我正在用Python创建一个滑动拼图游戏，其中有一个n x n网格，每个正方形都有一个数字，除了一个是空白的，玩家输入一个数字，这个数字在空白正方形的旁边或上面/下面，与它交换位置。每次有效移动后，网格都会更新并打印出来，然后询问玩家下一步的移动，除非谜题被解决。游戏的目的是让所有的数字按数字顺序排列，最后一个方块是空白的。我需要你帮我数一个玩家完成拼图所走的步数。
我试过使用初始化为零的number_of_moves，并在玩家每次移动时增加一个。但是，当打印移动次数时，它总是打印“移动次数= 0”，然后打印“移动次数= 1”。不管我走多少步都是这样。到目前为止我写的代码是：

def milestone_2(tiles):
    def valid_moves(tiles_list, grid_size):
        number_moves = 0
        while tiles_list[:-1] != sorted(tiles_list[:-1]) and tiles_list[-1] != "":    
            while True:
                choice = input("Your move: ")
                if choice == 'quit':
                    print("Quitting...")
                    return
                if choice not in tiles_list:
                    print(f"{choice} is not valid. Try again.")
                    number_moves += 1
                    continue
                blank_square_index = tiles_list.index("")
                choice_index = tiles_list.index(choice)
                if abs(choice_index - blank_square_index) == grid_size or ((choice_index // grid_size) == (blank_square_index // grid_size)) and (abs(choice_index - blank_square_index)) == 1 :
                    tiles_list[blank_square_index], tiles_list[choice_index] = tiles_list[choice_index], tiles_list[blank_square_index]
                    milestone_2(','.join(tiles_list))
                    number_moves += 1
                    break   
                else:
                    print(f"{choice} is not valid. Try again.")
                    number_moves += 1
        print(f"You won in {number_moves} moves. Congratulations!")
        
    def create_grid():          
        tiles_list = tiles.split(',')
        grid_size = int(len(tiles_list) ** 0.5)
        result = "┌" + ("────┬") * (grid_size -1) + "────┐\n" 
        for i in range(0, len(tiles_list), grid_size):
            result += "│"
            for j in range(grid_size):
                tile = tiles_list[i+j]
                if tile == "":
                    result += "    │"
                elif len(tile) == 1:
                    result += "  " + tile + " │"    
                elif len(tile) == 2:
                    result += " " + tile + " │"
            result += "\n"
            if i < len(tiles_list) - grid_size:
                result += "├" + (grid_size-1) * "────┼" + "────┤\n"
        result += "└" + (grid_size-1)*"────┴" + "────┘"
        print(result)
        valid_moves(tiles_list, grid_size)
    create_grid()

milestone_2("1,2,3,4,5,6,7,8,9,10,11,12,13,14,,15")