From f84134daa61e33d18e210f070aa897c3d2f33167 Mon Sep 17 00:00:00 2001
From: Param Tanna <paramtanna011@gmail.com>
Date: Wed, 28 Oct 2026 08:22:55 +0000
Subject: [PATCH] Fix quadratic time for unclosed inline-link parentheses

`LinkProcessor.parse_inline_anchor_or_image` runs two O(n) scans per
`[...](`: `Markdown._find_balanced` over the destination and
`_inline_link_title.search`. The link loop calls it once per `[`, so an
unclosed `(` makes conversion quadratic (`"[a](" * 4000`, 16 KB, ~3s;
`"![a](" * n` and `"[a](<u>" * n` the same).

Both scans only matter when the destination actually closes:

- `_find_balanced` returns `len(text)` immediately when no `close_c`
  remains, using a per-text cache of the last `close_c` position, instead
  of scanning to the end of the text.
- `parse_inline_anchor_or_image` skips the title search when the
  destination did not close (`text[end_idx - 1] != ")"`), because
  `_inline_link_title` ends in `\)$` and cannot match otherwise.

Both changes are output-preserving: the patched converter was compared
against the original over 320,000 random inputs and produced identical
output, and the existing test suite's results are unchanged (same set of
environment-related failures before and after). `"[a](" * 4000` goes from
~3s to ~0.01s; link-heavy documents are unaffected (`rfind` is linear and
cached per text).
---
 lib/markdown2.py                        |  18 ++
 test/tm-cases/unclosed_link_parens.html | 409 ++++++++++++++++++++++++
 test/tm-cases/unclosed_link_parens.text | 409 ++++++++++++++++++++++++
 3 files changed, 836 insertions(+)
 create mode 100644 test/tm-cases/unclosed_link_parens.html
 create mode 100644 test/tm-cases/unclosed_link_parens.text

diff --git a/lib/markdown2.py b/lib/markdown2.py
index 3632a54e..f63f2ef0 100755
--- a/lib/markdown2.py
+++ b/lib/markdown2.py
@@ -1581,6 +1581,20 @@ def _find_balanced(self, text: str, start: int, open_c: str, close_c: str) -> in
         out - the same number of open_c and close_c are encountered - or the
         end of string if it's reached before the balance point is found.
         """
+        # If no `close_c` remains in the text, the loop below runs to the end,
+        # and the link scanner calls this once per `[...](`, which is quadratic
+        # for unclosed brackets. Skip the scan using a per-text cache of the
+        # last `close_c` position.
+        cache = getattr(self, "_balanced_last_close", None)
+        if cache is None or cache[0] is not text:
+            cache = [text, {}]
+            self._balanced_last_close = cache
+        last = cache[1].get(close_c)
+        if last is None:
+            last = text.rfind(close_c)
+            cache[1][close_c] = last
+        if start > last:
+            return len(text)
         i = start
         l = len(text)
         count = 1
@@ -3157,6 +3171,10 @@ def parse_inline_anchor_or_image(self, text: str, _link_text: str, start_idx: in
         if has_anglebrackets:
             end_idx = self.md._find_balanced(text, end_idx+1, "<", ">")
         end_idx = self.md._find_balanced(text, end_idx, "(", ")")
+        if text[end_idx - 1] != ")":
+            # The destination's "(" never closed, so `_inline_link_title` (which
+            # ends in `\)$`) cannot match; skip the search.
+            return
         match = self.md._inline_link_title.search(text, idx, end_idx)
         if not match:
             return
diff --git a/test/tm-cases/unclosed_link_parens.html b/test/tm-cases/unclosed_link_parens.html
new file mode 100644
index 00000000..c6dee763
--- /dev/null
+++ b/test/tm-cases/unclosed_link_parens.html
@@ -0,0 +1,409 @@
+<p>[a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a](</p>
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+<p>![img](</p>
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+<p>![img](</p>
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+<p>![img](</p>
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+<p>![img](</p>
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+<p>![img](</p>
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+<p>![img](</p>
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+<p>![img](</p>
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+<p>![img](</p>
+
+<p>![img](</p>
+
+<p>![img](</p>
+
+<p>![img](</p>
+
+<p>![img](</p>
+
+<p>![img](</p>
+
+<p>![img](</p>
+
+<p>![img](</p>
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+<p>![img](</p>
+
+<p>![img](</p>
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+<p>![img](</p>
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+<p>![img](</p>
+
+<p>![img](</p>
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+<p>![img](</p>
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+<p>![img](</p>
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+<p>![img](</p>
+
+<p>[c](unclosed</p>
+
+<p>[d](<unclosed scheme</p>
+
+<p>[e](a(b)c</p>
+
+<p>A valid <a href="http://example.com">ok</a> link, a <a href="http://example.com" title="Title">titled</a> link, and a <a href="http://example.com/a(b)c">nested</a> one.</p>
diff --git a/test/tm-cases/unclosed_link_parens.text b/test/tm-cases/unclosed_link_parens.text
new file mode 100644
index 00000000..e4b5a263
--- /dev/null
+++ b/test/tm-cases/unclosed_link_parens.text
@@ -0,0 +1,409 @@
+[a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a]([a](
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+![img](
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+![img](
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+[c](unclosed
+
+[d](<unclosed scheme
+
+[e](a(b)c
+
+A valid [ok](http://example.com) link, a [titled](http://example.com "Title") link, and a [nested](http://example.com/a(b)c) one.