Optimize (AND FIX) mb_check_encoding (cut execution time by 50%+)

Previously, `mb_check_encoding` did an awful lot of unneeded work. In order to
determine whether a string was valid or not, it would convert the whole string
into wchar (code points), which required dynamically allocating a (potentially
large) buffer. Then it would turn right around and convert that big 'ol buffer
of code points back to the original encoding again. Finally, it would check
whether any invalid bytes were detected during that long and onerous process.

The thing is, mbstring _already_ has machinery for detecting whether a string
is valid in a certain encoding or not, and it doesn't require copying any data
around or allocating buffers. Better yet, it can fail fast when an invalid byte
is found. Why not use it? It's sure a lot faster!

Further, the legacy code was also badly broken. Why? Because aside from
checking whether illegal characters were detected, it would also check whether
the conversion to and from wchars was lossless. But, some encodings have
more than one valid encoding for the same character. In such cases, it is
not possible to make the conversion to and from wchars lossless for every
valid character. So `mb_check_encoding` would actually reject good strings
in a lot of encodings!

Author: alexdowad

ext/mbstring/mbstring.c

@@ -3856,84 +3856,50 @@ PHP_FUNCTION(mb_get_info)
 /* }}} */
 
 
-static inline mbfl_buffer_converter *php_mb_init_convd(const mbfl_encoding *encoding)
+MBSTRING_API int php_mb_check_encoding(const char *input, size_t length, const mbfl_encoding *encoding)
 {
-	mbfl_buffer_converter *convd;
-
-	convd = mbfl_buffer_converter_new(encoding, encoding, 0);
-	if (convd == NULL) {
-		return NULL;
-	}
-	mbfl_buffer_converter_illegal_mode(convd, MBFL_OUTPUTFILTER_ILLEGAL_MODE_NONE);
-	mbfl_buffer_converter_illegal_substchar(convd, 0);
-	return convd;
-}
-
-
-static inline int php_mb_check_encoding_impl(mbfl_buffer_converter *convd, const char *input, size_t length, const mbfl_encoding *encoding) {
-	mbfl_string string, result;
-
-	mbfl_string_init_set(&string, encoding);
-	mbfl_string_init(&result);
-
-	string.val = (unsigned char *) input;
-	string.len = length;
-
-	mbfl_string *ret = mbfl_buffer_converter_feed_result(convd, &string, &result);
-	size_t illegalchars = mbfl_buffer_illegalchars(convd);
-
-	if (ret != NULL) {
-		if (illegalchars == 0 && string.len == result.len && memcmp(string.val, result.val, string.len) == 0) {
-			mbfl_string_clear(&result);
-			return 1;
+	mbfl_identify_filter *ident = mbfl_identify_filter_new2(encoding);
+
+	while (length--) {
+		unsigned char c = *input++;
+		(ident->filter_function)(c, ident);
+		if (ident->flag) {
+			mbfl_identify_filter_delete(ident);
+			return 0;
 		}
-		mbfl_string_clear(&result);
 	}
-	return 0;
-}
-
-MBSTRING_API int php_mb_check_encoding(const char *input, size_t length, const mbfl_encoding *encoding)
-{
-	mbfl_buffer_converter *convd = php_mb_init_convd(encoding);
-	/* If this assertion fails this means some memory allocation failure which is a bug */
-	ZEND_ASSERT(convd != NULL);
 
-	int result = php_mb_check_encoding_impl(convd, input, length, encoding);
-	mbfl_buffer_converter_delete(convd);
+	/* String must not end in the middle of a multi-byte character */
+	int result = (ident->status == 0);
+	mbfl_identify_filter_delete(ident);
 	return result;
 }
 
 static int php_mb_check_encoding_recursive(HashTable *vars, const mbfl_encoding *encoding)
 {
-	mbfl_buffer_converter *convd;
 	zend_long idx;
 	zend_string *key;
 	zval *entry;
 	int valid = 1;
 
-	(void)(idx);
-
-	convd = php_mb_init_convd(encoding);
-	/* If this assertion fails this means some memory allocation failure which is a bug */
-	ZEND_ASSERT(convd != NULL);
+	(void)(idx); /* Suppress spurious compiler warning that `idx` is not used */
 
 	if (GC_IS_RECURSIVE(vars)) {
-		mbfl_buffer_converter_delete(convd);
 		php_error_docref(NULL, E_WARNING, "Cannot not handle circular references");
 		return 0;
 	}
 	GC_TRY_PROTECT_RECURSION(vars);
 	ZEND_HASH_FOREACH_KEY_VAL(vars, idx, key, entry) {
 		ZVAL_DEREF(entry);
 		if (key) {
-			if (!php_mb_check_encoding_impl(convd, ZSTR_VAL(key), ZSTR_LEN(key), encoding)) {
+			if (!php_mb_check_encoding(ZSTR_VAL(key), ZSTR_LEN(key), encoding)) {
 				valid = 0;
 				break;
 			}
 		}
 		switch (Z_TYPE_P(entry)) {
 			case IS_STRING:
-				if (!php_mb_check_encoding_impl(convd, Z_STRVAL_P(entry), Z_STRLEN_P(entry), encoding)) {
+				if (!php_mb_check_encoding(Z_STRVAL_P(entry), Z_STRLEN_P(entry), encoding)) {
 					valid = 0;
 					break;
 				}
@@ -3957,11 +3923,9 @@ static int php_mb_check_encoding_recursive(HashTable *vars, const mbfl_encoding
 		}
 	} ZEND_HASH_FOREACH_END();
 	GC_TRY_UNPROTECT_RECURSION(vars);
-	mbfl_buffer_converter_delete(convd);
 	return valid;
 }
 
-
 /* {{{ Check if the string is valid for the specified encoding */
 PHP_FUNCTION(mb_check_encoding)
 {