Commit | Line | Data |
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84d4ea48 JH |
1 | package sort; |
2 | ||
7a8ff2dd | 3 | our $VERSION = '1.02'; |
84d4ea48 | 4 | |
045ac317 RGS |
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; | |
84d4ea48 JH |
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; | |
84d4ea48 JH |
16 | |
17 | use strict; | |
18 | ||
19 | sub import { | |
20 | shift; | |
21 | if (@_ == 0) { | |
22 | require Carp; | |
23 | Carp::croak("sort pragma requires arguments"); | |
24 | } | |
84d4ea48 | 25 | local $_; |
045ac317 | 26 | no warnings 'uninitialized'; # bitops would warn |
84d4ea48 | 27 | while ($_ = shift(@_)) { |
c53fc8a6 | 28 | if (/^_q(?:uick)?sort$/) { |
045ac317 RGS |
29 | $hints &= ~$sort::sort_bits; |
30 | $hints |= $sort::quicksort_bit; | |
c53fc8a6 | 31 | } elsif ($_ eq '_mergesort') { |
045ac317 RGS |
32 | $hints &= ~$sort::sort_bits; |
33 | $hints |= $sort::mergesort_bit; | |
c53fc8a6 | 34 | } elsif ($_ eq 'stable') { |
045ac317 | 35 | $hints |= $sort::stable_bit; |
7a8ff2dd JL |
36 | } elsif ($_ 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; | |
84d4ea48 JH |
60 | } else { |
61 | require Carp; | |
71c4de84 | 62 | Carp::croak("sort: unknown subpragma '$_'"); |
84d4ea48 JH |
63 | } |
64 | } | |
65 | } | |
66 | ||
67 | sub current { | |
68 | my @sort; | |
045ac317 RGS |
69 | if ($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; | |
84d4ea48 JH |
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 | ||
c53fc8a6 JH |
87 | use sort 'stable'; # guarantee stability |
88 | use sort '_quicksort'; # use a quicksort algorithm | |
89 | use sort '_mergesort'; # use a mergesort algorithm | |
7a8ff2dd JL |
90 | use sort 'defaults'; # revert to default behavior |
91 | no sort 'stable'; # stability not important | |
84d4ea48 | 92 | |
c53fc8a6 | 93 | use sort '_qsort'; # alias for quicksort |
84d4ea48 | 94 | |
c53fc8a6 | 95 | my $current = sort::current(); # identify prevailing algorithm |
84d4ea48 JH |
96 | |
97 | =head1 DESCRIPTION | |
98 | ||
7a8ff2dd JL |
99 | With the C<sort> pragma you can control the behaviour of the builtin |
100 | C<sort()> function. | |
84d4ea48 JH |
101 | |
102 | In Perl versions 5.6 and earlier the quicksort algorithm was used to | |
7a8ff2dd | 103 | implement C<sort()>, but in Perl 5.8 a mergesort algorithm was also made |
c53fc8a6 JH |
104 | available, mainly to guarantee worst case O(N log N) behaviour: |
105 | the worst case of quicksort is O(N**2). In Perl 5.8 and later, | |
106 | quicksort defends against quadratic behaviour by shuffling large | |
107 | arrays before sorting. | |
108 | ||
109 | A stable sort means that for records that compare equal, the original | |
b0ae2885 | 110 | input ordering is preserved. Mergesort is stable, quicksort is not. |
c53fc8a6 JH |
111 | Stability will matter only if elements that compare equal can be |
112 | distinguished in some other way. That means that simple numerical | |
113 | and lexical sorts do not profit from stability, since equal elements | |
114 | are indistinguishable. However, with a comparison such as | |
115 | ||
116 | { substr($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 | |
7a8ff2dd JL |
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 | |
c53fc8a6 JH |
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. | |
7a8ff2dd JL |
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. | |
84d4ea48 | 152 | |
0e59b7c6 RGS |
153 | =head1 CAVEATS |
154 | ||
7a8ff2dd JL |
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. | |
0e59b7c6 | 199 | |
84d4ea48 JH |
200 | =cut |
201 |