Fix #74264: grapheme_strrpos() broken for negative offsets

We must not assume that `usearch_last()` gives the proper result for
negative offsets.  Instead we'd need to continue to search backwards
(`usearch_previous`) until we find a proper match.  However, apparently
searching backwards is broken, so we work around by searching forward
from the start of the string until we pass the `offset_pos`, and then
use the previous result.

Closes GH-7189.

Author: cmb69

NEWS

@@ -18,6 +18,7 @@ PHP                                                                        NEWS
   . Fixed bug #72809 (Locale::lookup() wrong result with canonicalize option).
     (cmb)
   . Fixed bug #68471 (IntlDateFormatter fails for "GMT+00:00" timezone). (cmb)
+  . Fixed bug #74264 (grapheme_strrpos() broken for negative offsets). (cmb)
 
 - OpenSSL:
   . Fixed bug #52093 (openssl_csr_sign truncates $serial). (cmb)

ext/intl/grapheme/grapheme_util.c

@@ -179,16 +179,29 @@ int32_t grapheme_strpos_utf16(char *haystack, size_t haystack_len, char *needle,
 			STRPOS_CHECK_STATUS(status, "Invalid search offset");
 		}
 		status = U_ZERO_ERROR;
-		usearch_setOffset(src, offset_pos, &status);
+		usearch_setOffset(src, last ? 0 : offset_pos, &status);
 		STRPOS_CHECK_STATUS(status, "Invalid search offset");
 	}
 
 
 	if(last) {
-		char_pos = usearch_last(src, &status);
-		if(char_pos < offset_pos) {
-			/* last one is beyound our start offset */
-			char_pos = USEARCH_DONE;
+		if (offset >= 0) {
+			char_pos = usearch_last(src, &status);
+			if(char_pos < offset_pos) {
+				/* last one is beyond our start offset */
+				char_pos = USEARCH_DONE;
+			}			
+		} else {
+			/* searching backwards is broken, so we search forwards, albeit it's less efficient */
+			int32_t prev_pos = USEARCH_DONE;
+			do {
+				char_pos = usearch_next(src, &status);
+				if (char_pos == USEARCH_DONE || char_pos > offset_pos) {
+					char_pos = prev_pos;
+					break;
+				}
+				prev_pos = char_pos;
+			} while(1);
 		}
 	} else {
 		char_pos = usearch_next(src, &status);

ext/intl/tests/bug74264.phpt

@@ -0,0 +1,26 @@
+--TEST--
+Bug #74264 (grapheme_sttrpos() broken for negative offsets)
+--SKIPIF--
+<?php
+if (!extension_loaded('intl')) die("skip intl extension not available");
+?>
+--FILE--
+<?php
+foreach (range(-5, -1) as $offset) {
+    var_dump(
+        grapheme_strrpos('déjàààà', 'à', $offset),
+        grapheme_strripos('DÉJÀÀÀÀ', 'à', $offset)
+    );        
+}
+?>
+--EXPECT--
+bool(false)
+bool(false)
+int(3)
+int(3)
+int(4)
+int(4)
+int(5)
+int(5)
+int(6)
+int(6)