/**
 * 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.hadoop.hive.ql.exec;

/**
 * Base class for all User-defined Aggregation Function (UDAF) classes.
 * 
 * UDAF classes are REQUIRED to inherit from this class.
 * 
 * Required for a UDAF class: 1. Implement the init() method, which reset the
 * status of the aggregation function. 2. Implement a single method called
 * "aggregate" that returns boolean. The method should always return "true" on
 * valid inputs, or the framework will throw an Exception. Following are some
 * examples: public boolean aggregate(double a); public boolean aggregate(int
 * b); public boolean aggregate(double c, double d); 3. Implement a single
 * method called "evaluate" that returns the FINAL aggregation result.
 * "evaluate" should never return "null" or an Exception will be thrown.
 * Following are some examples. public int evaluate(); public long evaluate();
 * public double evaluate(); public Double evaluate(); public String evaluate();
 * 
 * Optional for a UDAF class (by implementing these 2 methods, the user declares
 * that the UDAF support partial aggregations): 1. Implement a single method
 * called "evaluatePartial" that returns the PARTIAL aggregation result.
 * "evaluatePartial" should never return "null" or an Exception will be thrown.
 * 2. Implement a single method called "aggregatePartial" that takes a PARTIAL
 * aggregation result and returns a boolean. The method should always return
 * "true" on valid inputs, or the framework will throw an Exception.
 * 
 * Following are some examples: public int evaluatePartial(); public boolean
 * aggregatePartial(int partial);
 * 
 * public String evaluatePartial(); public boolean aggregatePartial(String
 * partial);
 * 
 */
public class UDAF {

  /**
   * The resolver used for method resolution.
   */
  UDAFEvaluatorResolver rslv;

  /**
   * The default constructor.
   */
  public UDAF() {
    rslv = new DefaultUDAFEvaluatorResolver(this.getClass());
  }

  /**
   * The constructor with a particular type of resolver.
   */
  public UDAF(UDAFEvaluatorResolver rslv) {
    this.rslv = rslv;
  }

  /**
   * Sets the resolver.
   * 
   * @param rslv
   *          The method resolver to use for method resolution.
   */
  public void setResolver(UDAFEvaluatorResolver rslv) {
    this.rslv = rslv;
  }

  /**
   * Gets the resolver.
   */
  public UDAFEvaluatorResolver getResolver() {
    return rslv;
  }
}