/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.flink.streaming.api.windowing.triggers;

import org.apache.flink.annotation.PublicEvolving;
import org.apache.flink.api.common.state.ValueState;
import org.apache.flink.api.common.state.ValueStateDescriptor;
import org.apache.flink.api.common.typeutils.TypeSerializer;
import org.apache.flink.streaming.api.functions.windowing.delta.DeltaFunction;
import org.apache.flink.streaming.api.windowing.windows.Window;

/**
 * A {@link Trigger} that fires based on a {@link DeltaFunction} and a threshold.
 *
 * <p>This trigger calculates a delta between the data point which triggered last
 * and the currently arrived data point. It triggers if the delta is higher than
 * a specified threshold.
 *
 * @param <W> The type of {@link Window Windows} on which this trigger can operate.
 */
@PublicEvolving
public class DeltaTrigger<T, W extends Window> extends Trigger<T, W> {
	private static final long serialVersionUID = 1L;

	private final DeltaFunction<T> deltaFunction;
	private final double threshold;
	private final ValueStateDescriptor<T> stateDesc;

	private DeltaTrigger(double threshold, DeltaFunction<T> deltaFunction, TypeSerializer<T> stateSerializer) {
		this.deltaFunction = deltaFunction;
		this.threshold = threshold;
		stateDesc = new ValueStateDescriptor<>("last-element", stateSerializer);

	}

	@Override
	public TriggerResult onElement(T element, long timestamp, W window, TriggerContext ctx) throws Exception {
		ValueState<T> lastElementState = ctx.getPartitionedState(stateDesc);
		if (lastElementState.value() == null) {
			lastElementState.update(element);
			return TriggerResult.CONTINUE;
		}
		if (deltaFunction.getDelta(lastElementState.value(), element) > this.threshold) {
			lastElementState.update(element);
			return TriggerResult.FIRE;
		}
		return TriggerResult.CONTINUE;
	}

	@Override
	public TriggerResult onEventTime(long time, W window, TriggerContext ctx) {
		return TriggerResult.CONTINUE;
	}

	@Override
	public TriggerResult onProcessingTime(long time, W window, TriggerContext ctx) throws Exception {
		return TriggerResult.CONTINUE;
	}

	@Override
	public void clear(W window, TriggerContext ctx) throws Exception {
		ctx.getPartitionedState(stateDesc).clear();
	}

	@Override
	public String toString() {
		return "DeltaTrigger(" +  deltaFunction + ", " + threshold + ")";
	}

	/**
	 * Creates a delta trigger from the given threshold and {@code DeltaFunction}.
	 *
	 * @param threshold The threshold at which to trigger.
	 * @param deltaFunction The delta function to use
	 * @param stateSerializer TypeSerializer for the data elements.
	 *
	 * @param <T> The type of elements on which this trigger can operate.
	 * @param <W> The type of {@link Window Windows} on which this trigger can operate.
	 */
	public static <T, W extends Window> DeltaTrigger<T, W> of(double threshold, DeltaFunction<T> deltaFunction, TypeSerializer<T> stateSerializer) {
		return new DeltaTrigger<>(threshold, deltaFunction, stateSerializer);
	}
}