Perl Programmers Reference Guide                          sort(3)

NAME
     sort - perl pragma to control sort() behaviour

SYNOPSIS
         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

DESCRIPTION
     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.

perl v5.8.0          Last change: 2002-06-01                    1

Perl Programmers Reference Guide                          sort(3)

CAVEATS
     This pragma is not lexically scoped : its effect is global
     to the program it appears in.  This may change in future
     versions.

perl v5.8.0          Last change: 2002-06-01                    2