/*
 * 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.transforms;

import static com.google.common.base.Preconditions.checkNotNull;

import javax.annotation.CheckForNull;
import javax.annotation.Nullable;
import org.apache.beam.sdk.coders.CannotProvideCoderException;
import org.apache.beam.sdk.coders.Coder;
import org.apache.beam.sdk.coders.CoderRegistry;
import org.apache.beam.sdk.coders.KvCoder;
import org.apache.beam.sdk.values.KV;
import org.apache.beam.sdk.values.PCollection;
import org.apache.beam.sdk.values.TypeDescriptor;

/**
 * {@code WithKeys<K, V>} takes a {@code PCollection<V>}, and either a
 * constant key of type {@code K} or a function from {@code V} to
 * {@code K}, and returns a {@code PCollection<KV<K, V>>}, where each
 * of the values in the input {@code PCollection} has been paired with
 * either the constant key or a key computed from the value.
 *
 * <p>Example of use:
 * <pre> {@code
 * PCollection<String> words = ...;
 * PCollection<KV<Integer, String>> lengthsToWords =
 *     words.apply(WithKeys.of(new SerializableFunction<String, Integer>() {
 *         public Integer apply(String s) { return s.length(); } }));
 * } </pre>
 *
 * <p>Each output element has the same timestamp and is in the same windows
 * as its corresponding input element, and the output {@code PCollection}
 * has the same
 * {@link org.apache.beam.sdk.transforms.windowing.WindowFn}
 * associated with it as the input.
 *
 * @param <K> the type of the keys in the output {@code PCollection}
 * @param <V> the type of the elements in the input
 * {@code PCollection} and the values in the output
 * {@code PCollection}
 */
public class WithKeys<K, V> extends PTransform<PCollection<V>,
                                               PCollection<KV<K, V>>> {
  /**
   * Returns a {@code PTransform} that takes a {@code PCollection<V>}
   * and returns a {@code PCollection<KV<K, V>>}, where each of the
   * values in the input {@code PCollection} has been paired with a
   * key computed from the value by invoking the given
   * {@code SerializableFunction}.
   *
   * <p>If using a lambda in Java 8, {@link #withKeyType(TypeDescriptor)} must
   * be called on the result {@link PTransform}.
   */
  public static <K, V> WithKeys<K, V> of(SerializableFunction<V, K> fn) {
    checkNotNull(fn,
        "WithKeys constructed with null function. Did you mean WithKeys.of((Void) null)?");
    return new WithKeys<>(fn, null);
  }

  /**
   * Returns a {@code PTransform} that takes a {@code PCollection<V>}
   * and returns a {@code PCollection<KV<K, V>>}, where each of the
   * values in the input {@code PCollection} has been paired with the
   * given key.
   */
  @SuppressWarnings("unchecked")
  public static <K, V> WithKeys<K, V> of(@Nullable final K key) {
    return new WithKeys<>(value -> key, (Class<K>) (key == null ? Void.class : key.getClass()));
  }


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

  private SerializableFunction<V, K> fn;
  @CheckForNull private transient Class<K> keyClass;

  private WithKeys(SerializableFunction<V, K> fn, Class<K> keyClass) {
    this.fn = fn;
    this.keyClass = keyClass;
  }

  /**
   * Return a {@link WithKeys} that is like this one with the specified key type descriptor.
   *
   * <p>For use with lambdas in Java 8, either this method must be called with an appropriate type
   * descriptor or {@link PCollection#setCoder(Coder)} must be called on the output
   * {@link PCollection}.
   */
  public WithKeys<K, V> withKeyType(TypeDescriptor<K> keyType) {
    // Safe cast
    @SuppressWarnings("unchecked")
    Class<K> rawType = (Class<K>) keyType.getRawType();
    return new WithKeys<>(fn, rawType);
  }

  @Override
  public PCollection<KV<K, V>> expand(PCollection<V> in) {
    PCollection<KV<K, V>> result =
        in.apply("AddKeys", MapElements.via(new SimpleFunction<V, KV<K, V>>() {
          @Override
          public KV<K, V> apply(V element) {
            return KV.of(fn.apply(element), element);
          }
        }));

    try {
      Coder<K> keyCoder;
      CoderRegistry coderRegistry = in.getPipeline().getCoderRegistry();
      if (keyClass == null) {
        keyCoder = coderRegistry.getOutputCoder(fn, in.getCoder());
      } else {
        keyCoder = coderRegistry.getCoder(TypeDescriptor.of(keyClass));
      }
      // TODO: Remove when we can set the coder inference context.
      result.setCoder(KvCoder.of(keyCoder, in.getCoder()));
    } catch (CannotProvideCoderException exc) {
      // let lazy coder inference have a try
    }

    return result;
  }
}
