/*
 * 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 java.util.List;
import java.util.Map;
import javax.annotation.Nullable;
import org.apache.beam.sdk.PipelineRunner;
import org.apache.beam.sdk.annotations.Internal;
import org.apache.beam.sdk.coders.Coder;
import org.apache.beam.sdk.coders.CoderException;
import org.apache.beam.sdk.coders.KvCoder;
import org.apache.beam.sdk.coders.VoidCoder;
import org.apache.beam.sdk.runners.TransformHierarchy.Node;
import org.apache.beam.sdk.util.CoderUtils;
import org.apache.beam.sdk.values.KV;
import org.apache.beam.sdk.values.PCollection;
import org.apache.beam.sdk.values.PCollectionView;
import org.apache.beam.sdk.values.PCollectionViews;

/**
 * Transforms for creating {@link PCollectionView PCollectionViews} from {@link PCollection
 * PCollections} (to read them as side inputs).
 *
 * <p>While a {@link PCollection PCollection&lt;ElemT&gt;} has many values of type {@code ElemT} per
 * window, a {@link PCollectionView PCollectionView&lt;ViewT&gt;} has a single value of type {@code
 * ViewT} for each window. It can be thought of as a mapping from windows to values of type {@code
 * ViewT}. The transforms here represent ways of converting the {@code ElemT} values in a window
 * into a {@code ViewT} for that window.
 *
 * <p>When a {@link ParDo} tranform is processing a main input element in a window {@code w} and a
 * {@link PCollectionView} is read via {@link DoFn.ProcessContext#sideInput}, the value of the view
 * for {@code w} is returned.
 *
 * <p>The SDK supports viewing a {@link PCollection}, per window, as a single value, a {@link List},
 * an {@link Iterable}, a {@link Map}, or a multimap (iterable-valued {@link Map}).
 *
 * <p>For a {@link PCollection} that contains a single value of type {@code T} per window, such as
 * the output of {@link Combine#globally}, use {@link View#asSingleton()} to prepare it for use as a
 * side input:
 *
 * <pre>{@code
 * PCollectionView<T> output = someOtherPCollection
 *     .apply(Combine.globally(...))
 *     .apply(View.<T>asSingleton());
 * }</pre>
 *
 * <p>For a small {@link PCollection} with windows that can fit entirely in memory, use {@link
 * View#asList()} to prepare it for use as a {@code List}. When read as a side input, the entire
 * list for a window will be cached in memory.
 *
 * <pre>{@code
 * PCollectionView<List<T>> output =
 *    smallPCollection.apply(View.<T>asList());
 * }</pre>
 *
 * <p>If a {@link PCollection} of {@code KV<K, V>} is known to have a single value per window for
 * each key, then use {@link View#asMap()} to view it as a {@code Map<K, V>}:
 *
 * <pre>{@code
 * PCollectionView<Map<K, V> output =
 *     somePCollection.apply(View.<K, V>asMap());
 * }</pre>
 *
 * <p>Otherwise, to access a {@link PCollection} of {@code KV<K, V>} as a {@code Map<K,
 * Iterable<V>>} side input, use {@link View#asMultimap()}:
 *
 * <pre>{@code
 * PCollectionView<Map<K, Iterable<V>> output =
 *     somePCollection.apply(View.<K, Iterable<V>>asMap());
 * }</pre>
 *
 * <p>To iterate over an entire window of a {@link PCollection} via side input, use {@link
 * View#asIterable()}:
 *
 * <pre>{@code
 * PCollectionView<Iterable<T>> output =
 *     somePCollection.apply(View.<T>asIterable());
 * }</pre>
 *
 * <p>Both {@link View#asMultimap()} and {@link View#asMap()} are useful for implementing lookup
 * based "joins" with the main input, when the side input is small enough to fit into memory.
 *
 * <p>For example, if you represent a page on a website via some {@code Page} object and have some
 * type {@code UrlVisits} logging that a URL was visited, you could convert these to more fully
 * structured {@code PageVisit} objects using a side input, something like the following:
 *
 * <pre>{@code
 * PCollection<Page> pages = ... // pages fit into memory
 * PCollection<UrlVisit> urlVisits = ... // very large collection
 * final PCollectionView<Map<URL, Page>> urlToPageView = pages
 *     .apply(WithKeys.of( ... )) // extract the URL from the page
 *     .apply(View.<URL, Page>asMap());
 *
 * PCollection<PageVisit> pageVisits = urlVisits
 *     .apply(ParDo.withSideInputs(urlToPageView)
 *         .of(new DoFn<UrlVisit, PageVisit>() {
 *             {@literal @}Override
 *             void processElement(ProcessContext context) {
 *               UrlVisit urlVisit = context.element();
 *               Map<URL, Page> urlToPage = context.sideInput(urlToPageView);
 *               Page page = urlToPage.get(urlVisit.getUrl());
 *               c.output(new PageVisit(page, urlVisit.getVisitData()));
 *             }
 *         }));
 * }</pre>
 *
 * <p>See {@link ParDo.SingleOutput#withSideInputs} for details on how to access this variable
 * inside a {@link ParDo} over another {@link PCollection}.
 */
public class View {

  // Do not instantiate
  private View() {}

  /**
   * Returns a {@link AsSingleton} transform that takes a {@link PCollection} with a single value
   * per window as input and produces a {@link PCollectionView} that returns the value in the main
   * input window when read as a side input.
   *
   * <pre>{@code
   * PCollection<InputT> input = ...
   * CombineFn<InputT, OutputT> yourCombineFn = ...
   * PCollectionView<OutputT> output = input
   *     .apply(Combine.globally(yourCombineFn))
   *     .apply(View.<OutputT>asSingleton());
   * }</pre>
   *
   * <p>If the input {@link PCollection} is empty, throws {@link java.util.NoSuchElementException}
   * in the consuming {@link DoFn}.
   *
   * <p>If the input {@link PCollection} contains more than one element, throws {@link
   * IllegalArgumentException} in the consuming {@link DoFn}.
   */
  public static <T> AsSingleton<T> asSingleton() {
    return new AsSingleton<>();
  }

  /**
   * Returns a {@link View.AsList} transform that takes a {@link PCollection} and returns a {@link
   * PCollectionView} mapping each window to a {@link List} containing all of the elements in the
   * window.
   *
   * <p>Unlike with {@link #asIterable}, the resulting list is required to fit in memory.
   */
  public static <T> AsList<T> asList() {
    return new AsList<>();
  }

  /**
   * Returns a {@link View.AsIterable} transform that takes a {@link PCollection} as input and
   * produces a {@link PCollectionView} mapping each window to an {@link Iterable} of the values in
   * that window.
   *
   * <p>The values of the {@link Iterable} for a window are not required to fit in memory, but they
   * may also not be effectively cached. If it is known that every window fits in memory, and
   * stronger caching is desired, use {@link #asList}.
   */
  public static <T> AsIterable<T> asIterable() {
    return new AsIterable<>();
  }

  /**
   * Returns a {@link View.AsMap} transform that takes a {@link PCollection PCollection&lt;KV&lt;K,
   * V&gt;&gt;} as input and produces a {@link PCollectionView} mapping each window to a {@link Map
   * Map&lt;K, V&gt;}. It is required that each key of the input be associated with a single value,
   * per window. If this is not the case, precede this view with {@code Combine.perKey}, as in the
   * example below, or alternatively use {@link View#asMultimap()}.
   *
   * <pre>{@code
   * PCollection<KV<K, V>> input = ...
   * CombineFn<V, OutputT> yourCombineFn = ...
   * PCollectionView<Map<K, OutputT>> output = input
   *     .apply(Combine.perKey(yourCombineFn))
   *     .apply(View.<K, OutputT>asMap());
   * }</pre>
   *
   * <p>Currently, the resulting map is required to fit into memory.
   */
  public static <K, V> AsMap<K, V> asMap() {
    return new AsMap<>();
  }

  /**
   * Returns a {@link View.AsMultimap} transform that takes a {@link PCollection
   * PCollection&lt;KV&lt;K, V&gt;&gt;} as input and produces a {@link PCollectionView} mapping each
   * window to its contents as a {@link Map Map&lt;K, Iterable&lt;V&gt;&gt;} for use as a side
   * input. In contrast to {@link View#asMap()}, it is not required that the keys in the input
   * collection be unique.
   *
   * <pre>{@code
   * PCollection<KV<K, V>> input = ... // maybe more than one occurrence of a some keys
   * PCollectionView<Map<K, Iterable<V>>> output = input.apply(View.<K, V>asMultimap());
   * }</pre>
   */
  public static <K, V> AsMultimap<K, V> asMultimap() {
    return new AsMultimap<>();
  }

  /**
   * <b><i>For internal use only; no backwards-compatibility guarantees.</i></b>
   *
   * <p>Public only so a {@link PipelineRunner} may override its behavior.
   *
   * <p>See {@link View#asList()}.
   */
  @Internal
  public static class AsList<T> extends PTransform<PCollection<T>, PCollectionView<List<T>>> {
    private AsList() {}

    @Override
    public PCollectionView<List<T>> expand(PCollection<T> input) {
      try {
        GroupByKey.applicableTo(input);
      } catch (IllegalStateException e) {
        throw new IllegalStateException("Unable to create a side-input view from input", e);
      }

      PCollection<KV<Void, T>> materializationInput =
          input.apply(new VoidKeyToMultimapMaterialization<>());
      PCollectionView<List<T>> view =
          PCollectionViews.listView(
              materializationInput, materializationInput.getWindowingStrategy());
      materializationInput.apply(CreatePCollectionView.of(view));
      return view;
    }
  }

  /**
   * <b><i>For internal use only; no backwards-compatibility guarantees.</i></b>
   *
   * <p>Public only so a {@link PipelineRunner} may override its behavior.
   *
   * <p>See {@link View#asIterable()}.
   */
  @Internal
  public static class AsIterable<T>
      extends PTransform<PCollection<T>, PCollectionView<Iterable<T>>> {
    private AsIterable() {}

    @Override
    public PCollectionView<Iterable<T>> expand(PCollection<T> input) {
      try {
        GroupByKey.applicableTo(input);
      } catch (IllegalStateException e) {
        throw new IllegalStateException("Unable to create a side-input view from input", e);
      }

      PCollection<KV<Void, T>> materializationInput =
          input.apply(new VoidKeyToMultimapMaterialization<>());
      PCollectionView<Iterable<T>> view =
          PCollectionViews.iterableView(
              materializationInput, materializationInput.getWindowingStrategy());
      materializationInput.apply(CreatePCollectionView.of(view));
      return view;
    }
  }

  /**
   * <b><i>For internal use only; no backwards-compatibility guarantees.</i></b>
   *
   * <p>Public only so a {@link PipelineRunner} may override its behavior.
   *
   * <p>See {@link View#asSingleton()}.
   */
  @Internal
  public static class AsSingleton<T> extends PTransform<PCollection<T>, PCollectionView<T>> {
    @Nullable private final T defaultValue;
    private final boolean hasDefault;

    private AsSingleton() {
      this.defaultValue = null;
      this.hasDefault = false;
    }

    private AsSingleton(T defaultValue) {
      this.defaultValue = defaultValue;
      this.hasDefault = true;
    }

    /** Returns whether this transform has a default value. */
    public boolean hasDefaultValue() {
      return hasDefault;
    }

    /** Returns the default value of this transform, or null if there isn't one. */
    public T defaultValue() {
      return defaultValue;
    }

    /** Default value to return for windows with no value in them. */
    public AsSingleton<T> withDefaultValue(T defaultValue) {
      return new AsSingleton<>(defaultValue);
    }

    @Override
    public PCollectionView<T> expand(PCollection<T> input) {
      try {
        GroupByKey.applicableTo(input);
      } catch (IllegalStateException e) {
        throw new IllegalStateException("Unable to create a side-input view from input", e);
      }

      Combine.Globally<T, T> singletonCombine =
          Combine.globally(new SingletonCombineFn<>(hasDefault, input.getCoder(), defaultValue));
      if (!hasDefault) {
        singletonCombine = singletonCombine.withoutDefaults();
      }
      return input.apply(singletonCombine.asSingletonView());
    }
  }

  private static class SingletonCombineFn<T> extends Combine.BinaryCombineFn<T> {
    private final boolean hasDefault;
    @Nullable private final Coder<T> valueCoder;
    @Nullable private final byte[] defaultValue;

    private SingletonCombineFn(boolean hasDefault, Coder<T> coder, T defaultValue) {
      this.hasDefault = hasDefault;
      if (hasDefault) {
        if (defaultValue == null) {
          this.defaultValue = null;
          this.valueCoder = coder;
        } else {
          this.valueCoder = coder;
          try {
            this.defaultValue = CoderUtils.encodeToByteArray(coder, defaultValue);
          } catch (CoderException e) {
            throw new IllegalArgumentException(
                String.format(
                    "Could not encode the default value %s with the provided coder %s",
                    defaultValue, coder));
          }
        }
      } else {
        this.valueCoder = null;
        this.defaultValue = null;
      }
    }

    @Override
    public T apply(T left, T right) {
      throw new IllegalArgumentException(
          "PCollection with more than one element "
              + "accessed as a singleton view. Consider using Combine.globally().asSingleton() to "
              + "combine the PCollection into a single value");
    }

    @Override
    public T identity() {
      if (hasDefault) {
        if (defaultValue == null) {
          return null;
        }
        try {
          return CoderUtils.decodeFromByteArray(valueCoder, defaultValue);
        } catch (CoderException e) {
          throw new IllegalArgumentException(
              String.format(
                  "Could not decode the default value with the provided coder %s", valueCoder));
        }
      } else {
        throw new IllegalArgumentException(
            "Empty PCollection accessed as a singleton view. "
                + "Consider setting withDefault to provide a default value");
      }
    }
  }

  /**
   * <b><i>For internal use only; no backwards-compatibility guarantees.</i></b>
   *
   * <p>Public only so a {@link PipelineRunner} may override its behavior.
   *
   * <p>See {@link View#asMultimap()}.
   */
  @Internal
  public static class AsMultimap<K, V>
      extends PTransform<PCollection<KV<K, V>>, PCollectionView<Map<K, Iterable<V>>>> {
    private AsMultimap() {}

    @Override
    public PCollectionView<Map<K, Iterable<V>>> expand(PCollection<KV<K, V>> input) {
      try {
        GroupByKey.applicableTo(input);
      } catch (IllegalStateException e) {
        throw new IllegalStateException("Unable to create a side-input view from input", e);
      }

      PCollection<KV<Void, KV<K, V>>> materializationInput =
          input.apply(new VoidKeyToMultimapMaterialization<>());
      PCollectionView<Map<K, Iterable<V>>> view =
          PCollectionViews.multimapView(
              materializationInput, materializationInput.getWindowingStrategy());
      materializationInput.apply(CreatePCollectionView.of(view));
      return view;
    }
  }

  /**
   * <b><i>For internal use only; no backwards-compatibility guarantees.</i></b>
   *
   * <p>Public only so a {@link PipelineRunner} may override its behavior.
   *
   * <p>See {@link View#asMap()}.
   */
  @Internal
  public static class AsMap<K, V>
      extends PTransform<PCollection<KV<K, V>>, PCollectionView<Map<K, V>>> {
    private AsMap() {}

    /** @deprecated this method simply returns this AsMap unmodified */
    @Deprecated()
    public AsMap<K, V> withSingletonValues() {
      return this;
    }

    @Override
    public PCollectionView<Map<K, V>> expand(PCollection<KV<K, V>> input) {
      try {
        GroupByKey.applicableTo(input);
      } catch (IllegalStateException e) {
        throw new IllegalStateException("Unable to create a side-input view from input", e);
      }

      PCollection<KV<Void, KV<K, V>>> materializationInput =
          input.apply(new VoidKeyToMultimapMaterialization<>());
      PCollectionView<Map<K, V>> view =
          PCollectionViews.mapView(
              materializationInput, materializationInput.getWindowingStrategy());
      materializationInput.apply(CreatePCollectionView.of(view));
      return view;
    }
  }

  ////////////////////////////////////////////////////////////////////////////
  // Internal details below

  /**
   * A {@link PTransform} which converts all values into {@link KV}s with {@link Void} keys.
   *
   * <p>TODO: Replace this materialization with specializations that optimize the various SDK
   * requested views.
   */
  @Internal
  static class VoidKeyToMultimapMaterialization<T>
      extends PTransform<PCollection<T>, PCollection<KV<Void, T>>> {

    private static class VoidKeyToMultimapMaterializationDoFn<T> extends DoFn<T, KV<Void, T>> {
      @ProcessElement
      public void processElement(@Element T element, OutputReceiver<KV<Void, T>> r) {
        r.output(KV.of((Void) null, element));
      }
    }

    @Override
    public PCollection<KV<Void, T>> expand(PCollection<T> input) {
      PCollection output = input.apply(ParDo.of(new VoidKeyToMultimapMaterializationDoFn<>()));
      output.setCoder(KvCoder.of(VoidCoder.of(), input.getCoder()));
      return output;
    }
  }

  /**
   * <b><i>For internal use only; no backwards-compatibility guarantees.</i></b>
   *
   * <p>Creates a primitive {@link PCollectionView}.
   *
   * @param <ElemT> The type of the elements of the input PCollection
   * @param <ViewT> The type associated with the {@link PCollectionView} used as a side input
   */
  @Internal
  public static class CreatePCollectionView<ElemT, ViewT>
      extends PTransform<PCollection<ElemT>, PCollection<ElemT>> {
    private PCollectionView<ViewT> view;

    private CreatePCollectionView(PCollectionView<ViewT> view) {
      this.view = view;
    }

    public static <ElemT, ViewT> CreatePCollectionView<ElemT, ViewT> of(
        PCollectionView<ViewT> view) {
      return new CreatePCollectionView<>(view);
    }

    /**
     * Return the {@link PCollectionView} that is returned by applying this {@link PTransform}.
     *
     * @deprecated This should not be used to obtain the output of any given application of this
     *     {@link PTransform}. That should be obtained by inspecting the {@link Node} that contains
     *     this {@link CreatePCollectionView}, as this view may have been replaced within pipeline
     *     surgery.
     */
    @Deprecated
    public PCollectionView<ViewT> getView() {
      return view;
    }

    @Override
    public PCollection<ElemT> expand(PCollection<ElemT> input) {
      return PCollection.createPrimitiveOutputInternal(
          input.getPipeline(), input.getWindowingStrategy(), input.isBounded(), input.getCoder());
    }
  }
}
