Enumerate all the subsets of a set of a given size

SubsetEnumerator.php

<?php
/**
 * @author Nicolò Martini - <[email protected]>
 *
 * Created on 05/02/2016, 19:52
 */

namespace NicMart;

/**
 * Class SubsetEnumerator
 * @package Dxi\Backend\Reporting\Index\Column
 */
class FixedSizeSubsetEnumerator
{
    public function getSubsets(array $set, $subsetsSize)
    {
        $setSize = count($set);

        if ($subsetsSize > $setSize) {
            return array();
        }

        $indexes = array();

        for ($i = 0; $i < $subsetsSize; $i++) {
            $indexes[$i] = $i;
        }

        $subsets = array();

        while (true) {
            $subsets[] = array_intersect_key($set, array_flip($indexes));

            $firstIndexToIncrease = $subsetsSize - 1;
            for (; $firstIndexToIncrease >= 0; $firstIndexToIncrease--) {
                $j = $subsetsSize - 1 - $firstIndexToIncrease;
                if ($indexes[$firstIndexToIncrease] != $setSize - $j - 1) {
                    break;
                }
            }

            if ($firstIndexToIncrease == -1) {
                break;
            }

            $indexes[$firstIndexToIncrease]++;

            for ($i = $firstIndexToIncrease + 1; $i < $subsetsSize; $i++) {
                $indexes[$i] = $indexes[$i - 1] + 1;
            }
        }

        return $subsets;
    }
}