David J. Oswald > Acme-Sort-Bogosort > Acme::Sort::Bogosort

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Module Version: 0.05   Source  

NAME ^

Acme::Sort::Bogosort - Implementation of a Bogosort (aka 'stupid sort' or 'slowsort').

VERSION ^

Version 0.05

SYNOPSIS ^

The Bogosort is a sort that is based on the "generate and test" paradigm. It works by first testing whether the input is in sorted order. If so, return the list. But if not, randomly shuffle the input and test again. Repeat until the shuffle comes back sorted.

    use Acme::Sort::Bogosort;

    my @unsorted = qw/ E B A C D /;
    my @ascending = bogosort( @unsorted );
    
    my @descending = bogosort(
        sub{ return $_[1] cmp $_[0]; },
        @unsorted
    );

The Bogosort has a worst case of O(INF), though as time approaches infinity the odds of not finding a solution decline toward zero (assuming a good random number generator). The average case is O( (n-1) * n! ). The n! term signifies how many shuffles will be required to obtain a sorted result in the average case. However, there is no guarantee that any particular sort will come in anywhere near average.

Keep in mind that a list of five items consumes an average of 5!, or 120 iterations. 10! is 3,628,800 shuffles. Also keep in mind that each shuffle itself is an O(n-1) operation. Unless you need to heat a cold office with your processor avoid sorts on large data sets.

EXPORT ^

Always exports one function: bogosort().

SUBROUTINES/METHODS ^

bogosort( @unsorted )

Accepts a list as a parameter and returns a sorted list.

If the first parameter is a reference to a subroutine, it will be used as the comparison function.

The Bogosort is probably mostly useful as a teaching example of a terrible sort algorithm. There are approximately 1e80 atoms in the universe. A sort list of 59 elements will gain an average case solution of 1e80 iterations, with a worst case approaching infinite iterations to find a solution. Anything beyond just a few items takes a considerable amount of work.

Each iteration checks first to see if the list is in order. Here a comparatively minor optimization is that the first out-of-order element will short-circuit the check. That step has a worst case of O(n), and average case of nearly O(1). That's the only good news. Once it is determined that the list is out of order, the entire list is shuffled (an O(n) operation). Then the test happens all over again, repeating until a solution is happened across by chance.

There is a potential for this sort to never finish, since a typical random number synthesizer does not generate an infinitely non-repeating series. Because this algorithm has the capability of producing O(INF) iterations, it would need an infinite source of random numbers to find a solution in any given dataset.

Small datasets are unlikely to encounter this problem, but as the dataset grows, so does the propensity for running through the entire set of pseudo-random numbers generated by Perl's rand() for a given seed. None of this really matters, of course, as no sane individual would ever use this for any serious sorting work.

Not every individual is sane.

compare( $a, $b )

By passing a subref as the first parameter to bogosort(), the user is able to manipulate sort orders just as is done with Perl's built in sort { code } @list routine.

The comparison function is easy to implement using Perl's <=> and cmp operators, but any amount of creativity is ok so long as return values are negative for "Order is ok", positive for "Order is not ok", and 0 for "Terms are equal (Order is ok)".

AUTHOR ^

David Oswald, <davido[at]cpan.org>

BUGS ^

Please report any bugs or feature requests to bug-acme-sort-bogosort at rt.cpan.org, or through the web interface at http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Acme-Sort-Bogosort. I will be notified, and then you'll automatically be notified of progress on your bug as I make changes.

SUPPORT ^

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

    perldoc Acme::Sort::Bogosort

You can also look for information at:

ACKNOWLEDGEMENTS ^

http://en.wikipedia.org/wiki/Bogosort - A nice Wikipedia article on the Bogosort.

LICENSE AND COPYRIGHT ^

Copyright 2011 David Oswald.

This program is free software; you can redistribute it and/or modify it under the terms of either: the GNU General Public License as published by the Free Software Foundation; or the Artistic License.

See http://dev.perl.org/licenses/ for more information.

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