library/src/main/java/com/datatorrent/lib/streamquery/function/AverageFunction.java

/*
 * Copyright (c) 2013 DataTorrent, Inc. ALL Rights Reserved.
 *
 * Licensed 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.datatorrent.lib.streamquery.function;

import java.util.ArrayList;
import java.util.Map;

import javax.validation.constraints.NotNull;

import org.apache.commons.lang.StringUtils;

/**
 * An implementation of function index that implements average function semantics. <br>
 * <p>
 *   e.g : sql => SELECT AVG(column_name) FROM table_name. <br>
 *   <br>
 *   <b> Properties : </b> <br>
 *   <b> column : </b> Aggregate over given column values.   <br>
 *   <b> alias  : </b> Alias name for aggregate output. <br>
 * @displayName: Average Function
 * @category: streamquery/function
 * @tag: sql average
 * @since 0.3.4
 */
public class AverageFunction  extends FunctionIndex
{
  /**
   * @param column Aggregate over given column values, must be non null. 
   * @param alias  Alias name for aggregate output.
   */
  public AverageFunction(@NotNull String column, String alias) 
  {
    super(column, alias);
  }

  /**
   * Compute average for given column values.
   */
  @Override
  public Object compute(@NotNull ArrayList<Map<String, Object>> rows) throws Exception
  {
    if (rows.size() == 0) return 0.0;
    double sum = 0.0;
    for (Map<String, Object> row : rows) {
      sum += ((Number)row.get(column)).doubleValue();
    }
    return sum/rows.size();
  }

  /**
   * Get aggregate name.
   * @return name.
   */
  @Override
  protected String aggregateName()
  {
    if (!StringUtils.isEmpty(alias)) return alias;
    return "AVG(" + column + ")";
  }
}