From d7731a95abaa084303a864a24bd4e1da1a59e3b5 Mon Sep 17 00:00:00 2001
From: "Yukihiro \"Matz\" Matsumoto" <matz@ruby.or.jp>
Date: Tue, 24 May 2022 08:47:07 +0900
Subject: [PATCH] string.c (mrb_str_rindex_m): avoid RSTRING_CHAR_LEN() if
 possible.

Since it scans the string. Use char_backtrack() instead for negative
offset (counting from the tail).
---
 src/string.c | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/src/string.c b/src/string.c
index efab10eb3..5f4fd4aa5 100644
--- a/src/string.c
+++ b/src/string.c
@@ -2042,21 +2042,23 @@ mrb_str_rindex_m(mrb_state *mrb, mrb_value str)
 
   mrb_value sub;
   mrb_int pos;
-  mrb_int len = RSTRING_CHAR_LEN(str);
 
   if (mrb_get_args(mrb, "S|i", &sub, &pos) == 1) {
-    pos = len;
+    pos = RSTRING_CHAR_LEN(str);
+    pos = chars2bytes(str, 0, pos);
+  }
+  else if (pos >= 0) {
+    pos = chars2bytes(str, 0, pos);
   }
   else {
-    if (pos < 0) {
-      pos += len;
-      if (pos < 0) {
-        return mrb_nil_value();
-      }
+    const char *p = RSTRING_PTR(str);
+    const char *e = RSTRING_END(str);
+    while (pos++ < 0 && p < e) {
+      e = char_backtrack(p, e);
     }
-    if (pos > len) pos = len;
+    if (p == e) return mrb_nil_value();
+    pos = (mrb_int)(e - p);
   }
-  pos = chars2bytes(str, 0, pos);
   pos = str_rindex(mrb, str, sub, pos);
   if (pos >= 0) {
     pos = bytes2chars(RSTRING_PTR(str), RSTRING_LEN(str), pos);