Debugging a stack level too deep error - Ruby

Question

I am building a Tic Tac Toe game in which the user can play a computer, or computers can play each other. While building the AI I am running into the below error. How can I debug this? I understand it is to do with a loop somewhere but I can't find it.

ttt-with-ai-project-cb-000/lib/players/computer.rb:13:in `each': stack level too deep (SystemStackError)
        from ttt-with-ai-project-cb-000/lib/players/computer.rb:13:in `detect'
        from ttt-with-ai-project-cb-000/lib/players/computer.rb:13:in `check_move'
        from ttt-with-ai-project-cb-000/lib/players/computer.rb:8:in `move'
        from ttt-with-ai-project-cb-000/lib/game.rb:61:in `wargame_turn'
        from ttt-with-ai-project-cb-000/lib/game.rb:64:in `wargame_turn'
        from ttt-with-ai-project-cb-000/lib/game.rb:64:in `wargame_turn'
        from ttt-with-ai-project-cb-000/lib/game.rb:64:in `wargame_turn'
        from ttt-with-ai-project-cb-000/lib/game.rb:64:in `wargame_turn'
         ... 11900 levels...
        from bin/tictactoe:37:in `block in run_game'
        from bin/tictactoe:35:in `times'
        from bin/tictactoe:35:in `run_game'
        from bin/tictactoe:79:in `<main>'

The Offending Methods

lib/players/computer.rb

def move(board)
    check_move(board)
end

def check_move(board)
    win_combo = Game::WIN_COMBINATIONS.detect do |indices|
    board.cells[indices[0]] == token && board.cells[indices[1]] == token || board.cells[indices[1]] == token && board.cells[indices[2]] == token || board.cells[indices[0]] == token && board.cells[indices[2]] == token
    end

    win_combo.detect {|index| board.cells[index] == " "}.join if win_combo
end

lib/game.rb

def wargame_turn
    input = current_player.move(board)

    if !board.valid_move?(input)
        wargame_turn
    else
        board.update(input, current_player)
    end
end

bin/tictactoe calls the method in the following block within #run_game:

100.times do 
    game = Game.new(Players::Computer.new("X"), player_2 = Players::Computer.new("O"))
    game.wargames
    if game.winner == "X"
        x_wins += 1
    elsif game.winner == "O"
        x_wins += 1
    elsif game.draw?
        draws += 1
    end
end

Answer 1

You can use the ruby Tracer class:

ruby -r tracer your_main_script.rb

Also, you can take a look at your code and see where the loop may happen:

def wargame_turn

  input = current_player.move(board)

  if !board.valid_move?(input)

    wargame_turn #### HERE'S A POTENTIAL INFINITE LOOP

So the heart of the problem seems at:

if !board.valid_move?(input)

It could be a good start.