London Racer: World Challenge unpacker

unpacker.py

'''
Unpacker for game archive of London Racer: World Challenge

Expects to run in the folder which contains files WR.idx and WR.img, creates
folder unpacked with the unpacked game resources.

See https://bunyk.github.io/posts/reverse_engineering_game_archive to know how this was made.
'''
import os
import struct

def main():
    items = read_index('WR.idx')

    with open('WR.img', 'rb') as img:
        create_folder('unpacked', items, img)

def read_index(filename):
    with open(filename, 'rb') as f:
        f.seek(48) # skip header
        return read_folder(f)

def read_folder(f):
    n = read_int(f)
    return [read_item(f) for i in range(n)]

def read_int(f):
    data = f.read(4)
    decoded = struct.unpack('<l', data) # unpack it as little-endian int
    return decoded[0]

def read_item(f):
    is_file = f.read(1)[0] # type of item
    f.read(3) # unknown bytes
    f.read(8) # zeroes
    name = read_string(f)
    if is_file == 1:
        size, offset = read_file_fields(f)
        return (name, size, offset)
    else:
        return (name, read_folder(f))

def read_string(f):
    res = []
    while True:
        c = f.read(1)[0]
        if c == 0:
            pad = f.tell() % 4
            if pad > 0:
                f.read(4 - pad) # skip padding
            return ''.join(res)
        res.append(chr(c))
    return res

def read_file_fields(f):
    data = f.read(16)
    size1, size2, fsize, offset = struct.unpack('<llll', data)
    f.read((fsize - 1) * 4) # skip some unknown values
    return size1, offset

def create_folder(path, items, img):
    os.mkdir(path)
    print("created folder", path)
    for i in items:
        np = path + '/' + i[0]
        if len(i) == 2: # another folder
            create_folder(np, i[1], img)
        if len(i) == 3:
            data = read_slice(img, i[1], i[2])
            create_file(np, data)

def read_slice(f, size, offset):
    f.seek(offset)
    return f.read(size)

def create_file(filename, data):
    with open(filename, 'wb') as f:
        f.write(data)
    print("created file  ", filename)

if __name__ == '__main__':
    main()