import struct

def calculate_checksum(data: bytearray) -> int:
    """
    Calculates the checksum for the given bytearray excluding the checksum field itself.
    Args:
        data (bytearray): The bytearray representing the file data.
    Returns:
        int: The calculated checksum value.
    """
    sum = 0
    for i in range(len(data)):
        ch = data[i]
        if 12 <= i < 16:
            ch = 0
        sum = (sum << 1) + ch
        # Ensure sum stays within 32 bits
        sum &= 0xFFFFFFFF
    return sum

def update_checksum(data: bytearray) -> None:
    """
    Updates the checksum field in the bytearray to reflect the calculated checksum.
    Args:
        data (bytearray): The bytearray representing the file data.
    """
    # Calculate the checksum excluding the current checksum bytes
    checksum = calculate_checksum(data)
    
    # Pack checksum into 4 bytes (little-endian)
    checksum_bytes = struct.pack('<I', checksum)
    
    # Update the checksum position (12-15 bytes)
    checksum_position = 12
    data[checksum_position:checksum_position+4] = checksum_bytes

def save_file(filename: str, data: bytearray) -> None:
    """
    Saves the bytearray to a file.
    Args:
        filename (str): The path to the file.
        data (bytearray): The bytearray to save.
    """
    with open(filename, 'wb') as file:
        file.write(data)

def main():
    filename = 'test.d2s'
    
    # Load file into bytearray
    with open(filename, 'rb') as file:
        data = bytearray(file.read())
    
    # Update the checksum in the data
    update_checksum(data)
    
    # Save the modified file
    save_file(filename, data)

if __name__ == "__main__":
    main()