2016-06-18 10:23 GMT+02:00 Dmitri Gribenko <griboz...@gmail.com>: > On Fri, Jun 17, 2016 at 12:37 PM, Svein Halvor Halvorsen > <svei...@lvor.halvorsen.cc> wrote: > > Ok. Good to hear that I'm not too far off :) > > > > Can you provide me with an example of a sequence type or two that has > some > > map or other function that would benefit from an optimized wrapper? Or: > > under what circumstances would the stdlib implementation outperform mine? > > Compare the default implementations of map() for Sequence and > Collection. In the Sequence case, we don't know the size of the > resulting array, so we have to grow the resulting array as we pull the > elements from the sequence. In the case of running Collection.map() > we know the final size from the count property. In the case of Array > we can do even better and eliminate a check (the _expectEnd() > call). However, certain collections where calculating the number of > elements might be expensive, can opt into the Sequence behavior (e.g., > various string views); this is up to the designer of the specific > collection.
Is this a problem in reality? The map() on Sequence uses underestimatedCount to reserve capacity, and if the sequence wraps a collection type, underestimatedCount is presumably O(1) and returns the same value as count, non?
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