NAME

List::Unique::DeterministicOrder - Store and access a list of keys using a deterministic order based on the sequence of insertions and deletions

VERSION

Version 0.01

SYNOPSIS

This module provides a structure to store a list of keys, without duplicates, and be able to access them by either key name or index.

    use List::Unique::DeterministicOrder;

    my $foo = List::Unique::DeterministicOrder->new(
        data => [qw /foo bar quux fetangle/]
    );
    
    print $foo->keys;
    #  foo bar quux fetangle
    
    $foo->delete ('bar')
    print $foo->keys;
    #  foo fetangle quux 
    
    print $foo->get_key_at_pos(2);
    #  quux
    print $foo->get_key_at_pos(20);
    #  undef
    
    $foo->push ('bardungle')
    print $foo->keys;
    #  foo fetangle quux bardungle
    
    #  keys are stored only once,
    #  just like with a normal hash
    $foo->push ('fetangle')
    print $foo->keys;
    #  foo fetangle quux bardungle
    
    print $foo->exists ('gelert');
    #  false
    
    print $foo->pop;
    #  bardungle
    

DISCUSSION

The algorithm used is from https://stackoverflow.com/questions/5682218/data-structure-insert-remove-contains-get-random-element-all-at-o1/5684892#5684892

The algorithm used inserts keys at the end, but swaps keys around on deletion. Hence it is deterministic and repeatable, but only if the sequence of insertions and deletions is replicated exactly.

So why would one use this in the first place? The motivating use-case was a randomisation process where keys would be selected from a pool of keys, and sometimes inserted. e.g. the process might select and remove the 10th key, then the 257th, then insert a new key, followed by more selections and removals. The randomisations needed to produce the same results same for the same given PRNG sequence for reproducibility purposes.

Using a hash to store the data provides rapid access, but getting the nth key requires the key list be generated each time, and Perl's hashes do not provide their keys in a deterministic order across all versions and platforms. Binary searches over sorted lists proved very effective for a while, but bottlenecks started to manifest when the data sets became much larger and the number of lists became both abundant and lengthy.

Since the order itself does not matter, only the ability to replicate it, this module was written.

One could also use Hash::Ordered, but it has the overhead of storing values, which are not needed here. I also wrote this module before I benchmarked against Hash::Ordered. Regardless, this module is faster for the example use-case described above - see the benchmarking results in bench.pl (which is part of this distribution). That said, some of the implementation details have been adapted/borrowed from Hash::Ordered.

METHODS

Note that most methods take a single argument (if any), so while the method names look hash-like, this is essentially cosmetic. In particular, it does not yet support splicing.

new

    $foo->new();
    $foo->new(data => [/a b c d e/]);

Create a new object. Optionally pass data using the data keyword. Duplicate keys are stored once only.

exists

True or false for if the key exists.

delete

    $foo->delete('some key');

Deletes the key passed as an argument. Returns the key name if successful, undef if the key was not found.

delete_key_at_pos

    $foo->delete_key_at_pos(1);

Removes a single key from the set at the specified position.

get_key_at_pos

    $foo->get_key_at_pos(5);

Returns the key at some position.

get_key_pos

    $foo->get_key_pos('quux');

Returns the position of a key.

keys

Returns the list of keys in list context, and the number of keys in scalar context.

pop

    $foo->pop;

Removes and returns the last key in the list.

push

    $foo->push('quux');

Appends the specified key to the end of the list, unless it is already in the list.

AUTHOR

Shawn Laffan, <shawnlaffan at gmail.com>

BUGS

Please report any bugs or feature requests via https://github.com/shawnlaffan/perl-List-Unique-DeterministicOrder/issues.

ACKNOWLEDGEMENTS

The algorithm used is from https://stackoverflow.com/questions/5682218/data-structure-insert-remove-contains-get-random-element-all-at-o1/5684892#5684892

Some implementation details have been borrowed/adapted from Hash::Ordered.

SEE ALSO

Hash::Ordered (and modules listed in its "See Also" section)

List::BinarySearch

List::MoreUtils

LICENSE AND COPYRIGHT

Copyright 2018 Shawn Laffan

This program is free software; you can redistribute it and/or modify it under the terms of the 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.