Squirrel Eiserloh presented strong evidence for using noise functions in place of more traditional RNGs. Random number generators typically are forward-only mutating algorithms; they can’t be threaded easily, can’t be rewound or skipped forward efficiently, and may have expensive seed or copy operations. Noise functions are stateless and so solve most of those issues. The most eye-opening bit of the talk were a set of benchmarks and tests showing that noise can outperform even well-regarded RNGs like Mersenne Twister by orders of magnitude, and these fast noise functions can produce a higher quality of randomness! Plus an RNG can be built from a noise function just by adding a generation value, which itself can be used from multiple threads safely via atomic increments, so a noise-based RNG is a very compelling option for game devs.
It all seems obvious in retrospect, but Squirrel’s talk made a case that perhaps really needed to be made.
# ~!~ coding: utf-8 ~!~
"""
squirrel3
~~~~~~~~~
Provides a noise function which is fast and had good distribution. It was
introduced by Squirrel Eiserloh at 'Math for Game Programmers: Noise-based
RNG', GDC17.
:copyright: (c) 2017 by Heungsub Lee.
:license: BSD, see LICENSE for more details.
"""
import random
__all__ = ['squirrel3', 'Squirrel3Random']
# The base bit-noise constants were crafted to have distinctive and interesting
# bits, and have so far produced excellent experimental test results.
NOISE1 = 0xb5297a4d # 0b0110'1000'1110'0011'0001'1101'1010'0100
NOISE2 = 0x68e31da4 # 0b1011'0101'0010'1001'0111'1010'0100'1101
NOISE3 = 0x1b56c4e9 # 0b0001'1011'0101'0110'1100'0100'1110'1001
CAP = 1 << 32
def squirrel3(n, seed=0):
"""Returns an unsigned integer containing 32 reasonably-well-scrambled
bits, based on a given (signed) integer input parameter `n` and optional
`seed`. Kind of like looking up a value in an infinitely large
non-existent table of previously generated random numbers.
"""
n *= NOISE1
n += seed
n ^= n >> 8
n += NOISE2
n ^= n << 8
n *= NOISE3
n ^= n >> 8
# Cast into uint32 like the original `Squirrel3`.
return n % CAP
class Squirrel3Random(random.Random):
_n = 0
def seed(self, a=None):
if a is None:
a = 0
self._seed = a
def random(self):
n = self._n
self._n += 1
return squirrel3(n, self._seed) / float(CAP)
