FM / perl5

/usr/share/perl5

Name	Size	Mode	Actions
B/	-	0755	rm
Class/	-	0755	rm
Config/	-	0755	rm
DBM_Filter/	-	0755	rm
encoding/	-	0755	rm
ExtUtils/	-	0755	rm
File/	-	0755	rm
Getopt/	-	0755	rm
I18N/	-	0755	rm
IPC/	-	0755	rm
Math/	-	0755	rm
Net/	-	0755	rm
overload/	-	0755	rm
Pod/	-	0755	rm
pod/	-	0755	rm
Search/	-	0755	rm
Term/	-	0755	rm
Text/	-	0755	rm
Thread/	-	0755	rm
Tie/	-	0755	rm
Time/	-	0755	rm
Unicode/	-	0755	rm
unicore/	-	0755	rm
URI/	-	0755	rm
User/	-	0755	rm
vendor_perl/	-	0755	rm
warnings/	-	0755	rm
AnyDBM_File.pm	2618	0644	edit dl rm
AutoLoader.pm	15797	0644	edit dl rm
AutoSplit.pm	19637	0644	edit dl rm
autouse.pm	4238	0644	edit dl rm
base.pm	10980	0644	edit dl rm
Benchmark.pm	31025	0644	edit dl rm
blib.pm	2086	0644	edit dl rm
bytes.pm	3754	0644	edit dl rm
bytes_heavy.pl	758	0644	edit dl rm
charnames.pm	20867	0644	edit dl rm
CORE.pod	3188	0644	edit dl rm
DB.pm	18922	0644	edit dl rm
DBM_Filter.pm	14385	0644	edit dl rm
deprecate.pm	3079	0644	edit dl rm
diagnostics.pm	19038	0644	edit dl rm
DirHandle.pm	1556	0644	edit dl rm
Dumpvalue.pm	17556	0644	edit dl rm
dumpvar.pl	15555	0644	edit dl rm
English.pm	4761	0644	edit dl rm
feature.pm	17085	0644	edit dl rm
fields.pm	9489	0644	edit dl rm
FileCache.pm	5572	0644	edit dl rm
FileHandle.pm	6784	0644	edit dl rm
filetest.pm	4003	0644	edit dl rm
FindBin.pm	4561	0644	edit dl rm
if.pm	3340	0644	edit dl rm
integer.pm	3254	0644	edit dl rm
Internals.pod	2576	0644	edit dl rm
less.pm	3204	0644	edit dl rm
locale.pm	4855	0644	edit dl rm
meta_notation.pm	2117	0644	edit dl rm
NEXT.pm	18846	0644	edit dl rm
overload.pm	53314	0644	edit dl rm
overloading.pm	1808	0644	edit dl rm
perl5db.pl	316420	0644	edit dl rm
PerlIO.pm	10459	0644	edit dl rm
Safe.pm	25082	0644	edit dl rm
SelectSaver.pm	1076	0644	edit dl rm
sigtrap.pm	7606	0644	edit dl rm
sort.pm	6081	0644	edit dl rm
strict.pm	4738	0644	edit dl rm
subs.pm	848	0644	edit dl rm
Symbol.pm	4799	0644	edit dl rm
Thread.pm	8287	0644	edit dl rm
UNIVERSAL.pm	6594	0644	edit dl rm
URI.pm	34790	0644	edit dl rm
utf8.pm	9116	0644	edit dl rm
utf8_heavy.pl	31615	0644	edit dl rm
vars.pm	2414	0644	edit dl rm
vmsish.pm	4313	0644	edit dl rm
warnings.pm	44756	0644	edit dl rm
XSLoader.pm	11267	0644	edit dl rm
_charnames.pm	33166	0644	edit dl rm

Edit: /usr/share/perl5/sort.pm (6081B)

package sort;

our $VERSION = '2.02';

# The hints for pp_sort are now stored in $^H{sort}; older versions
# of perl used the global variable $sort::hints. -- rjh 2005-12-19

$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 $_;
    $^H{sort} //= 0;
    while ($_ = shift(@_)) {
	if (/^_q(?:uick)?sort$/) {
	    $^H{sort} &= ~$sort::sort_bits;
	    $^H{sort} |=  $sort::quicksort_bit;
	} elsif ($_ eq '_mergesort') {
	    $^H{sort} &= ~$sort::sort_bits;
	    $^H{sort} |=  $sort::mergesort_bit;
	} elsif ($_ eq 'stable') {
	    $^H{sort} |=  $sort::stable_bit;
	} elsif ($_ eq 'defaults') {
	    $^H{sort} =   0;
	} else {
	    require Carp;
	    Carp::croak("sort: unknown subpragma '$_'");
	}
    }
}

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$/) {
	    $^H{sort} &= ~$sort::sort_bits;
	} elsif ($_ eq '_mergesort') {
	    $^H{sort} &= ~$sort::sort_bits;
	} elsif ($_ eq 'stable') {
	    $^H{sort} &= ~$sort::stable_bit;
	} else {
	    require Carp;
	    Carp::croak("sort: unknown subpragma '$_'");
	}
    }
}

sub current {
    my @sort;
    if ($^H{sort}) {
	push @sort, 'quicksort' if $^H{sort} & $sort::quicksort_bit;
	push @sort, 'mergesort' if $^H{sort} & $sort::mergesort_bit;
	push @sort, 'stable'    if $^H{sort} & $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;
    BEGIN {
	$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

As of Perl 5.10, this pragma is lexically scoped and takes effect
at compile time. In earlier versions its effect was global and took
effect at run-time; the documentation suggested using C<eval()> to
change the behaviour:

{ 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
  }

Such code no longer has the desired effect, for two reasons.
Firstly, the use of C<eval()> means that the sorting algorithm
is not changed until runtime, by which time it's too late to
have any effect. Secondly, C<sort::current> is also called at
run-time, when in fact the compile-time value of C<sort::current>
is the one that matters.

So now this code would be written:

{ use sort qw(defaults _quicksort); # force quicksort
    no sort "stable";      # stability not wanted
    my $current;
    BEGIN { $current = sort::current; }
    print "$current\n";
    @a = sort @b;
    # Pragmas go out of scope at the end of the block
  }
  { use sort qw(defaults stable);     # force stability
    my $current;
    BEGIN { $current = sort::current; }
    print "$current\n";
    @c = sort @d;
  }

=cut

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