David J. Oswald > Acme-Sort-Bozo-0.05 > Acme::Sort::Bozo

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NAME ^

Acme::Sort::Bozo - Implementation of a Bozo sort algorithm.

VERSION ^

Version 0.05

SYNOPSIS ^

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

    use Acme::Sort::Bozo;

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

The worst case for Bozo is difficult to determine, though one study suggests it probably approaches O(INF). The good news is that, as time (and computation) approaches infinity the odds of not finding a solution decline toward zero (assuming a good random number generator). So if you have an eternity to wait, you'll get your results soon enough. The average case is O( n * n! ). However, there is no guarantee that any particular sort will come in anywhere near average. Where the bogosort is a 'stateless' sort, the bozo sort maintains a list state from one iteration to the next, but its decision mechanism for swaps is stateless; it blindly swaps any random two elements.

Keep in mind that a list of five items consumes an average of 5 * 5!, or 600 iterations. 10! is 36,288,000 iterations on average. The universe will either collapse or expand to the point that it cannot sustain life long before the Bozo sort manages to sort a deck of cards, in the average case. In the worst case, all of the background radiation from our universe will have decayed to the point that there is no longer any trace of our existence before this sort manages to alphabetically sort your social networking friends list.

Test with short (4 to 7 element) lists, and be prepared to kill the process if you mistakenly hand it more elements than that.

EXPORT ^

Always exports one function: bozo().

SUBROUTINES/METHODS ^

bozo( @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 Bozo is probably mostly useful as a teaching example of a "perversely awful" sort algorithm. There are approximately 1e80 atoms in the universe. A sort list of 59 elements will gain an average case solution of 5.9e81 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, a pair of elements (not necessarily adjacent) are chosen at random, and swapped. 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.

Do you feel lucky today, chump?

compare( $a, $b )

By passing a subref as the first parameter to bozo(), 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-bozo at rt.cpan.org, or through the web interface at http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Acme-Sort-Bozo. 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::Bozo

You can also look for information at:

SEE ALSO ^

ACKNOWLEDGEMENTS ^

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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