+t @@ fun () -> hash max_int >= 0;;
+t @@ fun () -> hash min_int >= 0;;
+
+(* hash_to_int64 is non-negative *)
+t @@ fun () -> CCInt64.compare (hash_to_int64 0L) 0L >= 0;;
+t @@ fun () -> CCInt64.compare (hash_to_int64 (-1L)) 0L >= 0;;
+t @@ fun () -> CCInt64.compare (hash_to_int64 min_int) 0L >= 0;;
+
+(* hash is consistent with hash_to_int64 *)
+t @@ fun () -> hash 42L = Stdlib.(Int64.to_int (hash_to_int64 42L) land max_int)
+;;
+
+t @@ fun () ->
+hash (-1L) = Stdlib.(Int64.to_int (hash_to_int64 (-1L)) land max_int)
+;;
+
+(* different inputs produce different hashes *)
+t @@ fun () -> hash 0L <> hash 1L;;
+t @@ fun () -> hash 1L <> hash 2L;;
+t @@ fun () -> hash 1L <> hash (-1L);;
+t @@ fun () -> hash_to_int64 0L <> hash_to_int64 1L;;
+
+(* deterministic *)
+t @@ fun () -> hash 123L = hash 123L;;
+t @@ fun () -> hash_to_int64 123L = hash_to_int64 123L;;
+
+(* quickcheck: hash is always non-negative *)
+q Q.(map Int64.of_int int) (fun n -> hash n >= 0);;
+q Q.(map Int64.of_int int) (fun n -> CCInt64.compare (hash_to_int64 n) 0L >= 0)