From 6d9286a2021196370ae61ade3552f0382dbe9cc7 Mon Sep 17 00:00:00 2001
From: Calvin Rose <calsrose@gmail.com>
Date: Tue, 7 Dec 2021 08:36:08 -0600
Subject: [PATCH] Add some more changes to hashing to improve pointer hashing.

---
 src/core/value.c            | 12 +++++++-----
 tools/hashbench/ints1.janet | 24 ++++++++++++++++++++++++
 2 files changed, 31 insertions(+), 5 deletions(-)
 create mode 100644 tools/hashbench/ints1.janet

diff --git a/src/core/value.c b/src/core/value.c
index e1a82f32..a29c48d0 100644
--- a/src/core/value.c
+++ b/src/core/value.c
@@ -325,7 +325,7 @@ int32_t janet_hash(Janet x) {
             uint32_t lo = (uint32_t)(as.u & 0xFFFFFFFF);
             uint32_t hi = (uint32_t)(as.u >> 32);
             uint32_t hilo = (hi ^ lo) * 2654435769u;
-            hash = (int32_t)((hilo << 16) | ((hilo >> 16) & 0xFFFF));
+            hash = (int32_t)((hilo << 16) | (hilo >> 16));
             break;
         }
         case JANET_ABSTRACT: {
@@ -339,15 +339,17 @@ int32_t janet_hash(Janet x) {
         /* fallthrough */
         default:
             if (sizeof(double) == sizeof(void *)) {
-                /* Assuming 8 byte pointer */
+                /* Assuming 8 byte pointer (8 byte aligned) */
                 uint64_t i = janet_u64(x);
                 uint32_t lo = (uint32_t)(i & 0xFFFFFFFF);
                 uint32_t hi = (uint32_t)(i >> 32);
-                hash = (int32_t)(hi ^ (lo >> 3));
+                uint32_t hilo = (hi ^ lo) * 2654435769u;
+                hash = (int32_t)((hilo << 16) | (hilo >> 16));
             } else {
                 /* Assuming 4 byte pointer (or smaller) */
-                hash = (int32_t)((char *)janet_unwrap_pointer(x) - (char *)0);
-                hash >>= 2;
+                ptrdiff_t diff = ((char *)janet_unwrap_pointer(x) - (char *)0);
+                uint32_t hilo = (uint32_t) diff * 2654435769u;
+                hash = (int32_t)((hilo << 16) | (hilo >> 16));
             }
             break;
     }
diff --git a/tools/hashbench/ints1.janet b/tools/hashbench/ints1.janet
new file mode 100644
index 00000000..37219e13
--- /dev/null
+++ b/tools/hashbench/ints1.janet
@@ -0,0 +1,24 @@
+(def f @{})
+(var collisions 0)
+(loop [x :range [0 300] y :range [0 300]]  
+  (def key (hash (+ (* x 1000) y))) 
+  (if (in f key) 
+    (++ collisions)) 
+  (put f key true))
+(print "ints 1 collisions: " collisions)
+
+(def f @{})
+(var collisions 0)
+(loop [x :range [100000 101000] y :range [100000 101000]]
+  (def key (hash [x y]))
+  (if (in f key) (++ collisions))
+  (put f key true))
+(print "int pair 1 collisions: " collisions)
+
+(def f @{})
+(var collisions 0)
+(loop [x :range [10000 11000] y :range [10000 11000]]
+  (def key (hash [x y]))
+  (if (in f key) (++ collisions))
+  (put f key true))
+(print "int pair 2 collisions: " collisions)