Splittable pseudorandom number generators in Haskell: random v1.1 and v1.2
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Pseudorandom number generators (PRNGs) power property tests and Monte Carlo simulations. They are also used to model random behaviour in nature and to initialise machine learning models^{1}. Their performance and quality matter.
Unfortunately, up to version 1.1, the default PRNG of the random library for Haskell was neither very fast nor very high quality. The recently announced version 1.2 fixes this. Random version 1.2 is a collaboration between Dominic Steinitz, Alexey Kuleshevich, and myself, with invaluable support from Alexey Khudyakov and Andrew Lelechenko.
In this blog post, I will focus on how we ensured that the implementation used in random v1.2 is, indeed, a high quality PRNG.
Pseudorandom number generators
A PRNG produces from a seed a sequence of approximately uniformly distributed numbers.
init :: Seed > State
next :: State > (Output, State)
This allows us to generate a output sequence x1, x2, x3, …
as follows:
let g0 = init seed
(x1, g1) = next g0
(x2, g2) = next g1
(x3, g3) = next g2
This is not always enough though. Often you may want two or more parallel sequences that are still (approximately) random even when assembled. Such sequences are said to be uncorrelated.
split :: State > (State, State)
This can arise when you have different threads. But it’s of prime
importance in a lazy language like Haskell, since it makes it possible
to generate deterministic random numbers lazily. For instance, split
is used in
QuickCheck’s Gen monad.
An easy implementation of split
is to duplicate the current
state. But, the two parallel sequences will be correlated: indeed
they will be identical. A slightly more sophisticated implementation is to generate
the two split states randomly, but this will typically not yield
uncorrelated sequences either.
The split
function in random
up to version 1.1 creates generators that are correlated (#3575, #3620).
In a blog post comparing the performance of
Haskell PRNG implementations, Alexey Kuleshevich found splitmix
to
be the fastest pure PRNG written in Haskell, making it the top
candidate to replace the default PRNG in the proposed new version of
random
. But it also had to be good enough at generating random sequences.
In the following, I will go through how we tested the split
function in random
version 1.1 and splitmix0.1
, which we used for
version 1.2, in order to make sure that, while in version 1.1 split
generates correlated sequences, in version 1.2 split
generates
uncorrelated sequences.
Legacy and SplitMix.
The PRNG used by random
up to version 1.1 is one of the PRNG
algorithms proposed by L’Ecuyer, with an ad hoc split
function
tacked on without statistical justification. We shall refer to this
PRNG as Legacy. Since version 1.2, the PRNG is SplitMix, a PRNG
designed from the ground up to be splittable, as implemented in the
splitmix
package.
Every PRNG with a finite state is periodic: it will necessarily come back to the same state after a certain number of iterations, called its period. In contrast, long sequences of truly random numbers are unlikely to be periodic. As a result, every PRNG with a finite state is distinguishable from a source of true randomness, at least in theory.
Legacy produces numbers in the range $[1, 2^{31}5)$ and has a period of just under $2^{61}$; SplitMix produces numbers in the range $[0, 2^{64})$ and has a period of exactly $2^{64}$.
Considering the output range and period of Legacy and SplitMix, we see something interesting: Legacy’s period of roughly $2^{61}$ contains its range of roughly $2^{31}$ numbers many times over. For SplitMix, on the other hand, the output range and period are both $2^{64}$. SplitMix in fact generates each output in $[0, 2^{64})$ precisely once.
We actually expect repetitions much more often than that: observing $2^{64}$ numbers where each number in the range $[0, 2^{64})$ occurs exactly once is vanishingly unlikely in a truly random sequence of numbers. This is is a manifestation of the Birthday problem.
And indeed, SplitMix fails a test that checks specifically that repetitions in the output sequence occur as frequently as expected according to the Birthday problem.
How much of an issue is this? It depends on the application. Property tests, for example, don’t benefit from repetitions. And if, as is common, you only need values in a subrange of the full $[0, 2^{64})$, you will have many repetitions indeed.
Reproducible test environment
While certain PRNG properties like output range and period can be determined “by inspection”, there is no clear definition or test for its quality, that is, how similar to uniformly distributed numbers a PRNG’s output sequence is.
The closest thing we have are collections of tests, often called test batteries, which empirically check if a PRNG’s output sequence has certain properties expected of a random number sequence — like the distance between repetitions, the famous Birthday problem mentioned in the previous section.
To make it easy to test PRNGs, we created a repository with a Nix shell environment to run the most wellknown PRNG test batteries.
Setup
Nix is required to use the test environment. The easiest way to enter it is using this command:
$ nixshell https://github.com/tweag/randomquality/archive/master.tar.gz
You can test that it works by running PractRand on a truly terrible “PRNG”: /dev/zero
, which outputs zero bytes. The PractRand binary is called RNG_test
, passing “stdin32” makes it read unsigned 32bit integers from standard input:
$ RNG_test stdin32 < /dev/zero
PractRand should immediately show a large number of test failures. Don’t use /dev/zero
as a source of random numbers!
PRNGs
While this post is about testing Legacy and SplitMix, the environment we created can test any PRNG written in any language.
To demonstrate this, we have included programs in C, Python, Perl and Lua, each using the language’s standard PRNG to output a stream of unsigned 32bit integers^{2}.
To see the source code of the generator scripts, run e.g. cat $(which generate.pl)
.
You can run these programs as generate.c
, generate.py
, generate.pl
and generate.lua
within the Nix environment.
All the statistical tests in this environment consume binary input.
To convert the hexadecimal 32bit numbers to binary, pipe them through xxd r p
, e.g. $ generate.lua  xxd r p
.
Test batteries
The environment contains a collection of PRNG test batteries. All of them either support reading from standard input or were wrapped to support this. As a result, all the tests can be run as follows:
 To test a systemprovided randomness source, run
$ $TEST < $SOURCE
, e.g.$ RNG_test stdin32 < /dev/random
 To test a program that generates pseudorandom numbers, run
$ $PROGRAM  $TEST
, e.g.$ generate.py  xxd r p  RNG_test stdin32
The test batteries include:

PractRand, the most recent PRNG test in the collection. It is adaptive: by default, it will consume random numbers until a significant test failure occurs.
Example invocation:
$ RNG_test stdin32
Help:
$ RNG_test h

TestU01, containing the test batteries SmallCrush (
s
), Crush (c
) and BigCrush (b
). We have wrapped it in the executableTestU01_stdin
, which accepts input from standard input.Example invocation:
$ TestU01_stdin s
Help:
$ TestU01_stdin h
For a full list, see the README.
Split sequences
These test batteries test the randomness of individual sequences. But we want to test whether sequences generated by splits are uncorrelated or not, which involves at least two sequences at a time.
Rather than coming up with new test batteries dedicated to split
,
Schaathun proposes four sequences built of a
combination of next
and split
. These “split sequences”
are traditional (nonsplittable) PRNGs built off next
and
split
. The split sequences are then fed into regular PRNG tests.
Here is the API for splittable PRNGs again:
init :: Seed > State
next :: State > (Output, State)
split :: State > (State, State)
A naming convention for the result of split
will come in handy. Let’s say that a generator g
is split into gL
and gR
(for “left” and “right”), that is, (gL, gR) = split g
.
Then we get gLL
and gLR
by applying split
to gL
via (gLL, gLR) = split gL
, etc.
Let’s also call rX
the output of next gX
, i.e. (rLL, _) = next gLL
.
This lets us express concisely the first of the sequences proposed by Schaathun:
Sequence S_L: given g, output rL; repeat for g = gR
This sequence recurses into the right generator returned by split
while outputting a number generated by the left generator.
We can visualise the sequence as a binary tree where the nodes are split
operations.
Asterisk (*) stands for a generated output, ellipsis (…) is where the next iteration of the sequence continues:
split
/ \
* split
/ \
* …
The other sequences can be expressed as follows:
Sequence S_R: given g, output rR; repeat for g = gL
split
/ \
split *
/ \
… *
Sequence S_A: given g, output rR, rLL; repeat for g = gLR
split
/ \
split *
/ \
* …
Sequence S: given g, output rRLL, rRLR, rRRL, rRRR; repeat for g = gL
split
/ \
… split
/ \
split split
/ \ / \
* * * *
We implemented these sequences in a test program which uses Legacy or SplitMix as the underlying PRNG implementation and outputs the random numbers on stdout.
Test results for Legacy and SplitMix
The following table shows our test results. We tested with PractRand and TestU01.
The sequences S
, S_A
, S_L
and S_R
are those discussed above.
The test battery output is usually a pvalue per test indicating the probability of the input being random. We summarise the results as “Fail” or “Pass” here. Each table cell links to the full result, which includes the full command used to run them. Since the seeds of the sequences are fixed, you should be able to reproduce the results exactly.
PRNG / sequence  PractRand result  TestU01 result 

Legacy / S  Fail (8 MiB)  Fail (SmallCrush) 
SplitMix / S  Pass (2 TiB)  Pass (Crush) 
Legacy / S_A  Pass (1 TiB)  Pass (Crush) 
SplitMix / S_A  Pass (2 TiB)  Pass (BigCrush) 
Legacy / S_L  Fail (8 GiB)  Fail (Crush) 
SplitMix / S_L  Pass (2 TiB)  Pass (BigCrush) 
Legacy / S_R  Fail (64 GiB)  Fail (Crush) 
SplitMix / S_R  Pass (2 TiB)  Pass (BigCrush) 
Conclusion
In this post, I’ve described how we’ve made sure that the PRNG used in
the newly released version 1.2 of the random
library is of high
quality. The legacy PRNG used up to version 1.1 had a poor quality
implementation of the split
primitive, which we wanted to fix.
To this effect, we created a generalpurpose reproducible PRNG
testing environment, which makes it easy to run the most
commonly used PRNG tests using Nix with a minimal amount of setup
work. For testing, we used split sequences to reduce the quality of split
to
the quality of a traditional PRNG.
For further details, check the reddit announcement for random
1.2. And don’t hesitate to use our
repository to setup your own PRNG testing.
Further reading
Melissa O’Neill’s website on PCG, a family of PRNGs, contains a wealth of information on PRNGs and PRNG testing in general.
Peter Occil’s website is also a great starting point for further reading on PRNGs and PRNG testing, in particular the pages on PRNGs and on PRNG functions.

Securityrelevant applications also often require random data. They should not rely on the kinds of PRNGs discussed in this blog post. Instead, securityrelevant applications should use cryptographicallysecure pseudorandom number generators that have been vetted by the cryptographic community for this purpose.
↩ 
In the case of C, if
↩RAND_MAX
is not equal to 2^32, the program will not output the full 32bit range and will thus fail statistical tests. The other programs should in theory output numbers in the correct range.