FRACTRAN Prime Machine

from fractions import Fraction
from math import log

# 1/log(2) calculated because multiplicaiton is more efficient than division
ILOG2 = 1.0/log(2)
log2 = lambda x: log(x)*ILOG2

# Returns true if num is a power of 2
def isPowerOfTwo(num):
    return not num == 0 and ((num & (num - 1)) == 0)

# John Conway's FRACTRAN prime machine
tape = [
    Fraction(17,91),
    Fraction(78,85),
    Fraction(19,51),
    Fraction(23,38),
    Fraction(29,33),
    Fraction(77,29),
    Fraction(95,23),
    Fraction(77,19),
    Fraction(1,17),
    Fraction(11,13),
    Fraction(13,11),
    Fraction(15,14),
    Fraction(15,2),
    Fraction(55,1)
]

# Interpret a FRACTRAN program
def interpret(tape, n):
    index, output = 0, []
    while index < len(tape):
        # Print the next prime if we have it
        if isPowerOfTwo(n) and n not in output:
            output.append(n)
            print log2(n)

        # See if we get an integer, otherwise move on
        lens = n*tape[index]
        if (lens.denominator == 1):
            n, index = lens.numerator, 0
        else:
            index += 1

if __name__ == "__main__":
    interpret(tape, 2)