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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 'defaults';	  # revert to default behavior
      no  sort 'stable';	  # stability not important

      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 arrays of a few distinct values, repeated many times.	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.  The default algorithm is mergesort, which will be stable even if
  you do not explicitly demand it.  But the stability of the default sort is
  a side-effect that could change in later versions.  If stability is
  important, be sure to say so with a

    use sort 'stable';

  The "no sort" pragma doesn't forbid what follows, it just leaves the choice
  open.	 Thus, after

    no sort qw(_mergesort stable);

  a mergesort, which happens to be stable, will be employed anyway.  Note
  that

    no sort "_quicksort";
    no sort "_mergesort";

  have exactly the same effect, leaving the choice of sort algorithm open.

CAVEATS
  This pragma is not lexically scoped: its effect is global to the program it
  appears in.  That means the following will probably not do what you expect,
  because both pragmas take effect at compile time, before either "sort()"
  happens.

    { use sort "_quicksort";
      print sort::current . "\n";
      @a = sort @b;
    }
    { use sort "stable";
      print sort::current . "\n";
      @c = sort @d;
    }
    # prints:
    # quicksort stable
    # quicksort stable

  You can achieve the effect you probably wanted by using "eval()" to defer
  the pragmas until run time.  Use the quoted argument form of "eval()", not
  the BLOCK form, as in

    eval { use sort "_quicksort" }; # WRONG

  or the effect will still be at compile time.	Reset to default options
  before selecting other subpragmas (in case somebody carelessly left them
  on) and after sorting, as a courtesy to others.

    { eval 'use sort qw(defaults _quicksort)'; # force quicksort
      eval 'no sort "stable"';	    # stability not wanted
      print sort::current . "\n";
      @a = sort @b;
      eval 'use sort "defaults"';   # clean up, for others
    }
    { eval 'use sort qw(defaults stable)';     # force stability
      print sort::current . "\n";
      @c = sort @d;
      eval 'use sort "defaults"';   # clean up, for others
    }
    # prints:
    # quicksort
    # stable

  Scoping for this pragma may change in future versions.

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