RandomShuffle

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Revision as of 00:14, 4 September 2006 by Bfulgham (Talk | contribs)
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[edit] Problem

For some gaming applications, e.g. Poker, we would need a fair random shuffle. There are two basics algorithms for this both described by Knuth. The more well known Knuth/Fisher-Yates shuffle is O(n) but requires destructive updates. The shuffle listed is O(n log n) but works well for any functional language.

[edit] Solution

We associate each element in the list with a random number. The list is then sorted based on the generated number. We repeat this process log(n) times to ensure a fair shuffle. 

 
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%
%% shuffle(List1) -> List2
%% Takes a list and randomly shuffles it. Relies on random:uniform
%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

shuffle(List) ->
%% Determine the log n portion then randomize the list.
   randomize(round(math:log(length(List)) + 0.5), List).

randomize(1, List) ->
   randomize(List);
randomize(T, List) ->
   lists:foldl(fun(_E, Acc) ->
                  randomize(Acc)
               end, randomize(List), lists:seq(1, (T - 1))).

randomize(List) ->
   D = lists:map(fun(A) ->
                    {random:uniform(), A}
             end, List),
   {_, D1} = lists:unzip(lists:keysort(1, D)), 
   D1.
Retrieved from "http://www.trapexit.org/RandomShuffle"
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