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84d4ea48 JH |
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; | |

84d4ea48 JH |
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 $_; | |

726de688 | 22 | no warnings 'uninitialized'; # $^H{SORT} bitops would warn |

84d4ea48 | 23 | while ($_ = shift(@_)) { |

c53fc8a6 | 24 | if (/^_q(?:uick)?sort$/) { |

84d4ea48 JH |
25 | $^H{SORT} &= ~$sort::sort_bits; |

26 | $^H{SORT} |= $sort::quicksort_bit; | |

c53fc8a6 | 27 | } elsif ($_ eq '_mergesort') { |

84d4ea48 JH |
28 | $^H{SORT} &= ~$sort::sort_bits; |

29 | $^H{SORT} |= $sort::mergesort_bit; | |

c53fc8a6 JH |
30 | } elsif ($_ eq 'stable') { |

31 | $^H{SORT} |= $sort::stable_bit; | |

84d4ea48 JH |
32 | } else { |

33 | require Carp; | |

71c4de84 | 34 | Carp::croak("sort: unknown subpragma '$_'"); |

84d4ea48 JH |
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; | |

c53fc8a6 | 44 | push @sort, 'stable' if $^H{SORT} & $sort::stable_bit; |

84d4ea48 JH |
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 | ||

c53fc8a6 JH |
59 | use sort 'stable'; # guarantee stability |

60 | use sort '_quicksort'; # use a quicksort algorithm | |

61 | use sort '_mergesort'; # use a mergesort algorithm | |

84d4ea48 | 62 | |

c53fc8a6 | 63 | use sort '_qsort'; # alias for quicksort |

84d4ea48 | 64 | |

c53fc8a6 | 65 | my $current = sort::current(); # identify prevailing algorithm |

84d4ea48 JH |
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 | |

c53fc8a6 JH |
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 | |

b0ae2885 | 80 | input ordering is preserved. Mergesort is stable, quicksort is not. |

c53fc8a6 JH |
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. | |

84d4ea48 | 102 | |

0e59b7c6 RGS |
103 | =head1 CAVEATS |

104 | ||

105 | This pragma is not lexically scoped : its effect is global to the program | |

106 | it appears in. This may change in future versions. | |

107 | ||

84d4ea48 JH |
108 | =cut |

109 |