/*
 * Copyright (C) 2016 The Guava Authors
 *
 * Licensed 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 com.google.common.graph;

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

import com.google.common.collect.UnmodifiableIterator;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.util.AbstractSet;
import java.util.Iterator;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;
import org.checkerframework.checker.nullness.qual.Nullable;

A map-like data structure that wraps a backing map and caches values while iterating through unmodifiableKeySet(). By design, the cache is cleared when this structure is mutated. If this structure is never mutated, it provides a thread-safe view of the backing map. 
The MapIteratorCache assumes ownership of the backing map, and cannot guarantee correctness in the face of external mutations to the backing map. As such, it is strongly
recommended that the caller does not persist a reference to the backing map (unless the backing
map is immutable).
This class is tailored toward use cases in common.graph. It is *NOT* a general purpose map.
Author:
James Sexton/**
 * A map-like data structure that wraps a backing map and caches values while iterating through
 * {@link #unmodifiableKeySet()}. By design, the cache is cleared when this structure is mutated. If
 * this structure is never mutated, it provides a thread-safe view of the backing map.
 *
 * <p>The {@link MapIteratorCache} assumes ownership of the backing map, and cannot guarantee
 * correctness in the face of external mutations to the backing map. As such, it is <b>strongly</b>
 * recommended that the caller does not persist a reference to the backing map (unless the backing
 * map is immutable).
 *
 * <p>This class is tailored toward use cases in common.graph. It is *NOT* a general purpose map.
 *
 * @author James Sexton
 */
class MapIteratorCache<K, V> {
  private final Map<K, V> backingMap;

  // Per JDK: "the behavior of a map entry is undefined if the backing map has been modified after
  // the entry was returned by the iterator, except through the setValue operation on the map entry"
  // As such, this field must be cleared before every map mutation.
  private transient @Nullable Entry<K, V> entrySetCache;

  MapIteratorCache(Map<K, V> backingMap) {
    this.backingMap = checkNotNull(backingMap);
  }

  @CanIgnoreReturnValue
  public V put(@Nullable K key, @Nullable V value) {
    clearCache();
    return backingMap.put(key, value);
  }

  @CanIgnoreReturnValue
  public V remove(@Nullable Object key) {
    clearCache();
    return backingMap.remove(key);
  }

  public void clear() {
    clearCache();
    backingMap.clear();
  }

  public V get(@Nullable Object key) {
    V value = getIfCached(key);
    return (value != null) ? value : getWithoutCaching(key);
  }

  public final V getWithoutCaching(@Nullable Object key) {
    return backingMap.get(key);
  }

  public final boolean containsKey(@Nullable Object key) {
    return getIfCached(key) != null || backingMap.containsKey(key);
  }

  public final Set<K> unmodifiableKeySet() {
    return new AbstractSet<K>() {
      @Override
      public UnmodifiableIterator<K> iterator() {
        final Iterator<Entry<K, V>> entryIterator = backingMap.entrySet().iterator();

        return new UnmodifiableIterator<K>() {
          @Override
          public boolean hasNext() {
            return entryIterator.hasNext();
          }

          @Override
          public K next() {
            Entry<K, V> entry = entryIterator.next(); // store local reference for thread-safety
            entrySetCache = entry;
            return entry.getKey();
          }
        };
      }

      @Override
      public int size() {
        return backingMap.size();
      }

      @Override
      public boolean contains(@Nullable Object key) {
        return containsKey(key);
      }
    };
  }

  // Internal methods ('protected' is still package-visible, but treat as only subclass-visible)

  protected V getIfCached(@Nullable Object key) {
    Entry<K, V> entry = entrySetCache; // store local reference for thread-safety

    // Check cache. We use == on purpose because it's cheaper and a cache miss is ok.
    if (entry != null && entry.getKey() == key) {
      return entry.getValue();
    }
    return null;
  }

  protected void clearCache() {
    entrySetCache = null;
  }
}