MapCoder.java

/*
 * 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.beam.sdk.coders;

import com.google.common.collect.Maps;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.Arrays;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import org.apache.beam.sdk.util.common.ElementByteSizeObserver;
import org.apache.beam.sdk.values.TypeDescriptor;
import org.apache.beam.sdk.values.TypeParameter;

/**
 * A {@link Coder} for {@link Map Maps} that encodes them according to provided
 * coders for keys and values.
 *
 * @param <K> the type of the keys of the KVs being transcoded
 * @param <V> the type of the values of the KVs being transcoded
 */
public class MapCoder<K, V> extends StructuredCoder<Map<K, V>> {
  /**
   * Produces a MapCoder with the given keyCoder and valueCoder.
   */
  public static <K, V> MapCoder<K, V> of(
      Coder<K> keyCoder,
      Coder<V> valueCoder) {
    return new MapCoder<>(keyCoder, valueCoder);
  }

  public Coder<K> getKeyCoder() {
    return keyCoder;
  }

  public Coder<V> getValueCoder() {
    return valueCoder;
  }

  /////////////////////////////////////////////////////////////////////////////

  private Coder<K> keyCoder;
  private Coder<V> valueCoder;

  private MapCoder(Coder<K> keyCoder, Coder<V> valueCoder) {
    this.keyCoder = keyCoder;
    this.valueCoder = valueCoder;
  }

  @Override
  public void encode(Map<K, V> map, OutputStream outStream)
      throws IOException, CoderException {
    encode(map, outStream, Context.NESTED);
  }

  @Override
  public void encode(
      Map<K, V> map,
      OutputStream outStream,
      Context context)
      throws IOException, CoderException  {
    if (map == null) {
      throw new CoderException("cannot encode a null Map");
    }
    DataOutputStream dataOutStream = new DataOutputStream(outStream);

    int size = map.size();
    dataOutStream.writeInt(size);
    if (size == 0) {
      return;
    }

    // Since we handled size == 0 above, entry is guaranteed to exist before and after loop
    Iterator<Entry<K, V>> iterator = map.entrySet().iterator();
    Entry<K, V> entry = iterator.next();
    while (iterator.hasNext()) {
      keyCoder.encode(entry.getKey(), outStream);
      valueCoder.encode(entry.getValue(), outStream);
      entry = iterator.next();
    }

    keyCoder.encode(entry.getKey(), outStream);
    valueCoder.encode(entry.getValue(), outStream, context);
    // no flush needed as DataOutputStream does not buffer
  }

  @Override
  public Map<K, V> decode(InputStream inStream) throws IOException, CoderException {
    return decode(inStream, Context.NESTED);
  }

  @Override
  public Map<K, V> decode(InputStream inStream, Context context)
      throws IOException, CoderException {
    DataInputStream dataInStream = new DataInputStream(inStream);
    int size = dataInStream.readInt();
    if (size == 0) {
      return Collections.emptyMap();
    }

    Map<K, V> retval = Maps.newHashMapWithExpectedSize(size);
    for (int i = 0; i < size - 1; ++i) {
      K key = keyCoder.decode(inStream);
      V value = valueCoder.decode(inStream);
      retval.put(key, value);
    }

    K key = keyCoder.decode(inStream);
    V value = valueCoder.decode(inStream, context);
    retval.put(key, value);
    return retval;
  }

  /**
   * {@inheritDoc}
   *
   * @return a {@link List} containing the key coder at index 0 at the and value coder at index 1.
   */
  @Override
  public List<? extends Coder<?>> getCoderArguments() {
    return Arrays.asList(keyCoder, valueCoder);
  }

  /**
   * {@inheritDoc}
   *
   * @throws NonDeterministicException always. Not all maps have a deterministic encoding.
   * For example, {@code HashMap} comparison does not depend on element order, so
   * two {@code HashMap} instances may be equal but produce different encodings.
   */
  @Override
  public void verifyDeterministic() throws NonDeterministicException {
    throw new NonDeterministicException(this,
        "Ordering of entries in a Map may be non-deterministic.");
  }

  @Override
  public void registerByteSizeObserver(
      Map<K, V> map, ElementByteSizeObserver observer)
      throws Exception {
    observer.update(4L);
    if (map.isEmpty()) {
      return;
    }
    Iterator<Entry<K, V>> entries = map.entrySet().iterator();
    Entry<K, V> entry = entries.next();
    while (entries.hasNext()) {
      keyCoder.registerByteSizeObserver(entry.getKey(), observer);
      valueCoder.registerByteSizeObserver(entry.getValue(), observer);
      entry = entries.next();
    }
    keyCoder.registerByteSizeObserver(entry.getKey(), observer);
    valueCoder.registerByteSizeObserver(entry.getValue(), observer);
  }

  @Override
  public TypeDescriptor<Map<K, V>> getEncodedTypeDescriptor() {
    return new TypeDescriptor<Map<K, V>>() {}.where(
            new TypeParameter<K>() {}, keyCoder.getEncodedTypeDescriptor())
        .where(new TypeParameter<V>() {}, valueCoder.getEncodedTypeDescriptor());
  }
}