[naaz234]

Description

LinkInlineProcessor.getText (used by the link, image and reference processors) scans forward from a [ until the brackets balance. The treeprocessor calls it for every [ in the text, and when the brackets never close each call scans to the end, so conversion time grows with the square of the number of unclosed brackets. Timings for markdown.markdown(s * n) on CPython 3.11:

'[['   x8000  (16000 bytes): 55.91s -> 0.02s
'!['   x8000  (16000 bytes): 28.95s -> 0.01s
'[ ['  x8000  (24000 bytes): 68.13s -> 0.03s

This now finds the closing bracket for every index in one pass and caches the result for the current text, so repeated calls on the same text are a list lookup. I compared the new getText with the old loop on every index of about 30,000 random strings made of [, ], a and ( (about 400,000 calls) and the results were identical. The new tests pass with and without the change. test_many_unclosed_square_brackets takes about 27 seconds without the fix and finishes instantly with it.

getLink has a similar rescan for unclosed ( but its quote/title handling is more involved, so I left it out of this change.

AI Assistance Disclosure

• No AI tools were used in preparing this PR.

• AI tools were used and I have disclosed which ones, and have fully reviewed and verified their output.

  AI tools were use to assist with the following:

  • Code
  • Code Comments
  • Documentation
  • Changelog Entry
  • Commit Message
  • The Description Above
  • Other: tests, and finding the slow input with a timing script

  Tool used: Claude Code (Claude Opus 5.5). wrote the patch, tests, changelog entry, commit message and this description. I reviewed and checked all of it before submitting.

Checklist

• This PR follows the contribution guidelines.
• The code follows the Code Style Guide.
• The commit message follows the Commit Message Style Guide.
• I have added or updated relevant docs, including release notes if applicable which follow the Documentation Style Guide.
• I have added or updated relevant tests.
• I have not requested, and will not request, an automated AI review for this PR.

[waylan]

The spell checker seems to not like the word "rescan." It should probably be added to .spell-dict.

[waylan]

I am going to let @facelessuser review this as he is more familiar with the affected code.