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Posted to issues@commons.apache.org by "Gilles (JIRA)" <ji...@apache.org> on 2019/04/27 13:39:00 UTC
[jira] [Commented] (RNG-97) JumpableUniformRandomProvider
[ https://issues.apache.org/jira/browse/RNG-97?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16827603#comment-16827603 ]
Gilles commented on RNG-97:
---------------------------
In the interface's Javadoc, wouldn't it be better to replace
{noformat}
copy will be advanced an equivalent of {@code n} sequential calls
{noformat}
by something like
{noformat}
copy will be advanced by a fixed, implementation-dependent, number of sequential calls
{noformat}
?
In the helper method, I'm not sure that saying
{noformat}
Reuse of this generator may overlap with output from the jump series.
{noformat}
is correct; or at least, it is also true for all instances in the returned array (if {{next()}} is called sufficiently many times).
In the helper also, we should perhaps allow argument {{n}} to be zero (?).
I'm not too fond of the {{Series}} suffix in the name; IMHO it does not add sufficient clarity for having a longer name...
Perhaps {{jump}} is all that's needed (together with the self-documenting return value).
I'd think that the {{source}} argument would be advantageously renamed {{parent}}.
In order to facilitate usage (e.g. for launching parallel computations), we could add
{code:java}
public static UniformRandomProvider[] createJump(RandomSource source, int n) {
final UniformRandomProvider parent = source.create();
final UniformRandomProvider[] rngs = new UniformRandomProvider[n];
rngs[0] = parent;
final UniformRandomProvider[] children = jump(parent, n - 1);
for (int i = 0; i < children.length; i++) {
rngs[i + 1] = children[i];
}
return rngs;
}
{code}
With a variant where seed is user-provided (?).
> JumpableUniformRandomProvider
> -----------------------------
>
> Key: RNG-97
> URL: https://issues.apache.org/jira/browse/RNG-97
> Project: Commons RNG
> Issue Type: New Feature
> Components: client-api, core
> Affects Versions: 1.3
> Reporter: Alex D Herbert
> Assignee: Alex D Herbert
> Priority: Major
> Fix For: 1.3
>
>
> A feature of random number generators is their internal state is updated for every generation of a new random number. This is a single step. Some generators have the ability to compute the update to the state that is required to advance *{{n}}* steps. This is a jump. This can be supported using a new interface:
> {code:java}
> /**
> * Applies to generators that can be advanced a large number of
> * steps of the output sequence in a single operation.
> */
> public interface JumpableUniformRandomProvider
> extends UniformRandomProvider {
> /**
> * Creates a copy of the UniformRandomProvider and advances the
> * state of the copy. The state of the current instance is not altered.
> * The state of the copy will be advanced an equivalent of {@code n}
> * sequential calls to a method that updates the state of the provider.
> *
> * @return the copy with an advanced state
> */
> JumpableUniformRandomProvider jump();
> }
> {code}
> A suggestion for how to document an implementation is:
> {code:java}
> public class JumpableRNG implements JumpableUniformRandomProvider {
> /**
> * {@inheritDoc}
> *
> * <p>The jump size {@code n} is the equivalent of <pre>2<sup>32</sup></pre>
> * calls to {@link UniformRandomProvider#nextLong() nextLong()}.</p>
> */
> @Override
> public JumpableUniformRandomProvider jump() {
> return ...;
> }
> // etc.
> }
> {code}
> Notes on the interface:
> * A copy is returned
> * The original is not altered and so multiple calls to jump will return the same generator with an advanced state
> The intended use case is to create multiple copies of a RNG that will not overlap in sequence for use in parallel computations. A helper method can be added to {{RandomSource}} to facilitate this:
> {code:java}
> /**
> * Create a series of {@code n} generators by jumping from the source generator
> * {@code n} times. The resulting set of generators can be used in parallel
> * computations with a guarantee of no sequence overlap for at least the
> * length of the jump distance.
> *
> * <p>Note: The source generator state is not affected. Reuse of this
> * generator may overlap with output from the jump series.</p>
> *
> * @param source The source generator.
> * @param n The size of the series.
> */
> public static UniformRandomProvider[] createJumpSeries(
> JumpableUniformRandomProvider source, int n) {
> if (n <= 0) {
> throw new IllegalArgumentException("Size must be strictly positive");
> }
> final UniformRandomProvider[] rngs = new UniformRandomProvider[n];
> for (int i = 0; i < n; i++) {
> source = source.jump();
> // Option to wrap the JumpableUniformRandomProvider to
> // restrict to the functionality of UniformRandomProvider
> rngs[i] = source;
> }
> return rngs;
> }
> {code}
> Note: There is the possibility to wrap the jumped RNGs to restrict their functionality to UniformRandomProvider (or RestorableUniformRandomProvider). This prevents any of the series from being jumped again.
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