NAME
    SortKey - Reusable sort key generators

SPECIFICATION VERSION
    0.1

VERSION
    This document describes version 0.2.1 of SortKey (from Perl distribution
    SortKey), released on 2024-05-15.

SYNOPSIS
    Basic use with sort():

     use SortKey::Num::length;
     my $by_length = SortKey::Num::length::gen_keygen;

     my @sorted = sort { $by_length->($a) <=> $by_length->($b) } "food", "foo", "foolish";
     # => ('foo', 'food', 'foolish')

    Basic use with "Sort::Key":

     use Sort::Key qw(nkeysort);
     use SortKey::Num::length;
     my $by_length = SortKey::Num::length::gen_keygen;

     # use & prefix to get around prototype restriction
     my @sorted = &nkeysort($by_length, "food", "foo", "foolish");
     # => ('foo', 'food', 'foolish')

    Specifying arguments:

     use Sort::Key qw(nkeysort);
     use SortKey::Num::pattern_count;
     my $by_pattern_count = Sort::Key::Num::pattern_count::gen_keygen(pattern => qr/foo/);
     my @sorted = &nkeysort($by_pattern_count, ...);

    Specifying comparer on the command-line (for other CLI's):

     % customsort -k length ...
     % customsort -c pattern_count=pattern,foo ...

DESCRIPTION
    EXPERIMENTAL. SPEC MIGHT STILL CHANGE.

Glossary
    A sort key generator is a subroutine that accepts an item and converts
    it to a string/numeric key. The key then can be compared using the
    standard "<=>" or "cmp" operator.

    A SortKey::* module is a module that can return a sort key generator.

  Writing a SortKey module
     package SorKey::Num::pattern_count;

     # required. must return a hash (DefHash)
     sub meta {
         return +{
             v => 1,
             summary => 'Number of occurrences of a pattern, as sort key',
             args => {
                 pattern => {
                     schema => 're_from_str*', # Sah schema
                     req => 1,
                 },
             },
         };
     }

     sub gen_keygen {
         my %args = @_;

         ...
         return sub {

             # since one of the main usages is with Sort::Key, and Sort::Key does
             # not pass the argument to @_ but sets $_. So a sort key generator
             # subroutine must check if argument is passed and if not then use $_ as
             # the argument.
             my $arg = @_ ? $_[0] : $_;

             # convert $arg to key
             ...
         };
     }

     1;

  Namespace organization
    "SortKey::Num::*" are for modules that return a numeric key generator.
    Other subnamespaces are for modules that return a string key.

HOMEPAGE
    Please visit the project's homepage at
    <https://metacpan.org/release/SortKey>.

SOURCE
    Source repository is at <https://github.com/perlancar/perl-SortKey>.

SEE ALSO
  Base specifications
    DefHash

    Sah

  Related specifications
    Sorter

    Comparer

  Previous incarnation
    Sort::Sub

    "Sorter", "SortKey", and "Comparer" are meant to eventually supersede
    Sort::Sub. The main improvement over Sort::Sub is the split into three
    kinds of subroutines:

    1. sorter
        A subroutine that accepts a list of items to sort.

        "Sorter::*" modules are meant to generate sorters.

    2. sort key generator
        A subroutine that converts an item to a string/numeric key suitable
        for simple comparison using "eq" or "==" during sorting.

        "SortKey::*" modules are meant to generate sort key generators.

    3. comparer
        A subroutine that compares two items. Can be used in sort() as
        custom sort block.

        "Comparer::*" modules are meant to generate comparers.

        Perl's sort(), as mentioned above, allows us to specify a comparer,
        but oftentimes it's more efficient to sort by key using key
        generator, where the keys are often cached. And sometimes due to
        preprocessing and/or postprocessing it's more suitable to use the
        more generic sorter interface.

    Aside from the above, "SortKey" also makes Sort::Sub's special arguments
    "reverse" and "is_ci" become ordinary arguments, because they are not
    always applicable in all situation, especially "is_ci".

AUTHOR
    perlancar <perlancar@cpan.org>

CONTRIBUTING
    To contribute, you can send patches by email/via RT, or send pull
    requests on GitHub.

    Most of the time, you don't need to build the distribution yourself. You
    can simply modify the code, then test via:

     % prove -l

    If you want to build the distribution (e.g. to try to install it locally
    on your system), you can install Dist::Zilla,
    Dist::Zilla::PluginBundle::Author::PERLANCAR,
    Pod::Weaver::PluginBundle::Author::PERLANCAR, and sometimes one or two
    other Dist::Zilla- and/or Pod::Weaver plugins. Any additional steps
    required beyond that are considered a bug and can be reported to me.

COPYRIGHT AND LICENSE
    This software is copyright (c) 2024 by perlancar <perlancar@cpan.org>.

    This is free software; you can redistribute it and/or modify it under
    the same terms as the Perl 5 programming language system itself.

BUGS
    Please report any bugs or feature requests on the bugtracker website
    <https://rt.cpan.org/Public/Dist/Display.html?Name=SortKey>

    When submitting a bug or request, please include a test-file or a patch
    to an existing test-file that illustrates the bug or desired feature.