Debug recursive MinMax in TicTacToe

Question

I am trying to get the minmax algorithm (computer AI) to work in my tic-tac-toe game. I got stuck on this for a few days. In fact, I don’t understand why computer AI simply places its marker ( "O") in sequential order from the boards of the board 0-8.

For example, as human-human, if I choose 1, then the computer will choose 0:

 O| X| 2
--+---+--
 3| 4| 5
--+---+--
 6| 7| 8

Next, if I choose 4, then the computer will select 2:

 O| X| O
--+---+--
 3| X| 5
--+---+--
 6| 7| 8

Etc:

 O| X| O
--+---+--
 O| X| O
--+---+--
 X| 7| X

I debugged the minmax algorithm as much as possible, but it is very difficult for me to keep track of what is happening.

ComputerPlayer ( ). minmax , . ( 100% worst_score .)

class ComputerPlayer < Player
  def move(game_board)
    minmax(game_board) #minmax to create @best_move

    game_board.place_piece(@best_move, marker)
  end

  def minmax(board, player_tracker = 0) 
    if board.game_over?
      return score(board)
    else
      worst_score  = (1.0/0.0) #Infinity
      best_score  = -(1.0/0.0) #-Infinity
      @best_move  = board.get_available_positions.first

      new_marker = player_tracker.even? ? 'O' : 'X'
      player_tracker += 1

      board.get_available_positions.each do |move|
        new_board = board.place_piece(move, new_marker)
        current_score = minmax(new_board, player_tracker)
        if new_marker == marker #if the player is the computer player
          if current_score > best_score
            @best_move = move
            best_score = current_score
          end
        else
          if current_score < worst_score
            worst_score = current_score
          end
        end
      end
    end
    return best_score
  end

  def score(board)
    if board.winner == "O" #'O' == 'O', 'nil' == 'O'
      10
    elsif board.winner == "X" #'X' != 'O', 'nil' != 'O'
      -10
    elsif board.winner == nil
      0
    end
  end
end

Answer 1

, minmax best_score.

minmax . , - , -, .

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def minmax(board, player_tracker = 0, iteration = 0) #minmax
    if board.game_over?
        return score(board, iteration)
    end

    new_marker = player_tracker.even? ? 'O' : 'X'

    scores = {}
    board.get_available_positions.each do |move|
        new_board = board.place_piece(move, new_marker)
        scores[move] = minmax(new_board, player_tracker + 1, iteration + 1)
    end

    if player_tracker.even?
        @best_move = scores.sort_by {|_key, value| value}.reverse.to_h.keys[0]
    else
        @best_move = scores.sort_by {|_key, value| value}.to_h.keys[0]
    end

    return scores[@best_move]
end

, , , . 1- , , ?

def score(board, iteration)
    # "O", "X", "nil"
    if board.winner == "O" #'O' == 'O', 'nil' == 'O'
      10.0 / iteration
    elsif board.winner == "X" #'X' != 'O', 'nil' != 'O'
      -10.0 / iteration
    elsif board.winner == nil
      0
    else
      raise "ERROR"
    end
end

2 , , .