Modulo and inverse

gistfile1.txt

def power(x, y, p):
    res = 1
    x = x % p
    while y > 0:
        if y & 1:
            res = (res * x) % p
        y = y//2
        x = (x * x) % p
    return res

def modInverse(a, p):
    # Only if p is prime
    return power(a, p - 2, p)

def modFact(n, p):
    if p <= n:
        return 0
    res = p-1
    for i in range(n+1, p):
        res = (res * modInverse(i, p)) % p
    return res

n = 21
p = 23
print('{}! mod {} = {}'.format(n, p, modFact(n, p)))

print('inverse({}, {}) = {}'.format(22, 23, modInverse(22, 23)))