/*
 * 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.runtime.state;


import org.apache.flink.util.Preconditions;

import java.util.Collections;
import java.util.List;

/**
 * Handle to state handles for the operators in an operator chain.
 */
public class ChainedStateHandle<T extends StateObject> implements StateObject {

	private static final long serialVersionUID = 1L;

	/** The state handles for all operators in the chain */
	private final List<? extends T> operatorStateHandles;

	/**
	 * Wraps a list to the state handles for the operators in a chain. Individual state handles can be null.
	 *
	 * @param operatorStateHandles list with the state handles for the states in the operator chain.
	 */
	public ChainedStateHandle(List<? extends T> operatorStateHandles) {
		this.operatorStateHandles = Preconditions.checkNotNull(operatorStateHandles);
	}

	/**
	 * Check if there are any states handles present. Notice that this can be true even if {@link #getLength()} is
	 * greater than zero, because state handles can be null.
	 *
	 * @return true if there are no state handles for any operator.
	 */
	public boolean isEmpty() {
		for (T state : operatorStateHandles) {
			if (state != null) {
				return false;
			}
		}
		return true;
	}

	/**
	 * Returns the length of the operator chain. This can be different from the number of operator state handles,
	 * because the some operators in the chain can have no state and thus their state handle can be null.
	 *
	 * @return length of the operator chain
	 */
	public int getLength() {
		return operatorStateHandles.size();
	}

	/**
	 * Get the state handle for a single operator in the operator chain by it's index.
	 *
	 * @param index the index in the operator chain
	 * @return state handle to the operator at the given position in the operator chain. can be null.
	 */
	public T get(int index) {
		return operatorStateHandles.get(index);
	}

	@Override
	public void discardState() throws Exception {
		StateUtil.bestEffortDiscardAllStateObjects(operatorStateHandles);
	}

	@Override
	public long getStateSize() {
		long sumStateSize = 0;

		if (operatorStateHandles != null) {
			for (T state : operatorStateHandles) {
				if (state != null) {
					sumStateSize += state.getStateSize();
				}
			}
		}

		// State size as sum of all state sizes
		return sumStateSize;
	}

	@Override
	public boolean equals(Object o) {
		if (this == o) {
			return true;
		}
		if (o == null || getClass() != o.getClass()) {
			return false;
		}

		ChainedStateHandle<?> that = (ChainedStateHandle<?>) o;

		return operatorStateHandles.equals(that.operatorStateHandles);

	}

	@Override
	public int hashCode() {
		return operatorStateHandles.hashCode();
	}

	public static <T extends StateObject> ChainedStateHandle<T> wrapSingleHandle(T stateHandleToWrap) {
		return new ChainedStateHandle<T>(Collections.singletonList(stateHandleToWrap));
	}

	public static boolean isNullOrEmpty(ChainedStateHandle<?> chainedStateHandle) {
		return chainedStateHandle == null || chainedStateHandle.isEmpty();
	}
}