Re: [RFC] Make sorting stable

On Tue, May 12, 2020 at 4:29 PM Nicolas Grekas <nicolas.grekas+php@gmail.com> wrote: > Hi Nikita, > > Thanks for another nice RFC. > > This was previously discussed in > https://externals.io/message/108841, but >> I >> figured it would make sense to create an RFC for this change: >> https://wiki.php.net/rfc/stable_sorting >> >> As before, the implementation approach is to stick with the existing qsort >> and use a fallback comparison criterion, which is cheap to implement >> internally. The obvious alternative is to use a stable base sort like >> Timsort. I gave this a quick try in >> https://github.com/php/php-src/pull/5559, >> and it does better in some cases (already sorted data) and worse in others >> (random data). I don't plan to pursue this direction personally. (It may >> also be interesting to use pdqsort as the unstable base, which should make >> the "already sorted" case faster.) >> > > IIRC, in PHP 5 sorting was stable. Then PHP 7 made it unstable. That was > something to account for when migrating - BC break inside. > In PHP 5 sorting was unstable. In PHP 7 sorting became stable for arrays with 16 elements or less. In PHP 8 sorting would become stable for *all* arrays. > On my side, I would very much prefer a new sorting flag, to be 200% sure > of the behavior: > sort($array, SORT_STABLE) > > That would allow ppl that don't care about stability to let the engine go > with the fastest algorithm. > I'd prefer to avoid an extra option. PHP is a high-level language, and the user should not have to concern themselves with sort stability considerations. I'd go for a flag if stable sorting had a huge perf impact. But as it doesn't, I don't see the point in an option. Regards, Nikita