[naaz234]

Description

ReferenceProcessor.run records one reference definition, puts the rest of the block back at the front of blocks and returns. The next pass runs every block processor's test over the remainder again, and BlockQuoteProcessor, HashHeaderProcessor and HRProcessor search the whole block, so a block of n consecutive definitions costs O(n²). On CPython 3.12, 8000 definitions (247 KiB) take 18 s to convert with no extensions. FootnoteBlockProcessor.run in the footnotes extension has the same one-definition-per-pass shape, and 8000 footnote definitions (177 KiB) take 14 s.

Both processors now record a run of definitions in one pass. Only a definition that is directly followed by another definition is taken early. The last one, and any definition that is indented or spans lines, is left for the next pass as before, because the block starting at it can belong to another processor: an indented code block when tab_length is below 4, or a setext header such as [b]: /url followed by === (covered by a new test). The same inputs now take 0.01 s and 0.23 s.

Output is identical to master over about 1.2 million randomly generated documents rendered with no extensions, extra, footnotes, tables, def_list, admonition, and tab_length of 2 and 3. Definitions separated by other text are still handled one per pass, since what follows a definition can change how the earlier processors treat the block.

Timing script used:

import time, markdown
for n in (2000, 4000, 8000):
    s = ''.join(f'[{i}]: http://example.com/{i}\n' for i in range(n))
    t = time.perf_counter(); markdown.markdown(s); print('references', n, round(time.perf_counter() - t, 2))
md = markdown.Markdown(extensions=['footnotes'])
for n in (2000, 4000, 8000):
    s = ''.join(f'[^{i}]: Footnote {i}\n' for i in range(n))
    md.reset(); t = time.perf_counter(); md.convert(s); print('footnotes', n, round(time.perf_counter() - t, 2))

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 the randomized output comparison against master

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.