It’s SR-6693 <https://bugs.swift.org/browse/SR-6693> (“Collection.underestimatedCount shouldn't call .count when the latter isn't O(1).").
— Daryle Walker Mac, Internet, and Video Game Junkie darylew AT mac DOT com > On Jan 2, 2018, at 11:29 PM, Ben Cohen <ben_co...@apple.com> wrote: > > Hi Daryle, > > Good spot! This is definitely a bug. The documentation on Sequence should > read something different (though what it ought to say is somewhat debatable…). > > Could you file a bug on bugs.swift.org <http://bugs.swift.org/>? > > Thanks! > >> On Dec 29, 2017, at 17:54, Daryle Walker via swift-users >> <swift-users@swift.org <mailto:swift-users@swift.org>> wrote: >> >> Sequence.underestimatedCount is supposed to work at O(1) time, according to >> the docs on Apple’s developer website. On the other hand, Collection.count >> works at O(n) time, except for RandomAccessCollection where it can work at >> O(1) time. I’ve been playing around with creating various Collection types, >> and it seem that their underestimatedCount has a default implementation that >> just dumps to count, even when the latter is O(n)! >> >> I think that underestimatedCount should call count only for >> RandomAccessCollection, while Collection and BidirectionalCollection call >> isEmpty as needed. Is this a good idea? Should I post a bug report >> (somewhere)?
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