#!/usr/bin/env python3
# -*- coding: utf-8 -*-

import binascii
import struct
import argparse

def calculate_mpeg2_crc32(data):
    """
    Calculate MPEG-2 style CRC32 (Polynomial: 0x04C11DB7, Initial: 0xFFFFFFFF)
    Note: Result is NOT inverted (unlike standard CRC32)
    """
    crc = 0xFFFFFFFF
    for byte in data:
        crc ^= byte << 24
        for _ in range(8):
            if crc & 0x80000000:
                crc = (crc << 1) ^ 0x04C11DB7
            else:
                crc = crc << 1
        crc &= 0xFFFFFFFF
    return crc

def process_file(filename):
    try:
        with open(filename, 'rb+') as f:
            # Verify exact 256 bytes size
            f.seek(0, 2)
            if f.tell() != 256:
                raise ValueError(f"File must be exactly 256 bytes (got {f.tell()} bytes)")
            
            f.seek(0)
            data = f.read(252)
            if len(data) != 252:
                raise ValueError("Couldn't read 252 bytes from file")
            
            crc = calculate_mpeg2_crc32(data)
            f.seek(252)
            f.write(struct.pack('<I', crc))
            
            print(f"Success: Updated {filename} with CRC32 0x{crc:08X} at offset 252")
            return True
            
    except Exception as e:
        print(f"Error processing {filename}: {str(e)}")
        return False

if __name__ == "__main__":
    parser = argparse.ArgumentParser(
        description='Calculate MPEG-2 CRC32 for first 252 bytes and write to last 4 bytes of 256-byte file')
    parser.add_argument('filename', help='Binary file to process (must be 256 bytes)')
    args = parser.parse_args()
    
    if not process_file(args.filename):
        exit(1)