Checksum Calculation

import sys
from struct import unpack_from

with open(sys.argv[1], mode='rb') as file:
    content = file.read()
    end = 0

    # Find the offset of 'END\0'
    for i in range(len(content), 0, -1):
        if unpack_from("<3s", content, i-3)[0] == 'END'.encode('ascii'):
            end = i - 7
            break

    data = list(content[:end])

    # Set save size in bytes to default for checksum calculation
    data[4] = 0x42
    data[5] = 0x2E
    data[6] = 0x1E
    data[7] = 0x00

    checksum = 0
    for byte in data:
        checksum += byte

    print("%X" % checksum)