object: add b_string_substr

object/string.c

@@ -365,6 +365,28 @@ const char *b_string_ptr(const struct b_string *str)
 	return str->s_data.d_external;
 }
 
+struct b_string *b_string_substr(const struct b_string *str, size_t start, size_t len)
+{
+	if (start > b_string_get_size(str, B_STRLEN_NORMAL)) {
+		return NULL;
+	}
+
+	if (start + len > b_string_get_size(str, B_STRLEN_NORMAL)) {
+		len = b_string_get_size(str, B_STRLEN_NORMAL) - start;
+	}
+
+	struct b_string *newstr = b_string_create();
+	b_string_reserve(newstr, len);
+
+	const char *src = b_string_ptr(str) + start;
+	char *dest = string_ptr(newstr);
+
+	memcpy(dest, src, len);
+	newstr->s_len = len;
+
+	return newstr;
+}
+
 static void string_release(struct b_object *obj)
 {
 	struct b_string *str = B_STRING(obj);