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1 | package sort; |
2 | |
3 | our $VERSION = '1.00'; |
4 | |
5 | $sort::hint_bits = 0x00020000; # HINT_LOCALIZE_HH, really... |
6 | |
7 | $sort::quicksort_bit = 0x00000001; |
8 | $sort::mergesort_bit = 0x00000002; |
9 | $sort::sort_bits = 0x000000FF; # allow 256 different ones |
10 | $sort::stable_bit = 0x00000100; |
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11 | |
12 | use strict; |
13 | |
14 | sub import { |
15 | shift; |
16 | if (@_ == 0) { |
17 | require Carp; |
18 | Carp::croak("sort pragma requires arguments"); |
19 | } |
20 | $^H |= $sort::hint_bits; |
21 | local $_; |
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22 | no warnings 'uninitialized'; # $^H{SORT} bitops would warn |
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23 | while ($_ = shift(@_)) { |
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24 | if (/^_q(?:uick)?sort$/) { |
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25 | $^H{SORT} &= ~$sort::sort_bits; |
26 | $^H{SORT} |= $sort::quicksort_bit; |
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27 | } elsif ($_ eq '_mergesort') { |
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28 | $^H{SORT} &= ~$sort::sort_bits; |
29 | $^H{SORT} |= $sort::mergesort_bit; |
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30 | } elsif ($_ eq 'stable') { |
31 | $^H{SORT} |= $sort::stable_bit; |
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32 | } else { |
33 | require Carp; |
34 | Carp::croak("sort: unknown subpragma '@_'"); |
35 | } |
36 | } |
37 | } |
38 | |
39 | sub current { |
40 | my @sort; |
41 | if ($^H{SORT}) { |
42 | push @sort, 'quicksort' if $^H{SORT} & $sort::quicksort_bit; |
43 | push @sort, 'mergesort' if $^H{SORT} & $sort::mergesort_bit; |
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44 | push @sort, 'stable' if $^H{SORT} & $sort::stable_bit; |
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45 | } |
46 | push @sort, 'mergesort' unless @sort; |
47 | join(' ', @sort); |
48 | } |
49 | |
50 | 1; |
51 | __END__ |
52 | |
53 | =head1 NAME |
54 | |
55 | sort - perl pragma to control sort() behaviour |
56 | |
57 | =head1 SYNOPSIS |
58 | |
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59 | use sort 'stable'; # guarantee stability |
60 | use sort '_quicksort'; # use a quicksort algorithm |
61 | use sort '_mergesort'; # use a mergesort algorithm |
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62 | |
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63 | use sort '_qsort'; # alias for quicksort |
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64 | |
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65 | my $current = sort::current(); # identify prevailing algorithm |
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66 | |
67 | =head1 DESCRIPTION |
68 | |
69 | With the sort pragma you can control the behaviour of the builtin |
70 | sort() function. |
71 | |
72 | In Perl versions 5.6 and earlier the quicksort algorithm was used to |
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73 | implement sort(), but in Perl 5.8 a mergesort algorithm was also made |
74 | available, mainly to guarantee worst case O(N log N) behaviour: |
75 | the worst case of quicksort is O(N**2). In Perl 5.8 and later, |
76 | quicksort defends against quadratic behaviour by shuffling large |
77 | arrays before sorting. |
78 | |
79 | A stable sort means that for records that compare equal, the original |
80 | input ordering is preserved. Perl's mergesort is stable, quicksort is not. |
81 | Stability will matter only if elements that compare equal can be |
82 | distinguished in some other way. That means that simple numerical |
83 | and lexical sorts do not profit from stability, since equal elements |
84 | are indistinguishable. However, with a comparison such as |
85 | |
86 | { substr($a, 0, 3) cmp substr($b, 0, 3) } |
87 | |
88 | stability might matter because elements that compare equal on the |
89 | first 3 characters may be distinguished based on subsequent characters. |
90 | In Perl 5.8 and later, quicksort can be stabilized, but doing so will |
91 | add overhead, so it should only be done if it matters. |
92 | |
93 | The best algorithm depends on many things. On average, mergesort |
94 | does fewer comparisons than quicksort, so it may be better when |
95 | complicated comparison routines are used. Mergesort also takes |
96 | advantage of pre-existing order, so it would be favored for using |
97 | sort to merge several sorted arrays. On the other hand, quicksort |
98 | is often faster for small arrays, and on platforms with small memory |
99 | caches that are much faster than main memory. You can force the |
100 | choice of algorithm with this pragma, but this feels heavy-handed, |
101 | so the subpragmas beginning with a C<_> may not persist beyond Perl 5.8. |
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102 | |
103 | =cut |
104 | |