/*
 * 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.samza.operators.functions;

import java.io.Serializable;
import org.apache.samza.annotation.InterfaceStability;


/**
 * Incrementally updates the aggregated value as messages are added. Main usage is in {@link org.apache.samza.operators.windows.Window} operator.
 */
@InterfaceStability.Unstable
@FunctionalInterface
public interface FoldLeftFunction<M, WV> extends InitableFunction, ClosableFunction, Serializable {

  /**
   * Incrementally updates the aggregated value as messages are added.
   *
   * @param message the message being added to the aggregated value
   * @param oldValue the previous value
   * @return the new value
   */
  WV apply(M message, WV oldValue);
}