import mmap, sys, random

target = open(sys.argv[1],'rb+')
buffer = mmap.mmap(target.fileno(), 0)

# flip 1% of the bits
flip_ratio = .01

target.seek(2, 0)
file_size = target.tell()
target.seek(0, 0)

for _ in xrange(int(file_size * flip_ratio)):
    idx = random.randint(0, (file_size * 8) - 1)
    buffer[idx / 8] ^= 1 << (idx % 8)

buffer.close()
target.close()