common.py

# bunch of less exciting, common utilities we'll use in multiple files

# -----------------------------------------------------------------------------
# random number generation

# class that mimics the random interface in Python, fully deterministic,
# and in a way that we also control fully, and can also use in C, etc.
class RNG:
    def __init__(self, seed):
        self.state = seed

    def random_u32(self):
        # xorshift rng: https://en.wikipedia.org/wiki/Xorshift#xorshift.2A
        # doing & 0xFFFFFFFFFFFFFFFF is the same as cast to uint64 in C
        # doing & 0xFFFFFFFF is the same as cast to uint32 in C
        self.state ^= (self.state >> 12) & 0xFFFFFFFFFFFFFFFF
        self.state ^= (self.state << 25) & 0xFFFFFFFFFFFFFFFF
        self.state ^= (self.state >> 27) & 0xFFFFFFFFFFFFFFFF
        return ((self.state * 0x2545F4914F6CDD1D) >> 32) & 0xFFFFFFFF

    def random(self):
        # random float32 in [0, 1)
        return (self.random_u32() >> 8) / 16777216.0