Replace auto-linked URLs in a single pass

src/bbcode/__init__.py

@@ -27,6 +27,10 @@
     r"(?:com|net|org|edu|biz|gov|mil|info|io|name|me|tv|us|uk|mobi))"
 )
 
+# Matches the placeholder tokens used to hold links aside while escaping and cosmetic
+# replacement happen, so they can all be substituted back in a single pass.
+_link_token_re = re.compile(r"\{\{ bbcode-link-\d+ \}\}")
+
 
 # Taken from python-requests
 class CaseInsensitiveDict(MutableMapping):
@@ -634,29 +638,25 @@ def _transform(
         if self.replace_links and replace_links:
             # If we're replacing links in the text (i.e. not those in [url] tags) then
             # we need to be careful to pull them out before doing any escaping or
-            # cosmetic replacement.
-            pos = 0
-            while True:
-                match = _url_re.search(data, pos)
-                if not match:
-                    break
-                # Replace any link with a token that we can substitute back in after
-                # replacements.
+            # cosmetic replacement. Each link is swapped for a token here and put back
+            # at the end. Both halves are single passes: doing it a match at a time
+            # rebuilt the whole string once per link, so a post with many links cost
+            # time quadratic in its length.
+            def _tokenize_link(match):
                 token = "{{ bbcode-link-%s }}" % len(url_matches)
                 url_matches[token] = self._link_replace(match, **context)
-                start, end = match.span()
-                data = data[:start] + token + data[end:]
-                # To be perfectly accurate, this should probably be
-                # len(data[:start] + token), but start will work, because the token
-                # itself won't match as a URL.
-                pos = start
+                return token
+
+            data = _url_re.sub(_tokenize_link, data)
         if escape_html:
             data = self._replace(data, self.REPLACE_ESCAPE)
         if replace_cosmetic:
             data = self._replace(data, self.REPLACE_COSMETIC)
         # Now put the replaced links back in the text.
-        for token, replacement in url_matches.items():
-            data = data.replace(token, replacement)
+        if url_matches:
+            data = _link_token_re.sub(
+                lambda match: url_matches.get(match.group(0), match.group(0)), data
+            )
         if transform_newlines:
             data = data.replace("\n", "\r")
         return data

tests.py

@@ -321,6 +321,21 @@ def test_urls(self):
             num = len(bbcode._url_re.findall(line))
             self.assertEqual(num, 1, 'Found %d links in "%s"' % (num, line.strip()))
 
+    def test_many_links(self):
+        # Every link in a post with a lot of them should still be replaced, and the
+        # placeholder tokens used while escaping should not leak into the output.
+        src = "http://a.co/x " * 500
+        dst = bbcode.render_html(src)
+        self.assertEqual(dst.count('href="http://a.co/x"'), 500)
+        self.assertNotIn("bbcode-link", dst)
+
+    def test_link_token_in_source(self):
+        # Text that happens to look like an internal placeholder is left alone when
+        # there is no link to substitute for it.
+        self.assertEqual(
+            bbcode.render_html("{{ bbcode-link-0 }}"), "{{ bbcode-link-0 }}"
+        )
+
     def test_unicode(self):
         src = "[center]ƒünk¥ • §tüƒƒ[/center]"
         dst = '<div style="text-align:center;">ƒünk¥ • §tüƒƒ</div>'