/*
 * 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.api.java.operators;

import org.apache.flink.annotation.Internal;
import org.apache.flink.annotation.Public;
import org.apache.flink.api.common.InvalidProgramException;
import org.apache.flink.api.common.functions.Partitioner;
import org.apache.flink.api.common.operators.Keys;
import org.apache.flink.api.java.DataSet;

/**
 * Grouping is an intermediate step for a transformation on a grouped DataSet.<br>
 * The following transformation can be applied on Grouping:
 * <ul>
 * <li>{@link UnsortedGrouping#reduce(org.apache.flink.api.common.functions.ReduceFunction)},</li>
 * <li>{@link UnsortedGrouping#reduceGroup(org.apache.flink.api.common.functions.GroupReduceFunction)}, and</li>
 * <li>{@link UnsortedGrouping#aggregate(org.apache.flink.api.java.aggregation.Aggregations, int)}.</li>
 * </ul>
 *
 * @param <T> The type of the elements of the grouped DataSet.
 * 
 * @see DataSet
 */
@Public
public abstract class Grouping<T> {
	
	protected final DataSet<T> inputDataSet;
	
	protected final Keys<T> keys;
	
	protected Partitioner<?> customPartitioner;

	
	public Grouping(DataSet<T> set, Keys<T> keys) {
		if (set == null || keys == null) {
			throw new NullPointerException();
		}
		
		if (keys.isEmpty()) {
			throw new InvalidProgramException("The grouping keys must not be empty.");
		}

		this.inputDataSet = set;
		this.keys = keys;
	}
	
	/**
	 * Returns the input DataSet of a grouping operation, that is the one before the grouping. This means that
	 * if it is applied directly to the result of a grouping operation, it will cancel its effect. As an example, in the
	 * following snippet:
	 * <pre><code>
	 * DataSet<X> notGrouped = input.groupBy().getDataSet();
	 * DataSet<Y> allReduced = notGrouped.reduce()
	 * </pre></code>
	 * the <code>groupBy()</code> is as if it never happened, as the <code>notGrouped</code> DataSet corresponds
	 * to the input of the <code>groupBy()</code> (because of the <code>getDataset()</code>).
	 * */
	@Internal
	public DataSet<T> getInputDataSet() {
		return this.inputDataSet;
	}

	@Internal
	public Keys<T> getKeys() {
		return this.keys;
	}
	
	/**
	 * Gets the custom partitioner to be used for this grouping, or {@code null}, if
	 * none was defined.
	 * 
	 * @return The custom partitioner to be used for this grouping.
	 */
	@Internal
	public Partitioner<?> getCustomPartitioner() {
		return this.customPartitioner;
	}
}