import sys
import os

# Parámetros equivalentes
desp = 0x2000
numBytes = 0x2000
firstByte = desp + 0x200
lastByte = desp + numBytes - 1
checksum1 = desp + 0x270
checksum2 = desp + 0x271

buffer = bytearray()


def check_parameters():
    if len(sys.argv) != 2:
        exe = os.path.basename(sys.argv[0])
        print(f"Uso: {exe} FILE")
        sys.exit(1)
    return sys.argv[1]


def load_file(path):
    global buffer
    try:
        with open(path, "rb") as f:
            buffer = bytearray(f.read())
    except IOError:
        print(f"No se puede abrir el archivo: {path}")
        sys.exit(1)


def save_file(path):
    try:
        with open(path, "wb") as f:
            f.write(buffer)
        print(f"Fichero guardado: {path}")
    except IOError:
        print(f"No se puede abrir el archivo: {path}")
        sys.exit(1)


def calc_checksum():
    total = 0
    for i in range(firstByte, lastByte + 1):
        if i not in (checksum1, checksum2):
            total += buffer[i]

    total += 3328  # Corrección del checksum

    buffer[checksum1] = total % 256
    buffer[checksum2] = (total >> 8) % 256

    return total


def main():
    filePath = check_parameters()
    load_file(filePath)
    save_file(filePath + ".bak")
    calc_checksum()
    save_file(filePath)


if __name__ == "__main__":
    main()