Refactored recursive minimax with alpha/beta pruning (Flog score: 12)

gistfile1.rb

def minimax_with_alpha(max_player = true, ply = 0, alpha = -1000, beta = 1000)
  return(board.winner? ? winning_score(max_player, ply) : 0) if base_case_satisfied?
  ab_value            = max_player ? -1000 : 1000
  ab_value, best_move = gen_score_game_tree(max_player, ply, alpha, beta, ab_value)                                                                                           
  return( ply == 0 ? best_move : ab_value)
end 

def gen_score_game_tree(max_player, ply, alpha, beta, ab_value)
  best_move = 0 
  available_moves.each do |index|
    board[][index]                   = mark_curr_player_side(max_player)
    score                            = minimax_with_alpha(!max_player, ply + 1, alpha, beta)
    alpha, beta, ab_value, best_move = eval_score(max_player, index, score, alpha, beta, ab_value, best_move)
    undo_move(index)
    break if alpha_beta_swapped?(alpha, beta)
  end 
  [ab_value, best_move]
end 

def eval_score(max_player, index, score, alpha, beta, ab_value, best_move)
  best_move, alpha, ab_value = [index, score, score] if max_player && score > ab_value
  beta, ab_value                  = [score, score]        if !max_player && score < ab_value
  [alpha, beta, ab_value, best_move]
end 

def mark_curr_player_side(max_player)
  max_player ? side : opposite_side(side)
end
                                                                                                                                                                                                            
def opposite_side(side)
  side == "x" ? "o" : "x"
end

def alpha_beta_swapped?(alpha, beta)
  alpha >= beta
end

def base_case_satisfied?
  board.winner? || board.draw_game?
end
                                                                                                                                                                                                            
def winning_score(max_player, ply)
  max_player ? (-9999 + ply) : (9999 - ply)
end

def available_moves
  board[].each.with_index.map { |element, index| index if element == " " }.compact
end