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Tue, 07 Feb 2012

Mini-Challenge: Write Your Prisoner's Dilemma Strategy


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Here is a small task we considered for the Perl 6 Coding Contest, but not chose to not pursue. But it's a nice little challenge for your leisure time.

In the Prisoner's Dilemma, two suspected criminals can choose to not betray each other (which we call "cooperate"), or betraying the other ("defecting"). If only one suspect betrays the other, the traitor gets released and the betrayed one gets a long sentence; if both betray each other, both get a rather long sentence. If both cooperate, both get rather short sentences.

It becomes more interesting when the dilemma is repeated multiple times. Now instead of prison sentences the contestants are assigned scores, which add up over multiple rounds.

I challenge you to write one or two strategies for the iterated prisoner's dilemma, and send them to moritz@faui2k3.org no later than Friday February 17.

You'll find some basic strategies and a harness here. It runs on both newest Rakudo and Niecza.

The scoring is as follows, where True means cooperate and False means defect:

my %scoring =
    'True True' => [4, 4],
    'True False' => [0, 6],
    'False True' => [6, 0],
    'False False' => [1, 1],

Your strategy should be a subroutine or block that accepts the named parameters mine and theirs, which are lists of previous decisions of your own algorithm and of its opponents, and total, which is the number of laps to be played. It should return True if it wishes to cooperate, and False to defect.

Here is an example strategy that starts off with cooperating, and then randomly chooses a previous reaction of the current opponent:

sub example-strategy(:@theirs, *%) {
    @theirs.roll // True;
}

Your strategy or strategies will play against each other and against the example strategies in the gist above. It is not allowed to submit strategies that commit suicide to actively support another strategy.

I too have written two strategies that will take participate in the contest. Here is the checksum to convince you I won't alter the strategies in response to the submissions:

6d4ba99b66e4963a658c8dcfc72922dd0f74e0ad  prisoner-moritz.pl

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