## no critic (RequirePODUseEncodingUTF8)
package List::Breakdown;

# Force me to write this properly
use strict;
use warnings;
use utf8;

# Target reasonably old Perls
use 5.006;

# Import required modules
use Carp;

# Handle exporting in a way Perl v5.6 should tolerate
use base qw(Exporter);    ## no critic (ProhibitUseBase)
our @EXPORT_OK = 'breakdown';

# Specify package version
our $VERSION = '0.13';

# Dispatch table of functions to handle different ref types for the spec
# hashref's values
my %types = (

    # If it's a hash, apply breakdown() again as if it were another root-level
    # spec
    HASH => sub {
        my $spec = shift;
        return { breakdown( $spec, @_ ) };
    },

    # If it's a subroutine, return a arrayref of all elements for which it
    # returns true
    CODE => sub {
        my $sub = shift;
        return [ grep { $sub->() } @_ ];
    },

    # If it's a regular expression, return an arrayref of all elements it
    # matches
    Regexp => sub {
        my $re = shift;
        return [ grep { $_ =~ $re } @_ ];
    },
);

# Given a spec and a list of items, filter them into a hash of the same
# structure
sub breakdown {
    my ( $spec, @items ) = @_;

    # Check the spec is a hashref
    ref $spec eq 'HASH'
      or croak 'HASH reference expected for first argument';

    # Start building a results hash
    my %results;
    for my $key ( keys %{$spec} ) {

        # Check that the value for this key is a reference
        my $ref = ref $spec->{$key}
          or croak "Reference expected for '$key'";

        # Check it's a reference we understand
        exists $types{$ref}
          or croak "Unhandled ref type $ref for '$key'";

        # Apply the appropriate subroutine for this reference type to the list
        # of items
        $results{$key} = $types{$ref}->( $spec->{$key}, @items );
    }

    # Return the constructed result set
    return %results;
}

1;

__END__

=pod

=for stopwords
sublists Unhandled tradename licensable MERCHANTABILITY hashrefs

=head1 NAME

List::Breakdown - Build list sublists matching conditions

=head1 VERSION

Version 0.13

=head1 DESCRIPTION

This module "breaks down" a list--filtering elements from a list into a
specified bucket layout. It may be useful in situations where you have a big
list of things to generate reports on, or to otherwise filter into several
sublists.

It differs from the excellent C<List::Categorize> in the use of subroutine
references for each category and in not requiring only one final category for
any given item; an item can end up in the result set for more than one filter.

You could maybe think of this as a multi-C<grep> that returns named results.

=head1 SYNOPSIS

    my @words = qw(foo bar baz quux wibble florb);
    my $cats  = {
        all    => sub { 1 },
        has_b  => sub { m/ b /msx },
        has_w  => sub { m/ w /msx },
        length => {
            3    => sub { length == 3 },
            4    => sub { length == 4 },
            long => sub { length > 4 },
        },
        has_ba => qr/ba/msx,
    };
    my %filtered = breakdown $cats, @words;

This puts the following structure in C<%filtered>:

    (
        all    => ['foo', 'bar', 'baz', 'quux', 'wibble', 'florb'],
        has_b  => ['bar', 'baz', 'wibble', 'florb'],
        has_w  => ['wibble'],
        length => {
            3    => ['foo', 'bar', 'baz'],
            4    => ['quux'],
            long => ['wibble', 'florb'],
        },
        has_ba => ['bar', 'baz'],
    )

=head1 SUBROUTINES/METHODS

=head2 B<breakdown(\%spec, @items)>

Given a hash reference structure and a list of items, apply each of the
subroutines or regular expressions given as values of the hash reference,
returning a new hash in the same structure with the tests replaced with the
items for which the subroutine returns true, in the same way as C<grep>, or (as
a shortcut) for which the regular expression matched.

=head1 EXAMPLES

=head2 

Suppose you have a list of strings from a very legacy system that you need to
regularly check for problematic characters, alerting you to problems with an
imperfect Perl parser:

    my @records = (
        "NEW CUSTOMER John O''Connor\r 2017-01-01",
        "RETURNING CUSTOMER\tXah Zhang 2016-01-01",
        "CHECK ACCOUNT Pierre d'Alun 2016-12-01",
        "RETURNING CUSTOMER Aaron Carter 2016-05-01"
    );

You could have a bucket structure like this, which catches certain error types
you've seen before for review:

    my %buckets = (
        bad_whitespace     => qr/ [\r\t] /msx,
        apostrophes        => qr/ ' /msx,
        double_apostrophes => qr/ '' /msx,
        not_ascii          => qr/ [^[:ascii:]] /msx
    );

Notice that you don't have to wrap a quoted regular expression to match in a
`sub` subroutine reference, as a convenience shortcut.

Applying the bucket structure like so:

    my %results = breakdown \%buckets, @records;

The result set would look like this:

    my %expected = (
        bad_whitespace => [
            "NEW CUSTOMER John O''Connor\r 2017-01-01",
            "RETURNING CUSTOMER\tXah Lee 2016-01-01"
        ],
        apostrophes => [
            "NEW CUSTOMER John O''Connor\r 2017-01-01",
            'CHECK ACCOUNT Pierre d\'Alun 2016-12-01'
        ],
        double_apostrophes => [
            "NEW CUSTOMER John O''Connor\r 2017-01-01"
        ],
        not_ascii => [
        ]
    );

Notice that some of the lines appear in more than one list, and that the
C<not_ascii> bucket is empty because none of the items matched it.

=head2 Monitoring system check results

Suppose you ran a list of checks with your monitoring system, and you have a
list of hashrefs describing each check and its outcome:

    my @checks = (
        {
            hostname => 'webserver1',
            status   => 'OK',
        },
        {
            hostname => 'webserver2',
            status   => 'CRITICAL',
        },
        {
            hostname => 'webserver3',
            status   => 'WARNING',
        },
        {
            hostname => 'webserver4',
            status   => 'OK',
        }
    );

You would like to break the list down by status. Using C<List::Breakdown>, you
would lay out your buckets like so:

    my %buckets = (
        ok       => sub { $_->{status} eq 'OK' },
        problem  => {
            warning  => sub { $_->{status} eq 'WARNING' },
            critical => sub { $_->{status} eq 'CRITICAL' },
            unknown  => sub { $_->{status} eq 'UNKNOWN' },
        },
    );

And apply them like so:

    my %results = breakdown \%buckets, @checks;

You can then apply C<%buckets> to any other list you may need to check in the
same way to get the same structure.

For our sample data above, this would yield the following structure in
C<%results>:

    (
        ok => [
            {
                hostname => 'webserver1',
                status   => 'OK'
            },
            {
                hostname => 'webserver4',
                status   => 'OK'
            }
        ],
        problem => {
            warning => [
                {
                    hostname => 'webserver3',
                    status   => 'WARNING'
                }
            ],
            critical => [
                {
                    hostname => 'webserver2',
                    status   => 'CRITICAL'
                }
            ],
            unknown => []
        }
    )

Note the extra level of hash referencing beneath the C<problem> key.

=head1 AUTHOR

Tom Ryder C<< <tom@sanctum.geek.nz> >>

=head1 DIAGNOSTICS

=over 4

=item HASH reference expected for first argument

The first argument that B<breakdown()> saw wasn't the hash reference it expects.
That's the only format a spec is allowed to have.

=item Reference expected for '%s'

The value for the named key in the spec was not a reference, and one was
expected.

=item Unhandled ref type %s for '%s'

The value for the named key in the spec is of a type that makes no sense to
this module. Legal reference types are C<HASH>, C<CODE>, and C<Regexp>.

=back

=head1 DEPENDENCIES

Perl 5.6 and the core modules C<Carp> and C<Exporter>.

=head1 CONFIGURATION AND ENVIRONMENT

None required.

=head1 INCOMPATIBILITIES

None known.

=head1 BUGS AND LIMITATIONS

Definitely. This is a very early release. Please report any bugs or feature
requests to C<tom@sanctum.geek.nz>.

=head1 SUPPORT

You can find documentation for this module with the C<perldoc> command.

    perldoc List::Breakdown

=head1 LICENSE AND COPYRIGHT

Copyright (C) 2017 Tom Ryder

This program is free software; you can redistribute it and/or modify it under
the terms of the Artistic License (2.0). You may obtain a copy of the full
license at:

<http://www.perlfoundation.org/artistic_license_2_0>

Any use, modification, and distribution of the Standard or Modified Versions is
governed by this Artistic License. By using, modifying or distributing the
Package, you accept this license. Do not use, modify, or distribute the
Package, if you do not accept this license.

If your Modified Version has been derived from a Modified Version made by
someone other than you, you are nevertheless required to ensure that your
Modified Version complies with the requirements of this license.

This license does not grant you the right to use any trademark, service mark,
tradename, or logo of the Copyright Holder.

This license includes the non-exclusive, worldwide, free-of-charge patent
license to make, have made, use, offer to sell, sell, import and otherwise
transfer the Package with respect to any patent claims licensable by the
Copyright Holder that are necessarily infringed by the Package. If you
institute patent litigation (including a cross-claim or counterclaim) against
any party alleging that the Package constitutes direct or contributory patent
infringement, then this Artistic License to you shall terminate on the date
that such litigation is filed.

Disclaimer of Warranty: THE PACKAGE IS PROVIDED BY THE COPYRIGHT HOLDER AND
CONTRIBUTORS "AS IS' AND WITHOUT ANY EXPRESS OR IMPLIED WARRANTIES. THE IMPLIED
WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, OR
NON-INFRINGEMENT ARE DISCLAIMED TO THE EXTENT PERMITTED BY YOUR LOCAL LAW.
UNLESS REQUIRED BY LAW, NO COPYRIGHT HOLDER OR CONTRIBUTOR WILL BE LIABLE FOR
ANY DIRECT, INDIRECT, INCIDENTAL, OR CONSEQUENTIAL DAMAGES ARISING IN ANY WAY
OUT OF THE USE OF THE PACKAGE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
DAMAGE.

=cut