zstd: Fix corrupted output in "best" #876
Merged
+6
−5
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Regression from #784 and followup #793
Fixes #875
A 0 offset backreference was possible when "improve" was successful twice in a row in the "skipBeginning" part, only finding 2 (previously unmatched) length 4 matches, but where start offset decreased by 2 in both cases.
This would result in output where the end offset would equal to the next 's', thereby doing a self-reference.
Add a general check in "improve" and just reject these. Will also guard against similar issues in the future.
This also hints at some potentially suboptimal hash indexing - but I will take that improvement separately.
Fuzz test set updated.