From 903243643f987413f9546460ec7c5c3b0c664439 Mon Sep 17 00:00:00 2001
From: Simon Cruanes <simon.cruanes.2007@m4x.org>
Date: Sun, 4 Apr 2021 17:45:36 -0400
Subject: [PATCH] perf(hash): use FNV hashing

---
 src/util/Hash.ml | 64 +++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 53 insertions(+), 11 deletions(-)

diff --git a/src/util/Hash.ml b/src/util/Hash.ml
index 3d47a2b1..ee43b20c 100644
--- a/src/util/Hash.ml
+++ b/src/util/Hash.ml
@@ -3,24 +3,63 @@
 
 type 'a t = 'a -> int
 
-let bool b = if b then 1 else 2
+(* FNV hashing
+   https://en.wikipedia.org/wiki/Fowler%E2%80%93Noll%E2%80%93Vo_hash_function
+*)
+let fnv_offset_basis = 0xcbf29ce484222325L
+let fnv_prime = 0x100000001b3L
 
-let int i = i land max_int
+(* hash an integer *)
+let hash_int_ n =
+  let h = ref fnv_offset_basis in
+  for k = 0 to 7 do
+    h := Int64.(mul !h fnv_prime);
+    h := Int64.(logxor !h (of_int ((n lsr (k * 8)) land 0xff)));
+  done;
+  (Int64.to_int !h) land max_int (* truncate back to int and remove sign *)
 
-let string (s:string) = Hashtbl.hash s
+let combine2 a b =
+  let h = ref fnv_offset_basis in
+  (* we only do one loop, where we mix bytes of [a] and [b], so as
+     to simplify control flow *)
+  for k = 0 to 7 do
+    h := Int64.(mul !h fnv_prime);
+    h := Int64.(logxor !h (of_int ((a lsr (k * 8)) land 0xff)));
+    h := Int64.(mul !h fnv_prime);
+    h := Int64.(logxor !h (of_int ((b lsr (k * 8)) land 0xff)));
+  done;
+  Int64.to_int !h land max_int
 
-let combine f a b = Hashtbl.seeded_hash a (f b)
-
-let combine2 a b = Hashtbl.seeded_hash a b
+let[@inline] combine f s x =
+  combine2 s (f x)
 
 let combine3 a b c =
-  combine2 a b
-  |> combine2 c
+  let h = ref fnv_offset_basis in
+  (* we only do one loop, where we mix bytes of [a] [b] and [c], so as
+     to simplify control flow *)
+  for k = 0 to 7 do
+    h := Int64.(mul !h fnv_prime);
+    h := Int64.(logxor !h (of_int ((a lsr (k * 8)) land 0xff)));
+    h := Int64.(mul !h fnv_prime);
+    h := Int64.(logxor !h (of_int ((b lsr (k * 8)) land 0xff)));
+    h := Int64.(mul !h fnv_prime);
+    h := Int64.(logxor !h (of_int ((c lsr (k * 8)) land 0xff)));
+  done;
+  Int64.to_int !h land max_int
 
 let combine4 a b c d =
-  combine2 a b
-  |> combine2 c
-  |> combine2 d
+  let h = ref fnv_offset_basis in
+  for k = 0 to 7 do
+    h := Int64.(mul !h fnv_prime);
+    h := Int64.(logxor !h (of_int ((a lsr (k * 8)) land 0xff)));
+    h := Int64.(mul !h fnv_prime);
+    h := Int64.(logxor !h (of_int ((b lsr (k * 8)) land 0xff)));
+    h := Int64.(mul !h fnv_prime);
+    h := Int64.(logxor !h (of_int ((c lsr (k * 8)) land 0xff)));
+    h := Int64.(mul !h fnv_prime);
+    h := Int64.(logxor !h (of_int ((d lsr (k * 8)) land 0xff)));
+  done;
+  Int64.to_int !h land max_int
 
 let pair f g (x,y) = combine2 (f x) (g y)
 
@@ -28,9 +67,12 @@ let opt f = function
   | None -> 42
   | Some x -> combine2 43 (f x)
 
+let int = hash_int_
+let bool b = hash_int_ (if b then 1 else 2)
 let list f l = List.fold_left (combine f) 0x42 l
 let array f = Array.fold_left (combine f) 0x43
 let iarray f = IArray.fold (combine f) 0x44
+let string : string t = Hashtbl.hash
 let seq f seq =
   let h = ref 0x43 in
   seq (fun x -> h := combine f !h x);