/*
 * 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.functions.source;

import java.util.ArrayDeque;
import java.util.Deque;
import org.apache.flink.annotation.PublicEvolving;
import org.apache.flink.api.common.state.ListState;
import org.apache.flink.api.common.state.ListStateDescriptor;
import org.apache.flink.api.common.typeutils.base.LongSerializer;
import org.apache.flink.runtime.state.FunctionInitializationContext;
import org.apache.flink.runtime.state.FunctionSnapshotContext;
import org.apache.flink.streaming.api.checkpoint.CheckpointedFunction;
import org.apache.flink.util.Preconditions;

/**
 * A stateful streaming source that emits each number from a given interval exactly once,
 * possibly in parallel.
 *
 * <p>For the source to be re-scalable, the first time the job is run, we precompute all the elements
 * that each of the tasks should emit and upon checkpointing, each element constitutes its own
 * partition. When rescaling, these partitions will be randomly re-assigned to the new tasks.
 *
 * <p>This strategy guarantees that each element will be emitted exactly-once, but elements will not
 * necessarily be emitted in ascending order, even for the same tasks.
 */
@PublicEvolving
public class StatefulSequenceSource extends RichParallelSourceFunction<Long> implements CheckpointedFunction {

	private static final long serialVersionUID = 1L;

	private final long start;
	private final long end;

	private volatile boolean isRunning = true;

	private transient Deque<Long> valuesToEmit;

	private transient ListState<Long> checkpointedState;

	/**
	 * Creates a source that emits all numbers from the given interval exactly once.
	 *
	 * @param start Start of the range of numbers to emit.
	 * @param end End of the range of numbers to emit.
	 */
	public StatefulSequenceSource(long start, long end) {
		this.start = start;
		this.end = end;
	}

	@Override
	public void initializeState(FunctionInitializationContext context) throws Exception {

		Preconditions.checkState(this.checkpointedState == null,
			"The " + getClass().getSimpleName() + " has already been initialized.");

		this.checkpointedState = context.getOperatorStateStore().getListState(
			new ListStateDescriptor<>(
				"stateful-sequence-source-state",
				LongSerializer.INSTANCE
			)
		);

		this.valuesToEmit = new ArrayDeque<>();
		if (context.isRestored()) {
			// upon restoring

			for (Long v : this.checkpointedState.get()) {
				this.valuesToEmit.add(v);
			}
		} else {
			// the first time the job is executed

			final int stepSize = getRuntimeContext().getNumberOfParallelSubtasks();
			final int taskIdx = getRuntimeContext().getIndexOfThisSubtask();
			final long congruence = start + taskIdx;

			long totalNoOfElements = Math.abs(end - start + 1);
			final int baseSize = safeDivide(totalNoOfElements, stepSize);
			final int toCollect = (totalNoOfElements % stepSize > taskIdx) ? baseSize + 1 : baseSize;

			for (long collected = 0; collected < toCollect; collected++) {
				this.valuesToEmit.add(collected * stepSize + congruence);
			}
		}
	}

	@Override
	public void run(SourceContext<Long> ctx) throws Exception {
		while (isRunning && !this.valuesToEmit.isEmpty()) {
			synchronized (ctx.getCheckpointLock()) {
				ctx.collect(this.valuesToEmit.poll());
			}
		}
	}

	@Override
	public void cancel() {
		isRunning = false;
	}

	@Override
	public void snapshotState(FunctionSnapshotContext context) throws Exception {
		Preconditions.checkState(this.checkpointedState != null,
			"The " + getClass().getSimpleName() + " state has not been properly initialized.");

		this.checkpointedState.clear();
		for (Long v : this.valuesToEmit) {
			this.checkpointedState.add(v);
		}
	}

	private static int safeDivide(long left, long right) {
		Preconditions.checkArgument(right > 0);
		Preconditions.checkArgument(left >= 0);
		Preconditions.checkArgument(left <= Integer.MAX_VALUE * right);
		return (int) (left / right);
	}
}