util/StringUtil: add FindStringSuffix()

src/PlaylistFile.cxx

@@ -173,12 +173,9 @@ LoadPlaylistFileInfo(PlaylistInfo &info,
 		return false;
 
 	const auto *const name_fs_str = name_fs.c_str();
-	const size_t name_length = name_fs.length();
-
-	if (name_length < ARRAY_SIZE(PLAYLIST_FILE_SUFFIX))
-		return false;
-
-	if (!StringEndsWith(name_fs_str, PLAYLIST_FILE_SUFFIX))
+	const auto *const name_fs_end =
+		FindStringSuffix(name_fs_str, PLAYLIST_FILE_SUFFIX);
+	if (name_fs_end == nullptr)
 		return false;
 
 	FileInfo fi;
@@ -186,8 +183,7 @@ LoadPlaylistFileInfo(PlaylistInfo &info,
 	    !fi.IsRegular())
 		return false;
 
-	PathTraitsFS::string name(name_fs_str,
-				  name_length + 1 - ARRAY_SIZE(PLAYLIST_FILE_SUFFIX));
+	PathTraitsFS::string name(name_fs_str, name_fs_end);
 	std::string name_utf8 = PathToUTF8(name.c_str());
 	if (name_utf8.empty())
 		return false;

src/util/StringUtil.cxx

@@ -96,6 +96,21 @@ StringEndsWith(const char *haystack, const char *needle)
 		       needle, needle_length) == 0;
 }
 
+const char *
+FindStringSuffix(const char *p, const char *suffix)
+{
+	const size_t p_length = strlen(p);
+	const size_t suffix_length = strlen(suffix);
+
+	if (p_length < suffix_length)
+		return nullptr;
+
+	const char *q = p + p_length - suffix_length;
+	return memcmp(q, suffix, suffix_length) == 0
+		? q
+		: nullptr;
+}
+
 char *
 CopyString(char *gcc_restrict dest, const char *gcc_restrict src, size_t size)
 {

src/util/StringUtil.hxx

@@ -91,6 +91,14 @@ bool
 StringEndsWith(const char *haystack, const char *needle);
 
 /**
+ * Check if the given string ends with the specified suffix.  If yes,
+ * returns the position of the suffix, and nullptr otherwise.
+ */
+gcc_pure
+const char *
+FindStringSuffix(const char *p, const char *suffix);
+
+/**
  * Copy a string.  If the buffer is too small, then the string is
  * truncated.  This is a safer version of strncpy().
  *