unsigned long long int random_word(size_t n, size_t m)

# b = size of integer type
# n = number of set bits
# random_limited(x) is some function returning [0, x) sufficiently uniformly
def random_n_set_bits(b, n):
    assert b > 0
    assert n >= 0 and n <= b
    result = 0
    available = list(range(b))
    for i in range(n):
        index = random_limited(len(available))
        bit = available[index]
        available = available[:index] + available[index + 1:]
        result |= (1 << bit)
    return result