From a totally raw point of view, I would keep track of "x previous moves" and "current move".

Warning: Spoiler Code Ahead

#!/usr/bin/perl -w

# roshambo
# usage:  roshambo [look-behind]
#   look-behind    number of previous turns AI examines

use strict;

my $look_behind = shift || 10;
my @beat = (1, 2, 0);
my @names = qw( Rock Paper Scissors );
my @chars = qw( R P S );

my (%freq,@last,$uw,$cw,$t);


while (1) {
  my $rps;

  ### UPDATE
  ### FIXED SELECTION

  for my $i (0 .. $#last) {
    if (my $c = $freq{"@last[$i .. $#last]"}) {
      $rps = $beat[ weighted(@$c) ];
      last;
    }
  }

  $rps = int rand 3 if not defined $rps;

  print "Rock (1), Paper (2), Scissors (3):  ";
  chomp(my $user = <STDIN>);

  last if !$user or $user > 3;

  ### UPDATE
  ### FIXED WEIGHTING

  $user--;
  for (0 .. $#last) {
    $freq{"@last[0 .. $_]"}[$user]++;
  }

  shift @last if @last == $look_behind;
  push @last, $user;

  print "$names[$user] vs. $names[$rps] => ";

  if ($rps == $user) { $t++; print "tied\n" }
  elsif ($rps == $beat[$user]) { $cw++; print "AI wins\n" }
  else { $uw++; print "you win\n" }   
}


sub weighted {
  my ($choice,$sum);

  for my $i (0 .. $#_) {
    $_[$i] ||= 0;
    $choice = $i if rand($sum += $_[$i]) < $_[$i];
  }
    
  return $choice;
}
     
   
print "You won: $uw\n";
print "AI won:  $cw\n";
print "Ties:    $t\n\n";
  
for (sort { length($a) <=> length($b) } keys %freq) {
  for (split ' ') { print $chars[$_] }
  print " => ( ";
  for my $i (0 .. 2) {
    print "$chars[$i] = ", $freq{$_}[$i] || 0, ", ";
  }
  print ")\n";
}
[download]

All that code does is remember what move you made when you'd made a certain list of previous moves. It selects a weighted choice. The bottom section is output to show how well the AI did.

japhy -- Perl and Regex Hacker

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