UDAFCollectMap.java

/**
 * 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 com.facebook.hive.udf;

import org.apache.hadoop.hive.ql.exec.UDAF;
import org.apache.hadoop.hive.ql.exec.UDAFEvaluator;

import java.util.HashMap;

/**
 * Compute (normalized) entropy over a series of values.
 *
 */
public final class UDAFCollectMap extends UDAF {

    /**
     * Note that this is only needed if the internal state cannot be represented
     * by a primitive.
     *
     * The internal state can also contains fields with types like
     * ArrayList<String> and HashMap<String,Double> if needed.
     */
    public static class UDAFCollectMapState {
      private HashMap<String, String> elements;
    }

    /**
     * The actual class for doing the aggregation. Hive will automatically look
     * for all internal classes of the UDAF that implements UDAFEvaluator.
     */
    public static class UDAFCollectMapEvaluator implements UDAFEvaluator {

	UDAFCollectMapState state;

	public UDAFCollectMapEvaluator() {
	    super();
	    state = new UDAFCollectMapState();
	    init();
	}

	/**
	 * Reset the state of the aggregation.
	 */
	public void init() {
          state.elements = new HashMap<String, String>();
	}

	/**
	 * Iterate through one row of original data.
	 *
	 * The number and type of arguments need to the same as we call this UDAF
	 * from Hive command line.
	 *
	 * This function should always return true.
	 */
	public boolean iterate(String key, String val) {
	    if (key != null && val != null) {
		state.elements.put(key, val);
	    }
	    return true;
	}

	/**
	 * Terminate a partial aggregation and return the state. If the state is a
	 * primitive, just return primitive Java classes like Integer or String.
	 */
	public UDAFCollectMapState terminatePartial() {
	    // Return null if we have no data.
	    if (state.elements.size() == 0) {
		return null;
	    } else {
		return state;
	    }
	}

	/**
	 * Merge with a partial aggregation.
	 *
	 * This function should always have a single argument which has the same
	 * type as the return value of terminatePartial().
	 */
	public boolean merge(UDAFCollectMapState o) {
	    if (o != null) {
		state.elements.putAll(o.elements);
	    }
	    return true;
	}

	/**
	 * Terminates the aggregation and return the final result.
	 */
      public HashMap<String, String> terminate() {
	    return state.elements;
	}
    }

    private UDAFCollectMap() {
	// prevent instantiation
    }
}