From add28462a02bdd02bff3c36d1b7b796941a785c3 Mon Sep 17 00:00:00 2001
From: levlam <levlam@telegram.org>
Date: Fri, 14 May 2021 13:18:52 +0300
Subject: [PATCH] Fix Slice underflow.

---
 td/telegram/MessageEntity.cpp | 2 +-
 test/message_entities.cpp     | 1 +
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/td/telegram/MessageEntity.cpp b/td/telegram/MessageEntity.cpp
index 630e75332..9ea1be0d0 100644
--- a/td/telegram/MessageEntity.cpp
+++ b/td/telegram/MessageEntity.cpp
@@ -628,7 +628,7 @@ static vector<Slice> match_urls(Slice str) {
         }
         path_end_ptr = next_ptr;
       }
-      while (bad_path_end_chars.find(path_end_ptr[-1]) < bad_path_end_chars.size()) {
+      while (path_end_ptr > url_end_ptr + 1 && bad_path_end_chars.find(path_end_ptr[-1]) < bad_path_end_chars.size()) {
         path_end_ptr--;
       }
       if (url_end_ptr[0] == '/' || path_end_ptr > url_end_ptr + 1) {
diff --git a/test/message_entities.cpp b/test/message_entities.cpp
index 18d3a3f4c..e88139513 100644
--- a/test/message_entities.cpp
+++ b/test/message_entities.cpp
@@ -582,6 +582,7 @@ TEST(MessageEntities, url) {
   check_url("https://t…", {});
   check_url("👉http://ab.com/cdefgh-1IJ", {"http://ab.com/cdefgh-1IJ"});
   check_url("...👉http://ab.com/cdefgh-1IJ", {});  // TODO
+  check_url(".?", {});
 }
 
 static void check_fix_formatted_text(td::string str, td::vector<td::MessageEntity> entities,