#!/usr/bin/env python
#
# acpi_table_checksum - verify and calculate the checksum of ACPI tables
#
# Copyright (C) 2015  Antonio Ospite <ao2@ao2.it>
#
# This program is free software. It comes without any warranty, to
# the extent permitted by applicable law. You can redistribute it
# and/or modify it under the terms of the Do What The Fuck You Want
# To Public License, Version 2, as published by Sam Hocevar. See
# http://sam.zoy.org/wtfpl/COPYING for more details.

import sys

CHECKSUM_OFFSET = 9


# Two's complement byte checksum:
def acpi_table_checksum(buf, length):
    checksum = 0

    for i in range(0, length):
        checksum = (checksum + buf[i]) & 0xff

    return (0x100 - checksum) & 0xff


def calc_new_checksum(data):
    data_copy = data[:]
    data_copy[CHECKSUM_OFFSET] = 0
    return acpi_table_checksum(data_copy, len(data_copy))


def verify_checksum(data):
    return (acpi_table_checksum(data, len(data)) == 0)


def usage():
    print "%s <ACPI_TABLE>" % sys.argv[0]


if __name__ == "__main__":
    if len(sys.argv) != 2:
        usage()
        sys.exit(1)

    table = sys.argv[1]

    with open(table, 'rb') as f:
        data = bytearray(f.read())

        checksum_is_valid = verify_checksum(data)

        if checksum_is_valid:
            print "Checksum valid (0x%02x)." % data[CHECKSUM_OFFSET]
        else:
            print "Checksum not valid, calculating a new one..."
            new_checksum = calc_new_checksum(data)
            print "New checksum 0x%02x" % new_checksum
            data[CHECKSUM_OFFSET] = new_checksum
            print "Checksum vaiid %d" % verify_checksum(data)