gh-119105: Differ.compare is too slow [for degenerate cases]#119492
Merged
Uh oh!
There was an error while loading. Please reload this page.
Conversation
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters. Learn more about bidirectional Unicode characters
…window of 10 lines, and picks "the first" pair within the current window with a ratio > cutoff to synch on. This should make cubic-time cases impossible. It may change results in some cases, but I don't think I care - difflib is fundamentally opposed to synching on lines "far apart", and using a window enforces a notion of locality.
Stop tresting junk lines specially - if we synch on junk, so be it - it almost never occurs in real life anyway.
estyxx pushed a commit to estyxx/cpython that referenced this pull request Jul 17, 2024
…ython#119492) ``_fancy_replace()`` is no longer recursive. and a single call does a worst-case linear number of ratio() computations instead of quadratic. This renders toothless a universe of pathological cases. Some inputs may produce different output, but that's rare, and I didn't find a case where the final diff appeared to be of materially worse quality. To the contrary, by refusing to even consider synching on lines "far apart", there was more easy-to-digest locality in the output.
adacore-bot pushed a commit to AdaCore/e3-aws that referenced this pull request Dec 11, 2025
From python/cpython#119492, "This may change results in some cases, but no natural ones come to mind, and I don't care about contrived cases." and also "Correctness of diffs is not affected. Some similar lines far apart may be reported as deleting one and adding the other, where before they were displayed on adjacent output lines with markup showing the intraline differences.". This change breaks the test for Python3.14
Sign up for freeto join this conversation on GitHub. Already have an account? Sign in to comment
1 participant
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
Make it impossible for
Differ.compare()to degenerate to cubic-time. This may change results in some cases, but no natural ones come to mind, and I don't care about contrived cases.