package test;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;

public class SlidingWindowMap {
    private int maxCount;
    private long periodMs;
    private Map<String, List<Long>> usedBufferMap = new HashMap<String, List<Long>>();

    public SlidingWindowMap(Set<String> keys, int maxCount, long periodMs) {
        Iterator<String> iterator = keys.iterator();
        while(iterator.hasNext()){
            String key = iterator.next();
            usedBufferMap.put(key, new ArrayList<Long>());
        }
        this.maxCount = maxCount;
        this.periodMs = periodMs;
    }

    /**
     * @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() {
        for (Entry<String, List<Long>> entry: usedBufferMap.entrySet()){
            cleanEntry(entry.getValue());
            if (entry.getValue().size() < maxCount) {
                entry.getValue().add(System.currentTimeMillis());
                return entry.getKey();
            }
        }
        return null;
    }

    private void cleanEntry(List<Long> entry) {
        List<Long> toBeRemoved = new ArrayList<Long>();
        for (long used: entry)
            if (used < (System.currentTimeMillis() - periodMs))
                toBeRemoved.add(used);
        entry.removeAll(toBeRemoved);
    }
}