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clojure.data/diff uses set union on key seqs

Description

clojure.data/diff, on line 118, defines:

java.util.Map (diff-similar [a b] (diff-associative a b (set/union (keys a) (keys b))))

Since keys returns a key seq, this seems like an error. clojure.set/union has strange and inconsistent behavior with regard to non-sets, and in this case the two key seqs are concatenated. Based on a cursory benchmark, it seems that this bug is a slight performance gain when the maps have no common keys, and a significant performance loss when the maps have the same keys. The results are still correct because of the merging reduce in diff-associative.

The patch is easy (just call set on each key seq).

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Andy Fingerhut June 5, 2015 at 8:03 AM

I would suggest that perhaps bigger than the issue of performance here is that Clojure is relying on its own undocumented behavior that clojure.set/union works on some non-set arguments.

Andy Fingerhut October 15, 2012 at 8:52 PM

clj-1087-diff-perf-enhance-patch-v1.txt dated Oct 15 2012 implements Tom's suggested performance enhancement, although not exactly in the way he suggested. It does calculate the union of the two key sequences.

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Created October 15, 2012 at 9:59 AM
Updated June 5, 2015 at 8:03 AM