SlidingWindowMap code challenge FINAL

import java.util.Set;

// Quick experiment of the SlidingWindowMap code challenge
// Author: Thierry Kormann - thierry.kormann@me.com
//
public class SlidingWindowMap {

    private String[] _keys;
    private int _maxCount;
    private long _periodMs;

    // Used to store when a key has been used.
    private long[] _keyUsagesMs;
    // An index in _keyUsagesMs to know when a key will be available.
    private int _index;

    public SlidingWindowMap(Set<String> keys, int maxCount, long periodMs) {
        _keys = new String[keys.size()];
        keys.toArray(_keys);
        _maxCount = maxCount;
        _periodMs = periodMs;
        _keyUsagesMs = new long[keys.size() * maxCount];
    }

    /**
     * @return a key that has been used less than `maxCount` times during the
     * past `periodMs` milliseconds or null if no such key exists.
     */
    public String getNextKey() {
        if (_index == _keyUsagesMs.length) {
            _index = 0;
        }
        long now = System.currentTimeMillis();
        if (now >= _keyUsagesMs[_index] + _periodMs) {
            _keyUsagesMs[_index] = now;
            return _keys[_index++ % _keys.length];
        }
        return null;
    }
}