/**
 * A {@code TimestampAssigner} assigns event time timestamps to elements.
 * These timestamps are used by all functions that operate on event time,
 * for example event time windows.
 *
 * <p>Timestamps are represented in milliseconds since the Epoch
 * (midnight, January 1, 1970 UTC).
 *
 * @param <T> The type of the elements to which this assigner assigns timestamps.
 */
public interface TimestampAssigner<T> extends Function {

	/**
	 * Assigns a timestamp to an element, in milliseconds since the Epoch.
	 *
	 * <p>The method is passed the previously assigned timestamp of the element.
	 * That previous timestamp may have been assigned from a previous assigner,
	 * by ingestion time. If the element did not carry a timestamp before, this value is
	 * {@code Long.MIN_VALUE}.
	 *
	 * @param element The element that the timestamp will be assigned to.
	 * @param previousElementTimestamp The previous internal timestamp of the element,
	 *                                 or a negative value, if no timestamp has been assigned yet.
	 * @return The new timestamp.
	 */
	long extractTimestamp(T element, long previousElementTimestamp);
}

/**
 * The {@code AssignerWithPeriodicWatermarks} assigns event time timestamps to elements,
 * and generates low watermarks that signal event time progress within the stream.
 * These timestamps and watermarks are used by functions and operators that operate
 * on event time, for example event time windows.
 *
 * <p>Use this class to generate watermarks in a periodical interval.
 * At most every {@code i} milliseconds (configured via
 * {@link ExecutionConfig#getAutoWatermarkInterval()}), the system will call the
 * {@link #getCurrentWatermark()} method to probe for the next watermark value.
 * The system will generate a new watermark, if the probed value is non-null
 * and has a timestamp larger than that of the previous watermark (to preserve
 * the contract of ascending watermarks).
 *
 * <p>The system may call the {@link #getCurrentWatermark()} method less often than every
 * {@code i} milliseconds, if no new elements arrived since the last call to the
 * method.
 *
 * <p>Timestamps and watermarks are defined as {@code longs} that represent the
 * milliseconds since the Epoch (midnight, January 1, 1970 UTC).
 * A watermark with a certain value {@code t} indicates that no elements with event
 * timestamps {@code x}, where {@code x} is lower or equal to {@code t}, will occur any more.
 *
 * @param <T> The type of the elements to which this assigner assigns timestamps.
 *
 * @see org.apache.flink.streaming.api.watermark.Watermark
 */
public interface AssignerWithPeriodicWatermarks<T> extends TimestampAssigner<T> {

	/**
	 * Returns the current watermark. This method is periodically called by the
	 * system to retrieve the current watermark. The method may return {@code null} to
	 * indicate that no new Watermark is available.
	 *
	 * <p>The returned watermark will be emitted only if it is non-null and its timestamp
	 * is larger than that of the previously emitted watermark (to preserve the contract of
	 * ascending watermarks). If the current watermark is still
	 * identical to the previous one, no progress in event time has happened since
	 * the previous call to this method. If a null value is returned, or the timestamp
	 * of the returned watermark is smaller than that of the last emitted one, then no
	 * new watermark will be generated.
	 *
	 * <p>The interval in which this method is called and Watermarks are generated
	 * depends on {@link ExecutionConfig#getAutoWatermarkInterval()}.
	 *
	 * @see org.apache.flink.streaming.api.watermark.Watermark
	 * @see ExecutionConfig#getAutoWatermarkInterval()
	 *
	 * @return {@code Null}, if no watermark should be emitted, or the next watermark to emit.
	 */
	@Nullable
	Watermark getCurrentWatermark();
}

/**
 * This generator generates watermarks assuming that elements arrive out of order,
 * but only to a certain degree. The latest elements for a certain timestamp t will arrive
 * at most n milliseconds after the earliest elements for timestamp t.
 */
class BoundedOutOfOrdernessGenerator extends AssignerWithPeriodicWatermarks[MyEvent] {
    val maxOutOfOrderness = 3500L // 3.5 seconds
    var currentMaxTimestamp: Long = _
    override def extractTimestamp(element: MyEvent, previousElementTimestamp: Long): Long = {
        element.getCreationTime()
    }

    override def getCurrentWatermark(): Watermark = {
        // return the watermark as current highest timestamp minus the out-of-orderness bound
        new Watermark(currentMaxTimestamp - maxOutOfOrderness)
    }
}

class PunctuatedAssigner extends AssignerWithPunctuatedWatermarks[MyEvent] {

	override def extractTimestamp(element: MyEvent, previousElementTimestamp: Long): Long = {
		element.getCreationTime
	}

	override def checkAndGetNextWatermark(lastElement: MyEvent, extractedTimestamp: Long): Watermark = {
		if (lastElement.hasWatermarkMarker()) new Watermark(extractedTimestamp) else null
	}
}

stream
    .keyBy(<key selector>)
    .window(<window assigner>)
    .allowedLateness(<time>)
    .<windowed transformation>(<window function>)