1 | package sort;
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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
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6 | # $sort::hints. An improvement would be to store them in $^H{SORT} and have
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7 | # this information available somewhere in the listop OP_SORT, to allow lexical
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8 | # scoping of this pragma. -- rgs 2002-04-30
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9 |
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10 | our $hints = 0;
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11 |
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12 | $sort::quicksort_bit = 0x00000001;
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13 | $sort::mergesort_bit = 0x00000002;
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14 | $sort::sort_bits = 0x000000FF; # allow 256 different ones
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15 | $sort::stable_bit = 0x00000100;
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16 |
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17 | use strict;
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18 |
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19 | sub import {
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20 | shift;
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21 | if (@_ == 0) {
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22 | require Carp;
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23 | Carp::croak("sort pragma requires arguments");
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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;
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30 | $hints |= $sort::quicksort_bit;
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31 | } elsif ($_ eq '_mergesort') {
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32 | $hints &= ~$sort::sort_bits;
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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') {
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37 | $hints = 0;
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38 | } else {
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39 | require Carp;
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40 | Carp::croak("sort: unknown subpragma '$_'");
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41 | }
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42 | }
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43 | }
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44 |
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45 | sub unimport {
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46 | shift;
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47 | if (@_ == 0) {
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48 | require Carp;
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49 | Carp::croak("sort pragma requires arguments");
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50 | }
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51 | local $_;
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52 | no warnings 'uninitialized'; # bitops would warn
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53 | while ($_ = shift(@_)) {
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54 | if (/^_q(?:uick)?sort$/) {
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55 | $hints &= ~$sort::sort_bits;
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56 | } elsif ($_ eq '_mergesort') {
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57 | $hints &= ~$sort::sort_bits;
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58 | } elsif ($_ eq 'stable') {
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59 | $hints &= ~$sort::stable_bit;
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60 | } else {
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61 | require Carp;
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62 | Carp::croak("sort: unknown subpragma '$_'");
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63 | }
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64 | }
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65 | }
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66 |
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67 | sub current {
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68 | my @sort;
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69 | if ($hints) {
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70 | push @sort, 'quicksort' if $hints & $sort::quicksort_bit;
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71 | push @sort, 'mergesort' if $hints & $sort::mergesort_bit;
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72 | push @sort, 'stable' if $hints & $sort::stable_bit;
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73 | }
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74 | push @sort, 'mergesort' unless @sort;
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75 | join(' ', @sort);
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76 | }
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77 |
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78 | 1;
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79 | __END__
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80 |
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81 | =head1 NAME
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82 |
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83 | sort - perl pragma to control sort() behaviour
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84 |
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85 | =head1 SYNOPSIS
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86 |
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87 | use sort 'stable'; # guarantee stability
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88 | use sort '_quicksort'; # use a quicksort algorithm
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89 | use sort '_mergesort'; # use a mergesort algorithm
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90 | use sort 'defaults'; # revert to default behavior
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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 |
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97 | =head1 DESCRIPTION
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98 |
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99 | With the C<sort> pragma you can control the behaviour of the builtin
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100 | C<sort()> function.
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101 |
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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:
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105 | the worst case of quicksort is O(N**2). In Perl 5.8 and later,
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106 | quicksort defends against quadratic behaviour by shuffling large
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107 | arrays before sorting.
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108 |
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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
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112 | distinguished in some other way. That means that simple numerical
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113 | and lexical sorts do not profit from stability, since equal elements
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114 | are indistinguishable. However, with a comparison such as
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115 |
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116 | { substr($a, 0, 3) cmp substr($b, 0, 3) }
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117 |
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118 | stability might matter because elements that compare equal on the
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119 | first 3 characters may be distinguished based on subsequent characters.
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120 | In Perl 5.8 and later, quicksort can be stabilized, but doing so will
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121 | add overhead, so it should only be done if it matters.
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122 |
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123 | The best algorithm depends on many things. On average, mergesort
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124 | does fewer comparisons than quicksort, so it may be better when
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125 | complicated comparison routines are used. Mergesort also takes
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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
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128 | is often faster for small arrays, and on arrays of a few distinct
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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,
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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
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133 | you do not explicitly demand it.
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134 | But the stability of the default sort is a side-effect that could
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135 | change in later versions. If stability is important, be sure to
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136 | say so with a
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137 |
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138 | use sort 'stable';
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139 |
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140 | The C<no sort> pragma doesn't
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141 | I<forbid> what follows, it just leaves the choice open. Thus, after
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142 |
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143 | no sort qw(_mergesort stable);
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144 |
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145 | a mergesort, which happens to be stable, will be employed anyway.
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146 | Note that
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147 |
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148 | no sort "_quicksort";
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149 | no sort "_mergesort";
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150 |
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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
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154 |
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155 | This pragma is not lexically scoped: its effect is global to the program
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156 | it appears in. That means the following will probably not do what you
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157 | expect, because I<both> pragmas take effect at compile time, before
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158 | I<either> C<sort()> happens.
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159 |
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160 | { use sort "_quicksort";
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161 | print sort::current . "\n";
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162 | @a = sort @b;
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163 | }
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164 | { use sort "stable";
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165 | print sort::current . "\n";
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166 | @c = sort @d;
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167 | }
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168 | # prints:
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169 | # quicksort stable
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170 | # quicksort stable
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171 |
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172 | You can achieve the effect you probably wanted by using C<eval()>
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173 | to defer the pragmas until run time. Use the quoted argument
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174 | form of C<eval()>, I<not> the BLOCK form, as in
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175 |
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176 | eval { use sort "_quicksort" }; # WRONG
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177 |
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178 | or the effect will still be at compile time.
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179 | Reset to default options before selecting other subpragmas
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180 | (in case somebody carelessly left them on) and after sorting,
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181 | as a courtesy to others.
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182 |
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183 | { eval 'use sort qw(defaults _quicksort)'; # force quicksort
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184 | eval 'no sort "stable"'; # stability not wanted
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185 | print sort::current . "\n";
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186 | @a = sort @b;
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187 | eval 'use sort "defaults"'; # clean up, for others
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188 | }
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189 | { eval 'use sort qw(defaults stable)'; # force stability
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190 | print sort::current . "\n";
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191 | @c = sort @d;
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192 | eval 'use sort "defaults"'; # clean up, for others
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193 | }
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194 | # prints:
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195 | # quicksort
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196 | # stable
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197 |
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198 | Scoping for this pragma may change in future versions.
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199 |
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200 | =cut
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201 |
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