"""Chapter 6, exercise 2 in Allen Downey's Think Complexity book.

1. Write a program that implements one of the linear congruential generators
   described at http://en.wikipedia.org/wiki/Linear_congruential_generator

"""

class LCG(object):
    """A class to implement a Linear Congruential Generator (LCG) pseudo-random
    number generator.

    """
    def __init__(self, a=1103515245, c=12345, m=2**31,
            output=lambda x: x & 0x7fffffff):
        """The parameters a, c, and m are the LCG parameters:
            xn = (a * xn + c) % m

        The parameter output must be a function that transforms the next output
        value. This can be used to shift or mask off various bits to
        statistically improve the values produced.

        """
        self.a = a
        self.c = c
        self.m = m
        self.xn = 0
        self.output = output

    def seed(self, val):
        """Seed the pseudo-random number generator with the value val."""
        self.xn = val

    def rand(self):
        """Generate and return the next pseudo-rando number in the sequence."""
        self.xn = (self.a * self.xn + self.c) % self.m
        return self.output(self.xn)


def main(script, prng):
    """Rig up either our random number generator or Python's to produce output
    on stdout so we can test it with dieharder:

        $ python ch6ex2.py lcg | dieharder -a -g 200        # to test ours
        $ python ch6ex2.py python | dieharder -a -g 200     # to test Python

    """
    import struct
    import random

    lcg = LCG()
    py_rng = lambda : random.randint(0, 0xffffffff)

    if prng == 'python':
        r = py_rng
    else:
        r = lcg.rand

    while True:
        n = r()
        s = struct.pack('>L', n)
        sys.stdout.write(s)


if __name__ == '__main__':
    import sys
    main(*sys.argv)