scramble.py

GROUP_BITS = 31
PRIME = 7

def ddd(index):
    """
    Examples:

      >>> ddd(111)
      '00*00*0001***0*0*001****0**0**'
    """
    bitstring = bin(int(index))[2:].zfill(GROUP_BITS - 1)
    return encode(bitstring, GROUP_BITS, PRIME)

def encode(y, s, p):
    l = ["*"] * (s - 1)
    
    i = 1
    tmp = y[0]
    for t in range(s):
        l[i - 1] = tmp
        tmp = y[i - 1]
        i = i * p % s
    return ''.join(l)

def inv(s, p):
    for x in range(s):
        if x * p % s == 1:
            return x
    return None

def decode(y, s, p):
    return encode(y, s, inv(s, p))

def arr2int(arr):
    return int(''.join(arr), 2)