[deadstrobe5]

Description

The FencedBlockPreprocessor rebuilt the whole document text after every fenced code block it stashed (text = f'{text[:m.start()]}\n{placeholder}\n{text[m.end():]}'), so its cost grew with document size × number of blocks. This collects the output in pieces and joins them once.

Matching is unchanged: the search now resumes at m.end() in the original text instead of just after the placeholder in the rewritten text. The two differ only by the inserted \n, and since the fence pattern ends in [ ]*$, text[m.end()] is always \n or the end of the text, so no fence can begin at that position either way.

The project's own docs/*.md concatenated (250 KB, ~220 fenced blocks), and the same text ×4 (1 MB). Python 3.12, Apple M5, 7 alternating processes per side:

Case	Before	After
preprocessor alone, 250 KB	10.6 ms	1.95 ms (5.4x)
preprocessor alone, 1 MB	79.4 ms	8.2 ms (9.7x)
markdown(text, extensions=['extra']), 250 KB	81.9 ms	73.8 ms (~10%)
markdown(text, extensions=['extra']), 1 MB	371 ms	304 ms (~18%)
extra on a 10 KB document	2.40 ms	2.37 ms (unchanged)

Re-measured on CPython 3.14.7 (7 alternating processes per side, median): the docs concatenated with the extra extensions went from 89.3 ms to 78.7 ms (1.14x).

Output is byte-identical to the current code (same SHA-256) for 169 files (all tests/**/*.txt fixtures and docs/**/*.md) × 4 extension sets, plus 12,000 randomized documents with fence edge cases (unterminated fences, invalid attribute lists, missing trailing newline, fences inside blockquotes and lists) under fenced_code, fenced_code + attr_list, and extra. python -m unittest discover tests: 1127 OK; flake8 clean on the changed file. No new test, since the output does not change and the existing fenced_code tests cover the paths touched.

Sorry for dropping the template the first time.

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: the benchmark and the differential check (Claude Code)

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]

You deleted the PR template. Please provide the requested information or this issue will be closed.

[deadstrobe5]

Filled in the template, sorry about that.

[waylan]

The code looks good. I do find it curious that no tests are provided. Are we certain that the existing tests properly cover this?

Also, there is a merge conflict to resolve with the changelog.

[deadstrobe5]

Fair question. I checked it by mutating the new loop six ways (dropping the newline before or after the placeholder, dropping the text after the last block, resuming one character late, stopping after the first block, and not advancing the slice start): the existing test_fenced_code.py fails on every one, so the current tests do exercise this path.

Still, nothing in there says it directly, so I added four tests in db5968a: text before, between and after two blocks, a block that opens on the line right after another one closes, a block that ends the document without a trailing newline, and fifty blocks in one document. They pass on master as well, since the output is unchanged.

The changelog conflict is fixed too (merged master, entry kept under Unreleased / Fixed).

[waylan]

Thanks for doing the work on this.