From a3abf40bc224356c0ccfc65284bf649c98d0f62c Mon Sep 17 00:00:00 2001
From: Simon Cruanes <simon.cruanes.2007@m4x.org>
Date: Mon, 22 Aug 2022 17:48:21 +0000
Subject: [PATCH] add `CCInt64.{hash,hash_to_int64}`

---
 src/core/CCInt64.ml  | 18 +++++++++++++++++-
 src/core/CCInt64.mli |  9 +++++++--
 2 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/src/core/CCInt64.ml b/src/core/CCInt64.ml
index d8802983..e94cccc2 100644
--- a/src/core/CCInt64.ml
+++ b/src/core/CCInt64.ml
@@ -5,9 +5,25 @@ include Int64
 
 let min : t -> t -> t = Stdlib.min
 let max : t -> t -> t = Stdlib.max
-let hash x = Stdlib.abs (to_int x)
 let sign i = compare i zero
 
+(* use FNV:
+   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
+
+  let h = ref offset_basis in
+  for k = 0 to 7 do
+    h := mul !h prime;
+    (* h := h xor (k-th bit of n) *)
+    h := logxor !h (logand (shift_left n (k * 8)) 0xffL)
+  done;
+  logand !h max_int
+
+let[@inline] hash (n : t) : int =
+  to_int (hash_to_int64 n) land CCShims_.Stdlib.max_int
+
 (* see {!CCInt.popcount} for more details *)
 let[@inline] popcount (b : t) : int =
   let m1 = 0x5555555555555555L in
diff --git a/src/core/CCInt64.mli b/src/core/CCInt64.mli
index 56dbecae..5fdd6770 100644
--- a/src/core/CCInt64.mli
+++ b/src/core/CCInt64.mli
@@ -27,8 +27,13 @@ val max : t -> t -> t
     @since 3.0 *)
 
 val hash : t -> int
-(** [hash x] computes the hash of [x].
-    Like {!Stdlib.abs (to_int x)}. *)
+(** [hash x] computes the hash of [x], a non-negative integer.
+    Uses FNV since NEXT_RELEASE *)
+
+val hash_to_int64 : t -> t
+(** Like {!hash} but does not truncate.
+    Uses FNV.
+    @since NEXT_RELEASE *)
 
 val popcount : t -> int
 (** Number of bits set to 1.