library/src/main/java/com/datatorrent/lib/streamquery/function/FirstLastFunction.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 sql first,last function semantic. <br>
 * <p>
 *   e.g : sql => SELECT FIRST/LAST(column_name) FROM table_name. <br>
 *   <br>
 *   <b> Properties : </b> <br>
 *   <b> column : </b> column name for first/last value.   <br>
 *   <b> alias  : </b> Alias name for output. <br>
 *   <b> isFirst : </b> return first value if true.
 * @displayName: First Last Function
 * @category: streamquery/function
 * @tag: sql first, sql last
 * @since 0.3.4
 */
public class FirstLastFunction extends FunctionIndex
{
  /**
   * return first value if true.
   */
  private boolean isFirst;
  
  /**
   * @param column  column name for first/last value.
   * @param  alias   Alias name for output.
   * @param  isFirst return first value if true.
   */
  public FirstLastFunction(@NotNull String column,String alias, boolean isLast)
  {
    super(column, alias);
    isFirst = !isLast;
  }
  
  /**
   * Get first/last non null value for column.
   */
  @Override
  public Object compute(@NotNull ArrayList<Map<String, Object>> rows) throws Exception
  {
    if (rows.size() == 0) return null;
    if (isFirst) {
      for (int i=0; i < rows.size(); i++) {
        if (rows.get(i).get(column) != null) return rows.get(i).get(column);
      }
    } else {
      for (int i= (rows.size()-1); i >= 0;  i--) {
        if (rows.get(i).get(column) != null) return rows.get(i).get(column);
      }
    }
    return null;
  }

  /**
   * Aggregate output name.
   * @return name string.
   */
  @Override
  protected String aggregateName()
  {
    if (!StringUtils.isEmpty(alias)) return alias;
    if (isFirst) {
        return "FIRST(" + column + ")";
    }
    return "LAST(" + column + ")";
  }

  public boolean isFirst()
  {
    return isFirst;
  }

  public void setFirst(boolean isFirst)
  {
    this.isFirst = isFirst;
  }

}