edit_and_recalc.py

import glob


# from stackoverflow ref
def leftshift(int_in, shift_n, tot_bits):
    # returns an int.  take bin() to get  '0b10101'
    # bin(leftshift(int('10000000000000000000000000000000',2), 1, 32) )  ---> 0000...1
    return ((int_in << shift_n) % (1 << tot_bits)) | (int_in >> (tot_bits - shift_n))


def fix_checksum(bytarr):
    sum = 0 
    for i in range(len(bytarr)):
        # left shift for all idx
        sum = leftshift(sum, 1, 32) 
        sum += 0 if i in [12,13,14,15] else bytarr[i]

    bytarr[12:16] = sum.to_bytes(4, 'little')
    return bytarr


def open_and_edit(filename):
    # read .d2s char file bytes
    with open(filename, 'rb') as f:
        bytarr = bytearray(f.read())  # f.read() is bytes object (not bytes array)

    # save a backup of the .d2s char file
    with open(filename + '.bak', 'wb') as f:
        f.write(bytarr)

    # read patches
    with open('patches.txt', 'r') as f:
        lines = f.read().splitlines()

    # skip comments and blank lines and apply hex edits/patches
    for line in [l for l in lines if l.strip() and l[0] != '#']:
        # print(line)
        address = int(line.split(',')[0])
        if len(line.split(',')) > 2:  # decimal. convert to little endian hex e.g.  333deci -> 4d 01 00 00
            hexstr_list = int(line.split(',')[1].strip()).to_bytes(4, 'little').hex(' ').split()
        else:  # hex
            hexstr_list = line.split(',')[1].strip().split()
         
        # edit bytesarray       # orig 0:2  int85, 170 -> (b'U\xaa') -> 55aa
        for i in range(len(hexstr_list)):
            bytarr[address + i] = int(hexstr_list[i], 16)

    # write patched file with updated checksum
    with open(filename, 'wb') as f:
        f.write(fix_checksum(bytarr))


if __name__ == "__main__":
    filename = glob.glob('*.d2s')[0]
    open_and_edit(filename)
    print(f'd2s file {filename} saved successfully!')