Another name for that wonderfully wacky game rock, paper, scissors. There are groups dedicated to the worship and analysis of this game. There is even a news group alt.religion.roshambo. There is a Worship of Roshambo web page. There is a e-mail based version Roshambo Rampage at Brunching Shuttlecocks. There are roshambo tournaments.

Search the web you can find lots of info about it.

Roshambo or rock paper scissors is interesting from the point of view of AI. You can download the source code for the entries in the first Roshambo programming contest from http://www.cs.ualberta.ca/~darse/rsbpc.html.

You might think that a random strategy would be optimal for this game but because there are some opponents with a strategy that sucks

int rockbot ()  /* Good Ole Rock */
{
    /* "Good ole rock.  Nuthin' beats rock." */
    return(rock);
}          
there are opportunities for intelligent strategies to out-perform Random. The winner in the first tournament was Iocaine Powder by Dan Egnor. There's a detailed explanation of how it works in the source code but I'll outline the general idea.

IP tries to predict what its opponent will do. It has several predictive algorithms that it can use for this. These include Random Guess, Frequency Analysis and History Matching. But it also uses a metastrategy so that each predictive algorithm P gets tranformed into six variants. I won't list them all but these include

  1. Just Use P.
  2. Try to defeat second-guessing. Assume that your opponent guesses you are using P. Say P predicts their next move is paper. So according to P IP would play scissors. Since your opponent is on to you they will play rock instead of P's prediction. So to defeat them, this variant strategy will play paper.
As you can guess, defeating triple-guessing is next!

Finally, how does IP decide between each of (the six variants of) its different predicitve algorithms? Simple. It assumes that it's opponent will be using a consistent strategy and tests out how each of the variants of each P would have done over the last few rounds. It then chooses the variant that would have done best.

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