SFC64 Generator

class randomgen.sfc.SFC64(seed=None, w=1, k=1, *, mode='sequence')

Chris Doty-Humphrey’s Small Fast Chaotic PRNG with optional Weyl Sequence


A seed to initialize the BitGenerator. If None, then fresh, unpredictable entropy will be pulled from the OS. If an int or array_like[ints] is passed, then it will be passed to SeedSequence to derive the initial BitGenerator state. One may also pass in a SeedSequence instance.


The starting value of the Weyl sequence. If None, then the initial value is generated from the SeedSequence.


The increment to the Weyl sequence. Must be odd, and if even, 1 is added. If None, then k is generated from the `SeedSequence.


The default uses a seed sequence to initialize all unspecified values. When using “numpy” uses the seed sequence to initialize three values and checks that both w and k are 1.


SFC64 is a 256-bit implementation of Chris Doty-Humphrey’s Small Fast Chaotic PRNG ([1]). SFC64 has a few different cycles that one might be on, depending on the seed; the expected period will be about \(2^{255}\) ([2]). SFC64 incorporates a 64-bit counter which means that the absolute minimum cycle length is \(2^{64}\) and that distinct seeds will not run into each other for at least \(2^{64}\) iterations. SFC64 provides a capsule containing function pointers that produce doubles, and unsigned 32 and 64- bit integers. These are not directly consumable in Python and must be consumed by a Generator or similar object that supports low-level access.

State and Seeding

The SFC64 state vector consists of 4 unsigned 64-bit values. The last is a 64-bit counter that increments by 1 each iteration. The input seed is processed by SeedSequence to generate the first 3 values, then the SFC64 algorithm is iterated a small number of times to mix.

Compatibility Guarantee

SFC64 makes a guarantee that a fixed seed will always produce the same random integer stream.


SFC64 supports generating distinct streams using different Weyl increments. The recommend practice is to chose a set of distinct odd coefficients that have 32 or fewer bits set of 1 (i.e., <= 50%).

>>> import numpy as np
>>> from randomgen import SFC64, SeedSequence
>>> NUM_STREAMS = 8196
>>> seed_seq = SeedSequence(325874985469)
>>> bit_gen = SFC64(seed_seq)
>>> weyl_inc = bit_gen.weyl_increments(NUM_STREAMS)
>>> streams = [SFC64(seed_seq, k=k) for k in list(weyl_inc)]
>>> [stream.random_raw() for stream in streams[:3]]
[13020151409549081939, 8062752282355435850, 13933250373249421220]


Seeding and State


Seed the generator


Get or set the PRNG state



CFFI interface


ctypes interface


random_raw([size, output])

Return randoms as generated by the underlying BitGenerator


weyl_increments(n[, max_bits, min_bits])

Generate distinct Weyl increments to construct multiple streams