From ffc02a9877e86d1a69638aa44ac9d10f37e20d62 Mon Sep 17 00:00:00 2001
From: Matthew Flatt <mflatt@racket-lang.org>
Date: Sun, 7 Apr 2019 09:34:30 +0200
Subject: [PATCH] improve hash mixing

original commit: d7469cedd67a950931a561ce14388fe7e628770d
---
 c/intern.c   | 15 +++++++--------
 s/newhash.ss |  3 ++-
 2 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/c/intern.c b/c/intern.c
index acfee35393..ddc478b7e4 100644
--- a/c/intern.c
+++ b/c/intern.c
@@ -188,19 +188,18 @@ void S_resize_oblist(void) {
   S_G.oblist = new_oblist;
 }
 
-/* hash function: multiplier weights each character, h = n factors in the length */
-#define multiplier 3
+#define MIX_HASH(hc) (hc += (hc << 10), hc ^= (hc >> 6))
 
 static iptr hash(const unsigned char *s, iptr n) {
-  iptr h = n + 401887359;
-  while (n--) h = h * multiplier + *s++;
-  return h & most_positive_fixnum;
+  uptr h = (uptr)n + 401887359;
+  while (n--) { h += *s++; MIX_HASH(h); }
+  return (iptr)h & most_positive_fixnum;
 }
 
 static iptr hash_sc(const string_char *s, iptr n) {
-  iptr h = n + 401887359;
-  while (n--) h = h * multiplier + Schar_value(*s++);
-  return h & most_positive_fixnum;
+  uptr h = (uptr)n + 401887359;
+  while (n--) { h += Schar_value(*s++); MIX_HASH(h); }
+  return (iptr)h & most_positive_fixnum;
 }
 
 static iptr hash_uname(const string_char *s, iptr n) {
diff --git a/s/newhash.ss b/s/newhash.ss
index 7af992bbf2..5afea1cfc4 100644
--- a/s/newhash.ss
+++ b/s/newhash.ss
@@ -920,7 +920,8 @@ Documentation notes:
     (define (hcabs hc) (if (fx< hc 0) (fxnot hc) hc))
 
     (define (update hc k)
-      (fxlogxor (#3%fx+ (#3%fxsll hc 2) hc) k))
+      (let ([hc2 (#3%fx+ hc (#3%fxsll (#3%fx+ hc k) 10))])
+        (fxlogxor hc2 (fxsrl hc2 6))))
 
     (define bytevector-hash
       (lambda (bv)