Roulette Wheel

sub pick {
      #################
      #construct the triangular number sequence of the sums of the fitness _ranks_ of the (ascension sorted) population containing POP elements
      #pick a random point less than the total of the sums of said triangular sequence
      #determine which element's interval this falls into
      #select and return that element
      #################
      my ($pop) = @_; 
      #sum of the triangular sequence 1..POP
      state $tot = (POP+POP**2)/2;
      #found by solving x = (n^2-n)/2 + n for n
      my $elem = int( (sqrt(1 + 8*rand($tot)) - 1) / 2 );
      return $pop->[$elem];
    }