NAME

Algorithm::Numerical::Shuffle - Shuffle a list.

SYNOPSIS

use Algorithm::Numerical::Shuffle qw /shuffle/;

@shuffled = shuffle (1, 2, 3, 4, 5, 6, 7);

\$in_situ = [qw /one two three four five six/];
shuffle \$in_situ;

DESCRIPTION

shuffle performs a one pass, fair shuffle on a list. If the list is passed as a reference to an array, the shuffle is done in situ.

The subroutine returns the list in list context, and a reference to the list in scalar context.

COMPLEXITY

The running time of the algorithm is linear in the size of the list. For an in situ shuffle, the memory overhead is constant; otherwise, linear extra memory is used.

LITERATURE

The algorithm used is discussed by Knuth . It was first published by Fisher and Yates , and later by Durstenfeld .

CAVEAT

Salfi  points to a big caveat. If the outcome of a random generator is solely based on the value of the previous outcome, like a linear congruential method, then the outcome of a shuffle depends on exactly three things: the shuffling algorithm, the input and the seed of the random generator. Hence, for a given list and a given algorithm, the outcome of the shuffle is purely based on the seed. Many modern computers have 32 bit random numbers, hence a 32 bit seed. Hence, there are at most 2^32 possible shuffles of a list, foreach of the possible algorithms. But for a list of n elements, there are n! possible permutations. Which means that a shuffle of a list of 13 elements will not generate certain permutations, as 13! > 2^32.



R. Durstenfeld: CACM, 7, 1964. pp 420.



R. A. Fisher and F. Yates: Statistical Tables. London, 1938. Example 12.



D. E. Knuth: The Art of Computer Programming, Volume 2, Third edition. Section 3.4.2, Algorithm P, pp 145. Reading: Addison-Wesley, 1997. ISBN: 0-201-89684-2.



R. Salfi: COMPSTAT 1974. Vienna: 1974, pp 28 - 35.

List::Util also has a shuffle function which uses a similar algorithm. But since it's written in C, it's much faster. For all practical purposes, List::Util supersedes this module. Unless you really need in situ sorting.

DEVELOPMENT

The current sources of this module are found on github, git://github.com/Abigail/algorithm--numerical--shuffle.git.

AUTHOR

Abigail mailto:cpan@abigail.be.