/*
 * 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 org.apache.beam.sdk.values.KV;
import org.apache.beam.sdk.values.PCollection;
import org.apache.beam.sdk.values.TypeDescriptor;

/**
 * {@code Distinct<T>} takes a {@code PCollection<T>} and
 * returns a {@code PCollection<T>} that has all distinct elements of the
 * input. Thus, each element is unique within each window.
 *
 * <p>Two values of type {@code T} are compared for equality <b>not</b> by
 * regular Java {@link Object#equals}, but instead by first encoding
 * each of the elements using the {@code PCollection}'s {@code Coder}, and then
 * comparing the encoded bytes.  This admits efficient parallel
 * evaluation.
 *
 * <p>Optionally, a function may be provided that maps each element to a representative
 * value.  In this case, two elements will be considered duplicates if they have equal
 * representative values, with equality being determined as above.
 *
 * <p>By default, the {@code Coder} of the output {@code PCollection}
 * is the same as the {@code Coder} of the input {@code PCollection}.
 *
 * <p>Each output element is in the same window as its corresponding input
 * element, and has the timestamp of the end of that window.  The output
 * {@code PCollection} has the same
 * {@link org.apache.beam.sdk.transforms.windowing.WindowFn}
 * as the input.
 *
 * <p>Does not preserve any order the input PCollection might have had.
 *
 * <p>Example of use:
 * <pre> {@code
 * PCollection<String> words = ...;
 * PCollection<String> uniqueWords =
 *     words.apply(Distinct.<String>create());
 * } </pre>
 *
 * @param <T> the type of the elements of the input and output
 * {@code PCollection}s
 */
public class Distinct<T> extends PTransform<PCollection<T>,
                                                    PCollection<T>> {
  /**
   * Returns a {@code Distinct<T>} {@code PTransform}.
   *
   * @param <T> the type of the elements of the input and output
   * {@code PCollection}s
   */
  public static <T> Distinct<T> create() {
    return new Distinct<T>();
  }

  /**
   * Returns a {@code Distinct<T, IdT>} {@code PTransform}.
   *
   * @param <T> the type of the elements of the input and output
   * {@code PCollection}s
   * @param <IdT> the type of the representative value used to dedup
   */
  public static <T, IdT> WithRepresentativeValues<T, IdT> withRepresentativeValueFn(
      SerializableFunction<T, IdT> fn) {
    return new WithRepresentativeValues<T, IdT>(fn, null);
  }

  @Override
  public PCollection<T> expand(PCollection<T> in) {
    return in
        .apply("CreateIndex", MapElements.via(new SimpleFunction<T, KV<T, Void>>() {
          @Override
          public KV<T, Void> apply(T element) {
            return KV.of(element, (Void) null);
          }
        }))
        .apply(Combine.<T, Void>perKey(
            new SerializableFunction<Iterable<Void>, Void>() {
              @Override
              public Void apply(Iterable<Void> iter) {
                return null; // ignore input
                }
            }))
        .apply(Keys.<T>create());
  }

  /**
   * A {@link Distinct} {@link PTransform} that uses a {@link SerializableFunction} to
   * obtain a representative value for each input element.
   *
   * <p>Construct via {@link Distinct#withRepresentativeValueFn(SerializableFunction)}.
   *
   * @param <T> the type of input and output element
   * @param <IdT> the type of representative values used to dedup
   */
  public static class WithRepresentativeValues<T, IdT>
      extends PTransform<PCollection<T>, PCollection<T>> {
    private final SerializableFunction<T, IdT> fn;
    private final TypeDescriptor<IdT> representativeType;

    private WithRepresentativeValues(
        SerializableFunction<T, IdT> fn, TypeDescriptor<IdT> representativeType) {
      this.fn = fn;
      this.representativeType = representativeType;
    }

    @Override
    public PCollection<T> expand(PCollection<T> in) {
      WithKeys<IdT, T> withKeys = WithKeys.of(fn);
      if (representativeType != null) {
        withKeys = withKeys.withKeyType(representativeType);
      }
      return in
          .apply(withKeys)
          .apply(Combine.<IdT, T, T>perKey(
              new Combine.BinaryCombineFn<T>() {
                @Override
                public T apply(T left, T right) {
                  return left;
                }
              }))
          .apply(Values.<T>create());
    }

    /**
     * Return a {@code WithRepresentativeValues} {@link PTransform} that is like this one, but with
     * the specified output type descriptor.
     *
     * <p>Required for use of
     * {@link Distinct#withRepresentativeValueFn(SerializableFunction)}
     * in Java 8 with a lambda as the fn.
     *
     * @param type a {@link TypeDescriptor} describing the representative type of this
     *             {@code WithRepresentativeValues}
     * @return A {@code WithRepresentativeValues} {@link PTransform} that is like this one, but with
     *         the specified output type descriptor.
     */
    public WithRepresentativeValues<T, IdT> withRepresentativeType(TypeDescriptor<IdT> type) {
      return new WithRepresentativeValues<>(fn, type);
    }
  }
}