Fix hash function in xdiff library

Linus Torvalds · Junio C Hamano · commit 9de0834663f2 · 2006-10-16T21:27:44.000-07:00
Jim Mayering noticed that xdiff library took insanely long time when comparing files with many identical lines. This was because the hash function used in the library is broken on 64-bit architectures and caused too many collisions. http://thread.gmane.org/gmane.comp.version-control.git/28962/focus=28994 Acked-by: Davide Libenzi <davidel@xmaliserver.org> Signed-off-by: Junio C Hamano <junkio@cox.net>
diff --git a/xdiff/xmacros.h b/xdiff/xmacros.h
@@ -24,14 +24,15 @@
 #define XMACROS_H
 
 
-#define GR_PRIME 0x9e370001UL
 
 
 #define XDL_MIN(a, b) ((a) < (b) ? (a): (b))
 #define XDL_MAX(a, b) ((a) > (b) ? (a): (b))
 #define XDL_ABS(v) ((v) >= 0 ? (v): -(v))
 #define XDL_ISDIGIT(c) ((c) >= '0' && (c) <= '9')
-#define XDL_HASHLONG(v, b) (((unsigned long)(v) * GR_PRIME) >> ((CHAR_BIT * sizeof(unsigned long)) - (b)))
+#define XDL_ADDBITS(v,b)	((v) + ((v) >> (b)))
+#define XDL_MASKBITS(b)		((1UL << (b)) - 1)
+#define XDL_HASHLONG(v,b)	(XDL_ADDBITS((unsigned long)(v), b) & XDL_MASKBITS(b))
 #define XDL_PTRFREE(p) do { if (p) { xdl_free(p); (p) = NULL; } } while (0)
 #define XDL_LE32_PUT(p, v) \
 do { \
