"""
http://www.nxp.com/documents/user_manual/UM10360.pdf

32.3.1.1 Criterion for Valid User Code
The reserved Cortex-M3 exception vector location 7 (offset 0x1C in the vector table)
should contain the 2's complement of the check-sum of table entries 0 through 6. This
causes the checksum of the first 8 table entries to be 0. The boot loader code checksums
the first 8 locations in sector 0 of the flash. If the result is 0, then execution control is
transferred to the user code.
"""
from struct import unpack, pack


def patch(bin_path):
    with open(bin_path, 'r+b') as bin:
        # Read entries 0 through 6 (Little Endian 32bits words)
        vector = [unpack('<I', bin.read(4))[0] for _ in range(7)]
        
        # location 7 (offset 0x1C in the vector table) should contain the 2's
        # complement of the check-sum of table entries 0 through 6
        bin.seek(0x1C)
        bin.write(pack('<I', (~sum(vector) + 1) & 0xFFFFFFFF))


def is_patched(bin_path):
    with open(bin_path, 'rb') as bin:
        # The checksum of the first 8 table entries should be 0
        return (sum([unpack('<I', bin.read(4))[0] for _ in range(8)]) & 0xFFFFFFFF) == 0


if __name__ == '__main__':
    bin_path = "C:/Users/emimon01/releases/emilmont/build/test/LPC1768/ARM/MBED_A1/basic.bin"
    patch(bin_path)
    assert is_patched(bin_path), "The file is not patched"
