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NAME

List::Breakdown - Build sublist structures matching conditions

VERSION

Version 0.20

SYNOPSIS

use List::Breakdown 'breakdown';
...
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 %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'],
)

DESCRIPTION

This module assists you in making a breakdown of a list, copying and filtering its items into a structured bucket layout according to your specifications. Think of it as a syntax for grep that returns named and structured results from one list.

It differs from the excellent List::Categorize in the use of references to define 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.

If you want to divide or partition your list so that each item can only appear in one category, you may want either List::MoreUtils or possibly Set::Partition instead.

SUBROUTINES/METHODS

breakdown(\%spec, @items)

This is the only exportable subroutine. Given a hash reference structure and a list of items, it applies each of the referenced values as tests, returning a new hash in the same structure with the references replaced with the matching items, in the same way as grep.

There are two shortcut syntaxes for a value in the \%spec structure:

  • ARRAY

    If the referenced array has exactly two items, it will be interpreted as defining numeric bounds [lower,upper) for its values. undef can be used to denote negative or positive infinity. Any other number of items is a fatal error.

  • Regexp

    This will be interpreted as a pattern for the list items to match.

Additionally, if the value is a HASH reference, it can be used to make a sub-part of the structure, as demonstrated in the length key of the example \%spec given in SYNOPSIS.

EXAMPLES

Collecting troublesome records

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, using the pattern syntax, 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,
};

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 not_ascii bucket is empty, because none of the items matched it.

Monitoring system check results

Suppose you ran a list of checks with your monitoring system, and now you have a list of HASH references with keys 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. You would lay out your buckets like so, using the subroutine syntax:

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;

For our sample data above, this would yield the following structure in %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 references beneath the problem key.

Grouping numbers by size

Suppose you have a list of numbers from your volcanic activity reporting system, some of which might be merely worrisome, and some others an emergency, and they need to be filtered to know where to send them:

my @numbers = ( 1, 32, 3718.4, 0x56, 0777, 3.14, -5, 1.2e5 );

You could filter them into buckets like this, using the interval syntax: an ARRAY reference with exactly two elements: lower bound (inclusive) first, upper bound (exclusive) second:

my $filters = {
    negative => [ undef, 0 ],
    positive => {
        small  => [ 0,   10 ],
        medium => [ 10,  100 ],
        large  => [ 100, undef ],
    },
};

Applying the bucket structure like so:

my %filtered = breakdown $filters, @numbers;

The result set would look like this:

my %expected = (
    negative => [ -5 ],
    positive => {
        small  => [ 1, 3.14 ],
        medium => [ 32, 86 ],
        large  => [ 3_718.4, 511, 120_000 ],
    },
);

Notice that you can express infinity or negative infinity as undef. Note also this is a numeric comparison only.

AUTHOR

Tom Ryder <tom@sanctum.geek.nz>

DIAGNOSTICS

  • HASH reference expected for first argument

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

  • Reference expected for '%s'

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

  • 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 ARRAY, CODE, HASH, and Regexp.

DEPENDENCIES

CONFIGURATION AND ENVIRONMENT

None required.

INCOMPATIBILITIES

None known.

BUGS AND LIMITATIONS

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

SUPPORT

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

perldoc List::Breakdown

You can also look for information at:

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.

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