Index: src/string/charset/ascii.c
===================================================================
--- src/string/charset/ascii.c	(revision 44499)
+++ src/string/charset/ascii.c	(working copy)
@@ -596,42 +596,36 @@
 {
     ASSERT_ARGS(mixed_cs_index)
     String_iter src_iter, search_iter;
-    UINTVAL len, next_pos;
-    INTVAL found_at;
+    UINTVAL len, c1, c2, c2first, maxoffs;
 
     ENCODING_ITER_INIT(interp, src, &src_iter);
     src_iter.set_position(interp, &src_iter, offs);
     ENCODING_ITER_INIT(interp, search, &search_iter);
-    len = search->strlen;
-
-    found_at = -1;
-    next_pos = offs;
-
-    for (; len && offs < src->strlen ;) {
-        const UINTVAL c1 = src_iter.get_and_advance(interp, &src_iter);
-        const UINTVAL c2 = search_iter.get_and_advance(interp, &search_iter);
-
-        if (c1 == c2) {
-            --len;
-            if (found_at == -1)
-                found_at = offs;
-            ++offs;
+    if (search->strlen == 0)
+        return -1;  /* return 0 instead? */
+    c2first = search_iter.get_and_advance(interp, &search_iter);
+    if (src->strlen < search->strlen)
+        return -1;
+    maxoffs = src->strlen - search->strlen + 1;
+    while (offs < maxoffs) {
+        c1 = src_iter.get_and_advance(interp, &src_iter);
+        if (c1 != c2first) {
+            offs++;
+            continue;
         }
-        else {
-            len = search->strlen;
-            ++offs;
-            ++next_pos;
-            if (offs != next_pos) {
-                src_iter.set_position(interp, &src_iter, next_pos);
-                offs = next_pos;
-            }
-
-            found_at = -1;
-            search_iter.set_position(interp, &search_iter, 0);
+        len = search->strlen;
+        while (--len > 0) {
+            c1 = src_iter.get_and_advance(interp, &src_iter);
+            c2 = search_iter.get_and_advance(interp, &search_iter);
+            if (c1 != c2)
+                break;
         }
+        if (!len)
+            return offs;
+        offs++;
+        src_iter.set_position(interp, &src_iter, offs);
+        search_iter.set_position(interp, &search_iter, 1);
     }
-    if (len == 0)
-        return found_at;
     return -1;
 }