On Wed, Feb 3, 2021 at 8:50 AM Nikita Popov <nikita....@gmail.com> wrote: > > On Wed, Feb 3, 2021 at 4:38 PM Levi Morrison <le...@php.net> wrote: >> >> Hello, everyone! >> >> This proposal adds two new classes to the SPL: >> >> - `Spl\ReverseArrayIterator`. It iterates over an array in reverse >> order. It does not duplicate the array. >> - `Spl\ForwardArrayIterator`. It iterates over an array in forward >> (normal) order. It does not duplicate the array. >> >> They both implement Countable which returns the `count()` of the >> array. The [PR][1] has some examples and discusses why I am proposing >> `ForwardArrayIterator` when there is already `ArrayIterator`, the >> short of which is for performance. There are timing numbers in [one of >> the comments][2]. >> >> When it comes time to vote I may merge this into another RFC with >> [`CachedIterable` by Tyson Andre][3], which I recommend readers also >> take a look at. Whether we team up for the RFC vote or not, I wanted >> to get this out there for discussion and review. >> >> [1]: https://github.com/php/php-src/pull/6535 >> [2]: https://github.com/php/php-src/pull/6535#issuecomment-769179450 >> [3]: https://github.com/php/php-src/pull/6655 > > > Hey Levi, > > I like the general idea of having an "ArrayIterator but sane". > > That said, I don't think that the ReverseArrayIterator + ForwardArrayIterator > pair of iterators approaches this problem correctly. There are plenty of > iterators that could be run in reverse, and I think it would be silly to > create two classes for each of them. E.g. if we introduce an > ObjectPropertyIterator, should there be both ForwardObjectPropertyIterator > and ReverseObjectPropertyIterator? I don't think so. > > I think the correct abstraction for bidirectional iterators is to introduce > an interface > > // Or "ReversibleIterator" > interface BidrectionalIterator extends Iterator { > public function prev(): void; > public function end(): void; > } > > and then a class along the lines of: > > class ReverseIterator implements BidirectionalIterator { > public function __construct(private BidirectionalIterator $iter) {} > > public function next() { $this->iter->prev(); } > // etc. > } > > This would replace "new ReverseArrayIterator($array)" with "new > ReverseIterator(new ArrayIterator($array))", but in a way that is general, > and composes. > > Regards, > Nikita
I wrote an implementation of this idea two years ago: https://github.com/php/php-src/compare/master...morrisonlevi:BidirectionalArrayIterator It's fine for arrays, but not all structures that you can iterate through forward or reverse. For instance, tree structures can be traversed forward or reverse, but bidirectionally is far more difficult (and I did not find any pre-existing solutions in this space, btw). I do have repositories with tree structures, so it's not just theoretical. Another factor for abandonment was ergonomics. This is not great: `new ReverseIterator(new BidirectionalArrayIterator($array))`. It's mildly less performant as well, having to create two objects, and then having delegation through methods. Let me know if you think of any way to reconcile these issues, and I'll do the same. -- PHP Internals - PHP Runtime Development Mailing List To unsubscribe, visit: https://www.php.net/unsub.php
