animal_well_checksum_fix.py

#!/usr/bin/env python3
# vim: set expandtab tabstop=4 shiftwidth=4:

import struct
import argparse

def main():

    parser = argparse.ArgumentParser(
            description='Fix Animal Well savefile verification after edits',
            )

    parser.add_argument('filename',
            nargs=1,
            type=str,
            help='Save file to edit',
            )

    args = parser.parse_args()
    args.filename = args.filename[0]

    with open(args.filename, 'r+b') as df:

        # Read the data and at least check the first uint32 to see if
        # it's 9, which we believe is a version number for the savefile.
        data = df.read()
        df.seek(0)
        version = struct.unpack('<I', df.read(4))[0]
        if version != 9:
            raise RuntimeError(f'{args.filename} does not look like an Animal Well savefile that we understand')

        # Compute the XOR'd checksum (which lives at byte 0xD).  Be sure
        # to omit whatever checksum is currently living there.  (We could
        # substitute 0x00 but that wouldn't change the XOR, so we're just
        # skipping the byte entirely.)
        total = 0
        for byte in data[:13] + data[14:]:
            total ^= byte

        # Check to see if we even need to write it
        if total == data[0xD]:
            print(f'{args.filename}: No need to write checksum -- 0x{total:02X} is already correct')
        else:
            packed = struct.pack('<B', total)
            df.seek(0xD)
            df.write(packed)
            print(f'{args.filename}: Wrote new checksum 0x{total:02X} (previous: 0x{data[0xD]:02X})')

if __name__ == '__main__':
    main()