import random

def scramble(gen, buffer_size):
    buf = []
    i = iter(gen)
    while True:
        try:
            e = next(i)
            buf.append(e)
            if len(buf) >= buffer_size:
                choice = random.randint(0, len(buf)-1)
                buf[-1], buf[choice] = buf[choice], buf[-1]
                yield buf.pop()
        except StopIteration:
            random.shuffle(buf)
            yield from buf
            return