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   * 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.runners.dataflow.worker.windmill.state;
  
  import java.io.Closeable;
  import java.io.IOException;
  import java.util.Iterator;
  import java.util.Map;
  import java.util.function.Function;
  import javax.annotation.Nullable;
  import org.apache.beam.runners.dataflow.worker.windmill.Windmill;
  import org.apache.beam.sdk.state.ReadableState;
  import org.apache.beam.sdk.state.ReadableStates;
  import org.apache.beam.vendor.guava.v32_1_2_jre.com.google.common.base.Supplier;
  import org.apache.beam.vendor.guava.v32_1_2_jre.com.google.common.collect.Iterables;
  import org.apache.beam.vendor.guava.v32_1_2_jre.com.google.common.collect.Iterators;
  
  public class WindmillMapViaMultimap<KeyT, ValueT> extends AbstractWindmillMap<KeyT, ValueT> {
    final WindmillMultimap<KeyT, ValueT> multimap;
  
    WindmillMapViaMultimap(WindmillMultimap<KeyT, ValueT> multimap) {
      this.multimap = multimap;
    }
  
    @Override
    protected Windmill.WorkItemCommitRequest persistDirectly(WindmillStateCache.ForKeyAndFamily cache)
        throws IOException {
      return multimap.persistDirectly(cache);
    }
  
    @Override
    void initializeForWorkItem(
        WindmillStateReader reader, Supplier<Closeable> scopedReadStateSupplier) {
      super.initializeForWorkItem(reader, scopedReadStateSupplier);
      multimap.initializeForWorkItem(reader, scopedReadStateSupplier);
    }
  
    @Override
    void cleanupAfterWorkItem() {
      super.cleanupAfterWorkItem();
      multimap.cleanupAfterWorkItem();
    }
  
    @Override
    public void put(KeyT key, ValueT value) {
      multimap.remove(key);
      multimap.put(key, value);
    }
  
    @Override
    public ReadableState<ValueT> computeIfAbsent(
        KeyT key, Function<? super KeyT, ? extends ValueT> mappingFunction) {
      // Note that computeIfAbsent comments indicate that the read is lazy but this matches the
      // existing eager
      // behavior of WindmillMap.
      Iterable<ValueT> existingValues = multimap.get(key).read();
      if (Iterables.isEmpty(existingValues)) {
        ValueT inserted = mappingFunction.apply(key);
        multimap.put(key, inserted);
        return ReadableStates.immediate(inserted);
      } else {
        return ReadableStates.immediate(Iterables.getOnlyElement(existingValues));
      }
    }
  
    @Override
    public void remove(KeyT key) {
      multimap.remove(key);
    }
  
    private static class SingleValueIterableAdaptor<T> implements ReadableState<T> {
      final ReadableState<Iterable<T>> wrapped;
      final @Nullable T defaultValue;
  
      SingleValueIterableAdaptor(ReadableState<Iterable<T>> wrapped, @Nullable T defaultValue) {
        this.wrapped = wrapped;
        this.defaultValue = defaultValue;
      }
  
      @SuppressWarnings("nullness")
      @Override
      public T read() {
        Iterator<T> iterator = wrapped.read().iterator();
        if (!iterator.hasNext()) {
          return defaultValue;
        }
        return Iterators.getOnlyElement(iterator);
      }
  
      @Override
      public ReadableState<T> readLater() {
        wrapped.readLater();
        return this;
      }
    }
  
    @Override
    public ReadableState<ValueT> get(KeyT key) {
      return getOrDefault(key, null);
    }
  
    @Override
    public ReadableState<ValueT> getOrDefault(KeyT key, @Nullable ValueT defaultValue) {
      return new SingleValueIterableAdaptor<>(multimap.get(key), defaultValue);
    }
  
    @Override
    public ReadableState<Iterable<KeyT>> keys() {
      return multimap.keys();
    }
  
    private static class RemoveKeyAdaptor<K, V> implements ReadableState<Iterable<V>> {
      final ReadableState<Iterable<Map.Entry<K, V>>> wrapped;
  
      RemoveKeyAdaptor(ReadableState<Iterable<Map.Entry<K, V>>> wrapped) {
        this.wrapped = wrapped;
      }
  
      @Override
      public Iterable<V> read() {
        return Iterables.transform(wrapped.read(), Map.Entry::getValue);
      }
  
      @Override
      public ReadableState<Iterable<V>> readLater() {
        wrapped.readLater();
        return this;
      }
    }
  
    @Override
    public ReadableState<Iterable<ValueT>> values() {
      return new RemoveKeyAdaptor<>(multimap.entries());
    }
  
    @Override
    public ReadableState<Iterable<Map.Entry<KeyT, ValueT>>> entries() {
      return multimap.entries();
    }
  
    @Override
    public ReadableState<Boolean> isEmpty() {
      return multimap.isEmpty();
    }
  
    @Override
    public void clear() {
      multimap.clear();
    }
  }
  