Work on byte level when parsing markdown

Reasoning: instead encoding every character one by one as we
encounter them to use half their length as the correct offset,
we can simply encode the whole string at once as utf-16le and
work with that directly.

telethon/extensions/markdown.py

@@ -11,8 +11,6 @@ from ..tl.types import (
     MessageEntityPre, MessageEntityTextUrl
 )
 
-def tg_string_len(s):
-    return len(s.encode('utf-16le')) // 2
 
 class Mode(Enum):
     """Different modes supported by Telegram's Markdown"""
@@ -31,7 +29,10 @@ DEFAULT_DELIMITERS = {
     '```': Mode.PRE
 }
 
-DEFAULT_URL_RE = re.compile(r'\[(.+?)\]\((.+?)\)')
+# Regex used to match utf-16le encoded r'\[(.+?)\]\((.+?)\)',
+# reason why there's '\0' after every match-literal character.
+DEFAULT_URL_RE = re.compile(b'\\[\0(.+)\\]\0\\(\0(.+?)\\)\0')
+
 
 def parse(message, delimiters=None, url_re=None):
     """
@@ -40,40 +41,45 @@ def parse(message, delimiters=None, url_re=None):
     dictionary (or default if None).
 
     The url_re(gex) must contain two matching groups: the text to be
-    clickable and the URL itself.
+    clickable and the URL itself, and be utf-16le encoded.
     """
+    # Work on byte level with the utf-16le encoding to get the offsets right.
+    # The offset will just be half the index we're at.
     if url_re is None:
         url_re = DEFAULT_URL_RE
     elif url_re:
         if isinstance(url_re, str):
-            url_re = re.compile(url_re)
+            url_re = re.compile(url_re.encode('utf-16le'))
 
     if not delimiters:
         if delimiters is not None:
             return message, []
         delimiters = DEFAULT_DELIMITERS
 
+    delimiters = {k.encode('utf-16le'): v for k, v in delimiters.items()}
+
+    i = 0
     result = []
     current = Mode.NONE
-    offset = 0
-    i = 0
+    message = message.encode('utf-16le')
     while i < len(message):
         url_match = None
         if url_re and current == Mode.NONE:
             url_match = url_re.match(message, pos=i)
             if url_match:
-                message = ''.join((
+                message = b''.join((
                     message[:url_match.start()],
                     url_match.group(1),
                     message[url_match.end():]
                 ))
 
                 result.append((
-                    offset,
-                    offset + tg_string_len(url_match.group(1)),
-                    (Mode.URL, url_match.group(2))
+                    i // 2,
+                    (i + len(url_match.group(1))) // 2,
+                    (Mode.URL, url_match.group(2).decode('utf-16le'))
                 ))
                 i += len(url_match.group(1))
+
         if not url_match:
             for d, m in delimiters.items():
                 if message[i:i + len(d)] == d and current in (Mode.NONE, m):
@@ -82,21 +88,20 @@ def parse(message, delimiters=None, url_re=None):
 
                     message = message[:i] + message[i + len(d):]
                     if current == Mode.NONE:
-                        result.append(offset)
+                        result.append(i // 2)
                         current = m
                     else:
-                        result[-1] = (result[-1], offset, current)
+                        result[-1] = (result[-1], i // 2, current)
                         current = Mode.NONE
                     break
 
         if i < len(message):
-            offset += tg_string_len(message[i])
-            i += 1
+            i += 2
 
     if result and not isinstance(result[-1], tuple):
         result.pop()
 
-    return message, result
+    return message.decode('utf-16le'), result
 
 
 def parse_tg(message, delimiters=None):