/*
    * 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.giraph.function;
  
  import org.apache.giraph.function.vertex.ConsumerWithVertex;
  import org.apache.giraph.function.vertex.SupplierFromVertex;
  import org.apache.giraph.graph.Vertex;
  import org.apache.hadoop.io.Writable;
  import org.apache.hadoop.io.WritableComparable;
  
  /**
   * Default object transfer, intermediary between producers and consumers.
   *
   * Holds value temporarily in memory, so can be used only when producer and
   * consumer are in the same context. Nulls it out after supplying, so each
   * object is returned only once, and second consecutive call to 'get' will
   * return null.
   *
   * Useful for both:
   *
   * - passing data from vertexReceive function of WorkerReceivePiece of previous
   * Piece to vertexSend function WorkerSendPiece of next Piece, of the same
   * vertex.
   * - when value is set on the master, and later read in block logic
   * (RepeatUntilBlock), or in a different Piece, either on worker or master.
   * If it is read within the same piece - just use local field.
   *
   * @param <T> Type of object to transfer.
   */
  public class ObjectTransfer<T> implements Supplier<T>, Consumer<T> {
    /** value */
    private T value;
  
    /**
     * Constructor
     * @param value initial value
     */
    public ObjectTransfer(T value) {
      this.value = value;
    }
  
    /** Constructor */
    public ObjectTransfer() {
    }
  
    @Override
    public T get() {
      T result = value;
      value = null;
      return result;
    }
  
    @Override
    public void apply(T value) {
      this.value = value;
    }
  
    @Override
    public String toString() {
      return getClass() + " [value=" + value + "]";
    }
  
    /**
     * To be called when needing to pass it as a Supplier - making it
     * obvious that K, V and E on supplier side can be any types,
     * and to make code work without compile errors/warnings.
     *
     * In Java7, some callsites might need explicit types:
     * object.&lt;LongWritable, DoubleWritable, Writable&gt;castToSupplier()
     * In Java8, object.castToSupplier() is always going to be enough.
     *
     * @param <I> Vertex id type
     * @param <V> Vertex value type
     * @param <E> Edge value type
     * @return supplier from vertex
     */
    // TODO Java8: cleanup callers
    @SuppressWarnings("rawtypes")
    public <I extends WritableComparable, V extends Writable, E extends Writable>
    SupplierFromVertex<I, V, E, T> castToSupplier() {
      return new SupplierFromVertex<I, V, E, T>() {
        @Override
        public T get(Vertex<I, V, E> vertex) {
          return ObjectTransfer.this.get();
       }
     };
   }
 
   /**
    * To be called when needing to pass it as a Consumer - making it
    * obvious that K, V and E on consumer side can be any types,
    * and to make code work without compile errors/warnings.
    *
    * In Java7, some callsites might need explicit types:
    * object.&lt;LongWritable, DoubleWritable, Writable&gt;castToConsumer()
    * In Java8, object.castToConsumer() is always going to be enough.
    *
    * @param <I> Vertex id type
    * @param <V> Vertex value type
    * @param <E> Edge value type
    * @return consumer with vertex
    */
   // TODO Java8: cleanup callers
   @SuppressWarnings("rawtypes")
   public <I extends WritableComparable, V extends Writable, E extends Writable>
   ConsumerWithVertex<I, V, E, T> castToConsumer() {
     return new ConsumerWithVertex<I, V, E, T>() {
       @Override
       public void apply(Vertex<I, V, E> vertex, T value) {
         ObjectTransfer.this.apply(value);
       }
     };
   }
 }