compute fast-forward diffs much more efficiently #5989
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aryairani
approved these changes
Nov 10, 2025
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Overview
This PR swaps out the naive implementation of
diff.branch foo barwhenfooandbareither don't share a history at all (in which case we treatfooas the LCA), or when the LCA is eitherfooorbar(a fast-forward).Previously, we'd simply not bother to compare the LCA causal to either branch, and proceed to do a lot of unnecessary work in the case that it matches one of them. Now, we are careful to avoid any duplicate work (loading definitions, computing syntactic hashes, etc).
Test coverage
There's an existing transcript for this feature. I also tested manually at the command line.