Coding challenge: a sliding window map

SlidingWindowMap.groovy

import java.util.concurrent.DelayQueue
import java.util.concurrent.Delayed
import java.util.concurrent.TimeUnit

class SlidingWindowMap {
    private periodMs
    private available = new DelayQueue<DelayedKey<String>>() 
    
    SlidingWindowMap(Set<String> keys, int maxCount, long periodMs) {
        this.periodMs = periodMs
        keys.each { key -> maxCount.times { available << new DelayedKey<String>(payload: key) } }            
    }

    /**
     * @return a key that has been used less than `maxCount` times during the
     * past `periodMs` milliseconds or null if no such key exists.
     */
    String getNextKey() {
        def delayedKey = available.poll()
        if (delayedKey) available << delayedKey.blockFor(periodMs)  
        return delayedKey?.payload
    }

}

class DelayedKey<T> implements Delayed {
    T payload    
    long targetTimestamp
    long getDelay(TimeUnit unit) { unit.convert(targetTimestamp - System.currentTimeMillis(), TimeUnit.MILLISECONDS) } 
    int compareTo(Delayed o)     { getDelay(TimeUnit.MILLISECONDS) <=> o.getDelay(TimeUnit.MILLISECONDS) }
    DelayedKey<T> blockFor(long delayMs) { 
        targetTimestamp=System.currentTimeMillis() + delayMs 
        return this
    }
}