hash: Improve hash function for integers.

As previously defined, the following both returned the same value for
given values of 'basis':
	hash_int(0, hash_int(1, basis))
	hash_int(1, hash_int(0, basis))
because hash_int(0, basis) evaluated to basis and hash_int(1, basis)
evaluated to c + basis for some constant c.

This commit fixes the problem, by eliminating any simple linear
relationship between basis and the hash value.

We should write some tests for hash function quality.

diff --git a/lib/hash.h b/lib/hash.h
index 33c5c427a2e81ad4d44a70b440c4c0af8856cc94..3f140381c36104ce22eb558e08a9ea75ab52d6a4 100644 (file)
--- a/lib/hash.h
+++ b/lib/hash.h
@@ -65,10 +65,11 @@ static inline uint32_t hash_int(uint32_t x, uint32_t basis)
     x ^= x >> 17;
     x -= x << 9;
     x ^= x << 4;
+    x += basis;
     x -= x << 3;
     x ^= x << 10;
     x ^= x >> 15;
-    return x + basis;
+    return x;
 }
 
 /* An attempt at a useful 1-bit hash function.  Has not been analyzed for