/*
 * 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.assigners;

import java.util.ArrayList;
import java.util.Collection;
import java.util.List;
import org.apache.flink.api.common.ExecutionConfig;
import org.apache.flink.api.common.typeutils.TypeSerializer;
import org.apache.flink.streaming.api.environment.StreamExecutionEnvironment;
import org.apache.flink.streaming.api.windowing.time.Time;
import org.apache.flink.streaming.api.windowing.triggers.ProcessingTimeTrigger;
import org.apache.flink.streaming.api.windowing.triggers.Trigger;
import org.apache.flink.streaming.api.windowing.windows.TimeWindow;

/**
 * A {@link WindowAssigner} that windows elements into sliding windows based on the current
 * system time of the machine the operation is running on. Windows can possibly overlap.
 *
 * <p>For example, in order to window into windows of 1 minute, every 10 seconds:
 * <pre> {@code
 * DataStream<Tuple2<String, Integer>> in = ...;
 * KeyedStream<String, Tuple2<String, Integer>> keyed = in.keyBy(...);
 * WindowedStream<Tuple2<String, Integer>, String, TimeWindows> windowed =
 *   keyed.window(SlidingProcessingTimeWindows.of(Time.of(1, MINUTES), Time.of(10, SECONDS));
 * } </pre>
 */
public class SlidingProcessingTimeWindows extends WindowAssigner<Object, TimeWindow> {
	private static final long serialVersionUID = 1L;

	private final long size;

	private final long offset;

	private final long slide;

	private SlidingProcessingTimeWindows(long size, long slide, long offset) {
		if (offset < 0 || offset >= slide || size <= 0) {
			throw new IllegalArgumentException("SlidingProcessingTimeWindows parameters must satisfy 0 <= offset < slide and size > 0");
		}

		this.size = size;
		this.slide = slide;
		this.offset = offset;
	}

	@Override
	public Collection<TimeWindow> assignWindows(Object element, long timestamp, WindowAssignerContext context) {
		timestamp = context.getCurrentProcessingTime();
		List<TimeWindow> windows = new ArrayList<>((int) (size / slide));
		long lastStart = TimeWindow.getWindowStartWithOffset(timestamp, offset, slide);
		for (long start = lastStart;
			start > timestamp - size;
			start -= slide) {
			windows.add(new TimeWindow(start, start + size));
		}
		return windows;
	}

	public long getSize() {
		return size;
	}

	public long getSlide() {
		return slide;
	}

	@Override
	public Trigger<Object, TimeWindow> getDefaultTrigger(StreamExecutionEnvironment env) {
		return ProcessingTimeTrigger.create();
	}

	@Override
	public String toString() {
		return "SlidingProcessingTimeWindows(" + size + ", " + slide + ")";
	}

	/**
	 * Creates a new {@code SlidingProcessingTimeWindows} {@link WindowAssigner} that assigns
	 * elements to sliding time windows based on the element timestamp.
	 *
	 * @param size The size of the generated windows.
	 * @param slide The slide interval of the generated windows.
	 * @return The time policy.
	 */
	public static SlidingProcessingTimeWindows of(Time size, Time slide) {
		return new SlidingProcessingTimeWindows(size.toMilliseconds(), slide.toMilliseconds(), 0);
	}

	/**
	 * Creates a new {@code SlidingProcessingTimeWindows} {@link WindowAssigner} that assigns
	 * elements to time windows based on the element timestamp and offset.
	 *
	 * <p>For example, if you want window a stream by hour,but window begins at the 15th minutes
	 * of each hour, you can use {@code of(Time.hours(1),Time.minutes(15))},then you will get
	 * time windows start at 0:15:00,1:15:00,2:15:00,etc.
	 *
	 * <p>Rather than that,if you are living in somewhere which is not using UTC±00:00 time,
	 * such as China which is using UTC+08:00,and you want a time window with size of one day,
	 * and window begins at every 00:00:00 of local time,you may use {@code of(Time.days(1),Time.hours(-8))}.
	 * The parameter of offset is {@code Time.hours(-8))} since UTC+08:00 is 8 hours earlier than UTC time.
	 *
	 * @param size The size of the generated windows.
	 * @param slide  The slide interval of the generated windows.
	 * @param offset The offset which window start would be shifted by.
	 * @return The time policy.
	 */
	public static SlidingProcessingTimeWindows of(Time size, Time slide, Time offset) {
		return new SlidingProcessingTimeWindows(size.toMilliseconds(), slide.toMilliseconds(),
			offset.toMilliseconds() % slide.toMilliseconds());
	}

	@Override
	public TypeSerializer<TimeWindow> getWindowSerializer(ExecutionConfig executionConfig) {
		return new TimeWindow.Serializer();
	}

	@Override
	public boolean isEventTime() {
		return false;
	}
}