perl/lib/sort.pm

package sort;

our $VERSION = '1.02';

# Currently the hints for pp_sort are stored in the global variable
# $sort::hints. An improvement would be to store them in $^H{SORT} and have
# this information available somewhere in the listop OP_SORT, to allow lexical
# scoping of this pragma. -- rgs 2002-04-30

our $hints             = 0;

$sort::quicksort_bit   = 0x00000001;
$sort::mergesort_bit   = 0x00000002;
$sort::sort_bits       = 0x000000FF; # allow 256 different ones
$sort::stable_bit      = 0x00000100;

use strict;

sub import {
    shift;
    if (@_ == 0) {
        require Carp;
        Carp::croak("sort pragma requires arguments");
    }
    local $_;
    no warnings 'uninitialized';        # bitops would warn
    while ($_ = shift(@_)) {
        if (/^_q(?:uick)?sort$/) {
            $hints &= ~$sort::sort_bits;
            $hints |=  $sort::quicksort_bit;
        } elsif ($_ eq '_mergesort') {
            $hints &= ~$sort::sort_bits;
            $hints |=  $sort::mergesort_bit;
        } elsif ($_ eq 'stable') {
            $hints |=  $sort::stable_bit;
        } elsif ($_ eq 'defaults') {
            $hints =   0;
        } else {
            require Carp;
            Carp::croak("sort: unknown subpragma '$_'");
        }
    }
}

sub unimport {
    shift;
    if (@_ == 0) {
        require Carp;
        Carp::croak("sort pragma requires arguments");
    }
    local $_;
    no warnings 'uninitialized';        # bitops would warn
    while ($_ = shift(@_)) {
        if (/^_q(?:uick)?sort$/) {
            $hints &= ~$sort::sort_bits;
        } elsif ($_ eq '_mergesort') {
            $hints &= ~$sort::sort_bits;
        } elsif ($_ eq 'stable') {
            $hints &= ~$sort::stable_bit;
        } else {
            require Carp;
            Carp::croak("sort: unknown subpragma '$_'");
        }
    }
}

sub current {
    my @sort;
    if ($hints) {
        push @sort, 'quicksort' if $hints & $sort::quicksort_bit;
        push @sort, 'mergesort' if $hints & $sort::mergesort_bit;
        push @sort, 'stable'    if $hints & $sort::stable_bit;
    }
    push @sort, 'mergesort' unless @sort;
    join(' ', @sort);
}

1;
__END__

=head1 NAME

sort - perl pragma to control sort() behaviour

=head1 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

=head1 DESCRIPTION

With the C<sort> pragma you can control the behaviour of the builtin
C<sort()> function.

In Perl versions 5.6 and earlier the quicksort algorithm was used to
implement C<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
C<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 C<_> 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 C<no sort> pragma doesn't
I<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.

=head1 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 I<both> pragmas take effect at compile time, before
I<either> C<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 C<eval()>
to defer the pragmas until run time.  Use the quoted argument
form of C<eval()>, I<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.

=cut

Revision:	1.1
Committed:	Thu Jun 30 14:27:09 2005 UTC (21 years, 3 months ago) by root
Branch:	MAIN
CVS Tags:	PERL-5-8-7, HEAD
Branch point for:	PERL
Log Message:	* empty log message *
#	User	Rev	Content
1	root	1.1	package sort;
2
3			our $VERSION = '1.02';
4
5			# Currently the hints for pp_sort are stored in the global variable
6			# $sort::hints. An improvement would be to store them in $^H{SORT} and have
7			# this information available somewhere in the listop OP_SORT, to allow lexical
8			# scoping of this pragma. -- rgs 2002-04-30
9
10			our $hints = 0;
11
12			$sort::quicksort_bit = 0x00000001;
13			$sort::mergesort_bit = 0x00000002;
14			$sort::sort_bits = 0x000000FF; # allow 256 different ones
15			$sort::stable_bit = 0x00000100;
16
17			use strict;
18
19			sub import {
20			shift;
21			if (@_ == 0) {
22			require Carp;
23			Carp::croak("sort pragma requires arguments");
24			}
25			local $_;
26			no warnings 'uninitialized'; # bitops would warn
27			while ($_ = shift(@_)) {
28			if (/^_q(?:uick)?sort$/) {
29			$hints &= ~$sort::sort_bits;
30			$hints \|= $sort::quicksort_bit;
31			} elsif ($_ eq '_mergesort') {
32			$hints &= ~$sort::sort_bits;
33			$hints \|= $sort::mergesort_bit;
34			} elsif ($_ eq 'stable') {
35			$hints \|= $sort::stable_bit;
36			} elsif ($_ eq 'defaults') {
37			$hints = 0;
38			} else {
39			require Carp;
40			Carp::croak("sort: unknown subpragma '$_'");
41			}
42			}
43			}
44
45			sub unimport {
46			shift;
47			if (@_ == 0) {
48			require Carp;
49			Carp::croak("sort pragma requires arguments");
50			}
51			local $_;
52			no warnings 'uninitialized'; # bitops would warn
53			while ($_ = shift(@_)) {
54			if (/^_q(?:uick)?sort$/) {
55			$hints &= ~$sort::sort_bits;
56			} elsif ($_ eq '_mergesort') {
57			$hints &= ~$sort::sort_bits;
58			} elsif ($_ eq 'stable') {
59			$hints &= ~$sort::stable_bit;
60			} else {
61			require Carp;
62			Carp::croak("sort: unknown subpragma '$_'");
63			}
64			}
65			}
66
67			sub current {
68			my @sort;
69			if ($hints) {
70			push @sort, 'quicksort' if $hints & $sort::quicksort_bit;
71			push @sort, 'mergesort' if $hints & $sort::mergesort_bit;
72			push @sort, 'stable' if $hints & $sort::stable_bit;
73			}
74			push @sort, 'mergesort' unless @sort;
75			join(' ', @sort);
76			}
77
78			1;
79			__END__
80
81			=head1 NAME
82
83			sort - perl pragma to control sort() behaviour
84
85			=head1 SYNOPSIS
86
87			use sort 'stable'; # guarantee stability
88			use sort '_quicksort'; # use a quicksort algorithm
89			use sort '_mergesort'; # use a mergesort algorithm
90			use sort 'defaults'; # revert to default behavior
91			no sort 'stable'; # stability not important
92
93			use sort '_qsort'; # alias for quicksort
94
95			my $current = sort::current(); # identify prevailing algorithm
96
97			=head1 DESCRIPTION
98
99			With the C<sort> pragma you can control the behaviour of the builtin
100			C<sort()> function.
101
102			In Perl versions 5.6 and earlier the quicksort algorithm was used to
103			implement C<sort()>, but in Perl 5.8 a mergesort algorithm was also made
104			available, mainly to guarantee worst case O(N log N) behaviour:
105			the worst case of quicksort is O(N**2). In Perl 5.8 and later,
106			quicksort defends against quadratic behaviour by shuffling large
107			arrays before sorting.
108
109			A stable sort means that for records that compare equal, the original
110			input ordering is preserved. Mergesort is stable, quicksort is not.
111			Stability will matter only if elements that compare equal can be
112			distinguished in some other way. That means that simple numerical
113			and lexical sorts do not profit from stability, since equal elements
114			are indistinguishable. However, with a comparison such as
115
116			{ substr($a, 0, 3) cmp substr($b, 0, 3) }
117
118			stability might matter because elements that compare equal on the
119			first 3 characters may be distinguished based on subsequent characters.
120			In Perl 5.8 and later, quicksort can be stabilized, but doing so will
121			add overhead, so it should only be done if it matters.
122
123			The best algorithm depends on many things. On average, mergesort
124			does fewer comparisons than quicksort, so it may be better when
125			complicated comparison routines are used. Mergesort also takes
126			advantage of pre-existing order, so it would be favored for using
127			C<sort()> to merge several sorted arrays. On the other hand, quicksort
128			is often faster for small arrays, and on arrays of a few distinct
129			values, repeated many times. You can force the
130			choice of algorithm with this pragma, but this feels heavy-handed,
131			so the subpragmas beginning with a C<_> may not persist beyond Perl 5.8.
132			The default algorithm is mergesort, which will be stable even if
133			you do not explicitly demand it.
134			But the stability of the default sort is a side-effect that could
135			change in later versions. If stability is important, be sure to
136			say so with a
137
138			use sort 'stable';
139
140			The C<no sort> pragma doesn't
141			I<forbid> what follows, it just leaves the choice open. Thus, after
142
143			no sort qw(_mergesort stable);
144
145			a mergesort, which happens to be stable, will be employed anyway.
146			Note that
147
148			no sort "_quicksort";
149			no sort "_mergesort";
150
151			have exactly the same effect, leaving the choice of sort algorithm open.
152
153			=head1 CAVEATS
154
155			This pragma is not lexically scoped: its effect is global to the program
156			it appears in. That means the following will probably not do what you
157			expect, because I<both> pragmas take effect at compile time, before
158			I<either> C<sort()> happens.
159
160			{ use sort "_quicksort";
161			print sort::current . "\n";
162			@a = sort @b;
163			}
164			{ use sort "stable";
165			print sort::current . "\n";
166			@c = sort @d;
167			}
168			# prints:
169			# quicksort stable
170			# quicksort stable
171
172			You can achieve the effect you probably wanted by using C<eval()>
173			to defer the pragmas until run time. Use the quoted argument
174			form of C<eval()>, I<not> the BLOCK form, as in
175
176			eval { use sort "_quicksort" }; # WRONG
177
178			or the effect will still be at compile time.
179			Reset to default options before selecting other subpragmas
180			(in case somebody carelessly left them on) and after sorting,
181			as a courtesy to others.
182
183			{ eval 'use sort qw(defaults _quicksort)'; # force quicksort
184			eval 'no sort "stable"'; # stability not wanted
185			print sort::current . "\n";
186			@a = sort @b;
187			eval 'use sort "defaults"'; # clean up, for others
188			}
189			{ eval 'use sort qw(defaults stable)'; # force stability
190			print sort::current . "\n";
191			@c = sort @d;
192			eval 'use sort "defaults"'; # clean up, for others
193			}
194			# prints:
195			# quicksort
196			# stable
197
198			Scoping for this pragma may change in future versions.
199
200			=cut
201