From: "HonoreDB (Aaron Weiner)" Date: 2013-11-17T03:48:53+09:00 Subject: [ruby-core:58383] [ruby-trunk - Feature #9118] In Enumerable#to_a, use size to set array capa when possible Issue #9118 has been updated by HonoreDB (Aaron Weiner). It definitely breaks that usage, but that's bad usage--we're supposed to use Enumerable#count for that, not size. In cases where size doesn't correctly predict the array, this doesn't really break anything, it just switches out one bad guess at capa for another. ---------------------------------------- Feature #9118: In Enumerable#to_a, use size to set array capa when possible https://bugs.ruby-lang.org/issues/9118#change-42980 Author: HonoreDB (Aaron Weiner) Status: Open Priority: Normal Assignee: Category: Target version: Cross-post from https://github.com/ruby/ruby/pull/444. Enumerable#to_a works by creating an empty array with small capacity, then populating it and expanding the capacity as it goes. For large enumerables, this causes several resizes, which can hurt performance. When an enumerable exposes a size method, we can guess that the resulting array's size will usually be equal to the enumerable's size. If we're right, we only have to set capacity once, and if we're wrong, we don't lose anything. The attached file (or linked PR) adjusts enum.c's to_a method to take advantage of the size method when it's there. In my tests this makes Range#to_a about 10% faster, and doesn't have any significant effect on a vanilla enum with no size method. I couldn't find any existing benchmark that this consistently made better or worse. If you like this idea, this could also be done in other classes with custom to_a, like Hash. -- http://bugs.ruby-lang.org/