/*
 * 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.HashMap;
import java.util.List;
import java.util.Map;
import java.util.concurrent.ThreadLocalRandom;
import org.apache.beam.sdk.PipelineRunner;
import org.apache.beam.sdk.annotations.Internal;
import org.apache.beam.sdk.coders.BigEndianLongCoder;
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.io.range.OffsetRange;
import org.apache.beam.sdk.runners.TransformHierarchy.Node;
import org.apache.beam.sdk.transforms.windowing.BoundedWindow;
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;
import org.apache.beam.sdk.values.PCollectionViews.TypeDescriptorSupplier;
import org.apache.beam.sdk.values.PCollectionViews.ValueOrMetadata;
import org.apache.beam.sdk.values.PCollectionViews.ValueOrMetadataCoder;
import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * 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} transform 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>>asMultimap());
 * }</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>() }{
 *            {@code @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}.
 */
@SuppressWarnings({
  "nullness", // TODO(https://github.com/apache/beam/issues/20497)
  "rawtypes"
})
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>This view should only be used if random access and/or size of the PCollection is required.
   * {@link #asIterable()} will perform significantly better for sequential access.
   *
   * <p>Some runners may require that the view fits 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>Some runners may require that the view fits in memory.
   */
  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>Some runners may require that the view fits in 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>
   *
   * <p>Some runners may require that the view fits in memory.
   */
  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);
      }

      /**
       * The materialized format uses {@link Materializations#MULTIMAP_MATERIALIZATION_URN multimap}
       * access pattern where the key is a position and the index of the value in the iterable is a
       * sub-position. All keys are {@code long}s and all sub-positions are also considered {@code
       * long}s. A mapping from {@code [0, size)} to {@code (position, sub-position)} is used to
       * provide an ordering over all values in the {@link PCollection} per {@link BoundedWindow
       * window}. A total ordering is done by taking {@code (position, sub-position)} and ordering
       * first by {@code position} and then by {@code sub-position} where the smallest value in such
       * an ordering represents the index 0, and the next smallest 1, and so forth. The {@link
       * Long#MIN_VALUE} key is used to store all known {@link OffsetRange ranges} allowing us to
       * compute such an ordering.
       */
      Coder<T> inputCoder = input.getCoder();
      PCollection<KV<Long, ValueOrMetadata<T, OffsetRange>>> materializationInput =
          input
              .apply("IndexElements", ParDo.of(new ToListViewDoFn<>()))
              .setCoder(
                  KvCoder.of(
                      BigEndianLongCoder.of(),
                      ValueOrMetadataCoder.create(inputCoder, OffsetRange.Coder.of())));
      PCollectionView<List<T>> view =
          PCollectionViews.listView(
              materializationInput,
              (TypeDescriptorSupplier<T>) inputCoder::getEncodedTypeDescriptor,
              materializationInput.getWindowingStrategy());
      materializationInput.apply(CreatePCollectionView.of(view));
      return view;
    }
  }

  /**
   * Provides an index to value mapping using a random starting index and also provides an offset
   * range for each window seen. We use random offset ranges to minimize the chance that two ranges
   * overlap increasing the odds that each "key" represents a single index.
   */
  @Internal
  public static class ToListViewDoFn<T> extends DoFn<T, KV<Long, ValueOrMetadata<T, OffsetRange>>> {
    private Map<BoundedWindow, OffsetRange> windowsToOffsets = new HashMap<>();

    private OffsetRange generateRange(BoundedWindow window) {
      long offset =
          ThreadLocalRandom.current()
              .nextLong(Long.MIN_VALUE + 1, Long.MAX_VALUE - Integer.MAX_VALUE);
      return new OffsetRange(offset, offset);
    }

    @ProcessElement
    public void processElement(ProcessContext c, BoundedWindow window) {
      OffsetRange range = windowsToOffsets.computeIfAbsent(window, this::generateRange);
      c.output(KV.of(range.getTo(), ValueOrMetadata.create(c.element())));
      windowsToOffsets.put(window, new OffsetRange(range.getFrom(), range.getTo() + 1));
    }

    @FinishBundle
    public void finishBundle(FinishBundleContext c) {
      for (Map.Entry<BoundedWindow, OffsetRange> entry : windowsToOffsets.entrySet()) {
        c.output(
            KV.of(Long.MIN_VALUE, ValueOrMetadata.createMetadata(entry.getValue())),
            entry.getKey().maxTimestamp(),
            entry.getKey());
      }
      windowsToOffsets.clear();
    }
  }

  /**
   * <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);
      }

      Coder<T> inputCoder = input.getCoder();
      // HACK to work around https://github.com/apache/beam/issues/20873:
      // There are bugs in "composite" vs "primitive" transform distinction
      // in TransformHierachy. This noop transform works around them and should be zero
      // cost.
      PCollection<T> materializationInput =
          input.apply(MapElements.via(new SimpleFunction<T, T>(x -> x) {}));
      PCollectionView<Iterable<T>> view =
          PCollectionViews.iterableView(
              materializationInput,
              (TypeDescriptorSupplier<T>) inputCoder::getEncodedTypeDescriptor,
              materializationInput.getWindowingStrategy());
      input.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>> {
    private final @Nullable 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;
    private final @Nullable Coder<T> valueCoder;
    private final byte @Nullable [] 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);
      }

      KvCoder<K, V> kvCoder = (KvCoder<K, V>) input.getCoder();
      Coder<K> keyCoder = kvCoder.getKeyCoder();
      Coder<V> valueCoder = kvCoder.getValueCoder();
      // HACK to work around https://github.com/apache/beam/issues/20873:
      // There are bugs in "composite" vs "primitive" transform distinction
      // in TransformHierachy. This noop transform works around them and should be zero
      // cost.
      PCollection<KV<K, V>> materializationInput =
          input.apply(MapElements.via(new SimpleFunction<KV<K, V>, KV<K, V>>(x -> x) {}));
      PCollectionView<Map<K, Iterable<V>>> view =
          PCollectionViews.multimapView(
              materializationInput,
              (TypeDescriptorSupplier<K>) keyCoder::getEncodedTypeDescriptor,
              (TypeDescriptorSupplier<V>) valueCoder::getEncodedTypeDescriptor,
              materializationInput.getWindowingStrategy());
      input.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);
      }

      KvCoder<K, V> kvCoder = (KvCoder<K, V>) input.getCoder();
      Coder<K> keyCoder = kvCoder.getKeyCoder();
      Coder<V> valueCoder = kvCoder.getValueCoder();

      PCollection<KV<K, V>> materializationInput =
          input.apply(MapElements.via(new SimpleFunction<KV<K, V>, KV<K, V>>(x -> x) {}));
      PCollectionView<Map<K, V>> view =
          PCollectionViews.mapView(
              materializationInput,
              (TypeDescriptorSupplier<K>) keyCoder::getEncodedTypeDescriptor,
              (TypeDescriptorSupplier<V>) valueCoder::getEncodedTypeDescriptor,
              materializationInput.getWindowingStrategy());
      input.apply(CreatePCollectionView.of(view));
      return view;
    }
  }

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

  /**
   * <b><i>For internal use only; no backwards-compatibility guarantees.</i></b>
   *
   * <p>Placeholder transform for runners to have a hook to materialize a {@link PCollection} as a
   * side input. The metadata included in the {@link PCollectionView} is how the {@link PCollection}
   * will be read as a side input.
   *
   * @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());
    }
  }
}
