Problems with converting alpha-beta pruning algorithm from C++ to Ruby

So, alpha-beta pruning seems to be the most efficient algorithm out there aside from hard coding (for tic-tac-toe). However, I'm having problems converting the algorithm from the C++ example given in the link: http://www.webkinesia.com/games/gametree.php.

Players are开发者_开发问答 either 1 or 0, so doing 1-player will switch the player.

WIN = 1
LOSS = -1
DRAW = 0
INFINITY = 100
def calculate_ai_next_move
  best_move = -1
  best_score = -INFINITY

  cur_player = COMPUTER
  self.remaining_moves.each do |move|
    self.make_move_with_index(move, cur_player)
    score = -self.alphabeta(-INFINITY,INFINITY, 1 - cur_player)
    self.undo_move(move)

    if score > best_score
      best_score = score
      best_move = move
    end
  end

  return best_move
end

def alphabeta(alpha, beta, player)
  best_score = -INFINITY
  if not self.has_available_moves?
    return WIN if self.has_this_player_won?(player) == player
    return LOSS if self.has_this_player_won?(1 - player) == 1 - player
    return DRAW
  else
    self.remaining_moves.each do |move|
      break if alpha > beta

      self.make_move_with_index(move, player)
      move_score = -alphabeta(-beta, -alpha, 1 - player)
      self.undo_move(move)

      if move_score > alpha
        alpha = move_score
        next_move = move
      end
      best_score = alpha
    end
  end
  return best_score
end

Currently, the algorithm is playing terribly. It will initially pick the last space, and then choose the first (from left to right) available space after that.

Any idea with what's wrong with it?

Also, I've been doing TDD, so I know that self.has_this_player_won?, self.undo_move and self.remaining_moves is correct.

Hint: where is beta updated?

I guess you should swap alpha and beta in every level of the tree.