nsmb-checksums: Updates all checksums in a New Super Mario Bros. savefile (.sav, .dsv, etc).

nsmb-checksums.py

# nsmb-checksums.py
"""
MIT License

Copyright (c) 2017 RoadrunnerWMC

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
"""


import argparse
import struct


def nsmbChecksum(data):
    """
    Calculate the checksum of `data` using NSMB's savefile checksum
    algorithm.
    """
    checksum = sum(b'Mario2d') & 0xFFFF  # = 654

    for b in data:
        checksum = b ^ ((2 * checksum & 0xFFFE) | (checksum >> 15) & 1)
        checksum &= 0xFFFF
    
    return checksum


def recalculateSavefileChecksums(savefile):
    """
    Recalculate all checksums in the given savefile data.
    """
    savefile = bytearray(savefile)

    def doChecksum(checksumPos, dataLen):
        """
        Recalculate the checksum at `checksumPos`, of data of length
        `dataLen`.
        """
        checksum = nsmbChecksum(
            savefile[checksumPos + 10 : checksumPos + 10 + dataLen])
        struct.pack_into('<H', savefile, checksumPos, checksum)

    # Primary data / backup data
    for base in [0, 0x1000]:

        # Header
        doChecksum(base + 0x000, 0xF4)

        # Slots 1-3
        doChecksum(base + 0x100, 0x248)
        doChecksum(base + 0x380, 0x248)
        doChecksum(base + 0x600, 0x248)

        # Footer?
        doChecksum(base + 0x880, 0x14)

    return savefile


def main():
    parser = argparse.ArgumentParser(
        prog='nsmb-checksums',
        formatter_class=argparse.RawDescriptionHelpFormatter,
        description='Updates all checksums in a New Super Mario Bros. savefile (.sav, .dsv, etc).',
        epilog=__doc__)

    parser.add_argument('infile',
                        help='the input save file')
    parser.add_argument('outfile',
                        help='the output save file',
                        nargs='?')

    args = parser.parse_args()

    with open(args.infile, 'rb') as f:
        sav = f.read()

    fixedSav = recalculateSavefileChecksums(sav)

    with open(args.outfile if args.outfile else args.infile, 'wb') as f:
        f.write(fixedSav)

    print('Done!')


if __name__ == '__main__':
    main()