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1 | package sort; |
2 | |
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3 | our $VERSION = '1.02'; |
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4 | |
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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; |
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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; |
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16 | |
17 | use strict; |
18 | |
19 | sub import { |
20 | shift; |
21 | if (@_ == 0) { |
22 | require Carp; |
23 | Carp::croak("sort pragma requires arguments"); |
24 | } |
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25 | local $_; |
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26 | no warnings 'uninitialized'; # bitops would warn |
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27 | while ($_ = shift(@_)) { |
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28 | if (/^_q(?:uick)?sort$/) { |
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29 | $hints &= ~$sort::sort_bits; |
30 | $hints |= $sort::quicksort_bit; |
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31 | } elsif ($_ eq '_mergesort') { |
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32 | $hints &= ~$sort::sort_bits; |
33 | $hints |= $sort::mergesort_bit; |
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34 | } elsif ($_ eq 'stable') { |
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35 | $hints |= $sort::stable_bit; |
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36 | } elsif ($_ 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; |
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60 | } else { |
61 | require Carp; |
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62 | Carp::croak("sort: unknown subpragma '$_'"); |
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63 | } |
64 | } |
65 | } |
66 | |
67 | sub current { |
68 | my @sort; |
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69 | if ($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; |
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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 | |
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87 | use sort 'stable'; # guarantee stability |
88 | use sort '_quicksort'; # use a quicksort algorithm |
89 | use sort '_mergesort'; # use a mergesort algorithm |
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90 | use sort 'defaults'; # revert to default behavior |
91 | no sort 'stable'; # stability not important |
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92 | |
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93 | use sort '_qsort'; # alias for quicksort |
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94 | |
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95 | my $current = sort::current(); # identify prevailing algorithm |
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96 | |
97 | =head1 DESCRIPTION |
98 | |
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99 | With the C<sort> pragma you can control the behaviour of the builtin |
100 | C<sort()> function. |
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101 | |
102 | In Perl versions 5.6 and earlier the quicksort algorithm was used to |
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103 | implement C<sort()>, but in Perl 5.8 a mergesort algorithm was also made |
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104 | available, mainly to guarantee worst case O(N log N) behaviour: |
105 | the worst case of quicksort is O(N**2). In Perl 5.8 and later, |
106 | quicksort defends against quadratic behaviour by shuffling large |
107 | arrays before sorting. |
108 | |
109 | A stable sort means that for records that compare equal, the original |
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110 | input ordering is preserved. Mergesort is stable, quicksort is not. |
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111 | Stability will matter only if elements that compare equal can be |
112 | distinguished in some other way. That means that simple numerical |
113 | and lexical sorts do not profit from stability, since equal elements |
114 | are indistinguishable. However, with a comparison such as |
115 | |
116 | { substr($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 |
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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 |
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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. |
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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. |
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152 | |
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153 | =head1 CAVEATS |
154 | |
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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. |
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199 | |
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200 | =cut |
201 | |