package sort;
-our $VERSION = '1.00';
+our $VERSION = '2.01';
-$sort::hint_bits = 0x00020000; # HINT_LOCALIZE_HH, really...
+# 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;
require Carp;
Carp::croak("sort pragma requires arguments");
}
- $^H |= $sort::hint_bits;
local $_;
- no warnings 'uninitialized'; # $^H{SORT} bitops would warn
+ $^H{sort} //= 0;
while ($_ = shift(@_)) {
if (/^_q(?:uick)?sort$/) {
- $^H{SORT} &= ~$sort::sort_bits;
- $^H{SORT} |= $sort::quicksort_bit;
+ $^H{sort} &= ~$sort::sort_bits;
+ $^H{sort} |= $sort::quicksort_bit;
} elsif ($_ eq '_mergesort') {
- $^H{SORT} &= ~$sort::sort_bits;
- $^H{SORT} |= $sort::mergesort_bit;
+ $^H{sort} &= ~$sort::sort_bits;
+ $^H{sort} |= $sort::mergesort_bit;
} elsif ($_ eq 'stable') {
- $^H{SORT} |= $sort::stable_bit;
+ $^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} & $sort::stable_bit;
+ if ($^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);
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
+ my $current;
+ BEGIN {
+ $current = sort::current(); # identify prevailing algorithm
+ }
=head1 DESCRIPTION
-With the sort pragma you can control the behaviour of the builtin
-sort() function.
+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 sort(), but in Perl 5.8 a mergesort algorithm was also made
+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
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
+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. This may change in future versions.
+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 { $current = print sort::current; }
+ print "$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 { $current = print sort::current; }
+ print "$current\n";
+ @c = sort @d;
+ }
=cut
https://perl5.git.perl.org / perl5.git / blobdiff