From d18922065b186a88a8b37b19779a20a6eb1d9993 Mon Sep 17 00:00:00 2001
From: matz <matz@b2dd03c8-39d4-4d8f-98ff-823fe69b080e>
Date: Fri, 31 Jan 2003 06:48:54 +0000
Subject: [PATCH] * string.c (rb_str_index): search using Karp-Rabin algolithm.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@3429 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
---
 ChangeLog |  4 ++++
 string.c  | 26 ++++++++++++++++++++++++--
 2 files changed, 28 insertions(+), 2 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 9e6cd66e01..19a04fcad2 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Fri Jan 31 14:16:59 2003  Yukihiro Matsumoto  <matz@ruby-lang.org>
+
+	* string.c (rb_str_index): search using Karp-Rabin algolithm.
+
 Fri Jan 31 12:45:11 2003  Yukihiro Matsumoto  <matz@ruby-lang.org>
 
 	* variable.c (rb_obj_classname): new function.
diff --git a/string.c b/string.c
index c126ee9e8a..c864f1fc11 100644
--- a/string.c
+++ b/string.c
@@ -840,6 +840,7 @@ rb_str_index(str, sub, offset)
 {
     char *s, *e, *p;
     long len;
+    int d, hx, hy, i;
 
     if (offset < 0) {
 	offset += RSTRING(str)->len;
@@ -851,10 +852,31 @@ rb_str_index(str, sub, offset)
     len = RSTRING(sub)->len;
     if (len == 0) return offset;
     e = RSTRING(str)->ptr + RSTRING(str)->len - len + 1;
+
+    /* seach using Karp-Rabin algolithm described in:
+
+       EXACT STRING MATCHING ALGORITHMS
+       http://www-igm.univ-mlv.fr/~lecroq/string/index.html
+    */
+
+#define KR_REHASH(a, b, h) ((((h) - (a)*d) << 1) + (b))
+
+    /* Preprocessing */
+    /* computes d = 2^(m-1) with
+       the left-shift operator */
+    for (d = i = 1; i < len; ++i)
+	d = (d<<1);
+
+    for (hy = hx = i = 0; i < len; ++i) {
+	hx = ((hx<<1) + p[i]);
+	hy = ((hy<<1) + s[i]);
+    }
+
+    /* Searching */
     while (s < e) {
-	if (rb_memcmp(s, p, len) == 0) {
+	if (hx == hy && rb_memcmp(p, s, len) == 0)
 	    return (s-(RSTRING(str)->ptr));
-	}
+	hy = KR_REHASH(*s, *(s+len), hy);
 	s++;
     }
     return -1;