[jira] [Created] (RNG-184) ArraySampler to shuffle all primitive array types and generic T[] arrays

["Alex Herbert (Jira)" <ji...@apache.org>]

Alex Herbert created RNG-184:
--------------------------------

             Summary: ArraySampler to shuffle all primitive array types and generic T[] arrays
                 Key: RNG-184
                 URL: https://issues.apache.org/jira/browse/RNG-184
             Project: Commons RNG
          Issue Type: New Feature
          Components: sampling
    Affects Versions: 1.5
            Reporter: Alex Herbert
             Fix For: 1.6


The sampling module contains code to shuffle int[] in the PermutationSampler:
{code:java}
public static void shuffle(UniformRandomProvider rng, int[] list)
public static void shuffle(UniformRandomProvider rng,
   int[] list,
   int start,
   boolean towardHead) {code}
Shuffling support should be expanded to all primitive types and a generic array type.

The non-integer domain is out of scope for the PermutationSampler. The shuffle method is present as a utility for permuting arrays of indices.

I suggest a new API in ArraySampler with a shuffle that can handle sub-ranges as:
{code:java}
public static void shuffle(UniformRandomProvider rng, int[] data);
public static void shuffle(UniformRandomProvider rng, int[] data,
                           int from, int to);
{code}
Note that there is a ListSampler that offers similar functionality for a List:
{code:java}
public static <T> void shuffle(UniformRandomProvider rng,
                               List<T> list)
public static <T> void shuffle(UniformRandomProvider rng,
                               List<T> list,
                               int start,
                               boolean towardHead)

// Also sampling
public static <T> List<T> sample(UniformRandomProvider rng,
                                 List<T> collection,
                                 int k) 
{code}
I do not think supporting a range for shuffling here is required as this can be achieved using:
{code:java}
int from = ...
int to = ...
ListSampler.shuffle(rng, list.subList(from, to));
{code}
This is the how the half-shuffle method (towards head/tail) is implemented. The origin of the half-shuffle API is unknown but it is not as flexible a sub-range shuffle and I do not propose to implement it for all array types.

Note that consistency between Lists and arrays would require a sampling method for all arrays, e.g.:
{code:java}
public static double[] sample(UniformRandomProvider rng,
    double[] array,
    int k) {
public static <T> T[] sample(UniformRandomProvider rng,
    T[] array,
    int k)
{code}
Since this static method uses a new PermutationSampler per sample the method is not suitable for repeat invocation. Development of a sampling API for arrays should be under another ticket, e.g.
{code:java}
// Sampler returns k elements from the given array
SharedStateObjectSampler<double[]> createSampler(UniformRandomProvider rng,
                                                 double[] array, int k).{code}
Such a sampler would internally maintain the PermutationSampler between invocations.

This ticket will only cover shuffling support in ArraySampler.



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