sort - perl pragma to control sort() behaviour
use sort 'stable'; # guarantee stability
use sort '_quicksort'; # use a quicksort algorithm
use sort '_mergesort'; # use a mergesort algorithm
use sort '_qsort'; # alias for quicksort
my $current = sort::current(); # identify prevailing algorithm
With the sort pragma you can control the behaviour of the builtin sort() function.
In Perl versions 5.6 and earlier the quicksort algorithm was used to implement sort(), but in Perl 5.8 a mergesort algorithm was also made available, mainly to guarantee worst case O(N log N) behaviour: the worst case of quicksort is O(N**2). In Perl 5.8 and later, quicksort defends against quadratic behaviour by shuffling large arrays before sorting.
A stable sort means that for records that compare equal, the original input ordering is preserved. Mergesort is stable, quicksort is not. Stability will matter only if elements that compare equal can be distinguished in some other way. That means that simple numerical and lexical sorts do not profit from stability, since equal elements are indistinguishable. However, with a comparison such as
{ substr($a, 0, 3) cmp substr($b, 0, 3) }
stability might matter because elements that compare equal on the first 3 characters may be distinguished based on subsequent characters. In Perl 5.8 and later, quicksort can be stabilized, but doing so will add overhead, so it should only be done if it matters.
The best algorithm depends on many things. On average, mergesort
does fewer comparisons than quicksort, so it may be better when
complicated comparison routines are used. Mergesort also takes
advantage of pre-existing order, so it would be favored for using
sort to merge several sorted arrays. On the other hand, quicksort
is often faster for small arrays, and on platforms with small memory
caches that are much faster than main memory. You can force the
choice of algorithm with this pragma, but this feels heavy-handed,
so the subpragmas beginning with a _ may not persist beyond Perl 5.8.
This pragma is not lexically scoped : its effect is global to the program it appears in. This may change in future versions.