/*
 * Copyright (C) 2011 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.cache;

import com.google.common.annotations.GwtCompatible;
import com.google.common.annotations.J2ktIncompatible;
import com.google.common.collect.ImmutableMap;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.Map.Entry;
import java.util.concurrent.Callable;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.ExecutionException;

/**
 * This class provides a skeletal implementation of the {@code Cache} interface to minimize the
 * effort required to implement this interface.
 *
 * <p>To implement a cache, the programmer needs only to extend this class and provide an
 * implementation for the {@link #put} and {@link #getIfPresent} methods. {@link #getAllPresent} is
 * implemented in terms of {@link #getIfPresent}; {@link #putAll} is implemented in terms of {@link
 * #put}, {@link #invalidateAll(Iterable)} is implemented in terms of {@link #invalidate}. The
 * method {@link #cleanUp} is a no-op. All other methods throw an {@link
 * UnsupportedOperationException}.
 *
 * @author Charles Fry
 * @since 10.0
 */
@GwtCompatible
public abstract class AbstractCache<K, V> implements Cache<K, V> {

  /** Constructor for use by subclasses. */
  protected AbstractCache() {}

  /**
   * @since 11.0
   */
  @Override
  public V get(K key, Callable<? extends V> valueLoader) throws ExecutionException {
    throw new UnsupportedOperationException();
  }

  /**
   * {@inheritDoc}
   *
   * <p>This implementation of {@code getAllPresent} lacks any insight into the internal cache data
   * structure, and is thus forced to return the query keys instead of the cached keys. This is only
   * possible with an unsafe cast which requires {@code keys} to actually be of type {@code K}.
   *
   * @since 11.0
   */
  /*
   * <? extends Object> is mostly the same as <?> to plain Java. But to nullness checkers, they
   * differ: <? extends Object> means "non-null types," while <?> means "all types."
   */
  @Override
  public ImmutableMap<K, V> getAllPresent(Iterable<? extends Object> keys) {
    Map<K, V> result = new LinkedHashMap<>();
    for (Object key : keys) {
      if (!result.containsKey(key)) {
        @SuppressWarnings("unchecked")
        K castKey = (K) key;
        V value = getIfPresent(key);
        if (value != null) {
          result.put(castKey, value);
        }
      }
    }
    return ImmutableMap.copyOf(result);
  }

  /**
   * @since 11.0
   */
  @Override
  public void put(K key, V value) {
    throw new UnsupportedOperationException();
  }

  /**
   * @since 12.0
   */
  @Override
  public void putAll(Map<? extends K, ? extends V> m) {
    for (Entry<? extends K, ? extends V> entry : m.entrySet()) {
      put(entry.getKey(), entry.getValue());
    }
  }

  @Override
  public void cleanUp() {}

  @Override
  public long size() {
    throw new UnsupportedOperationException();
  }

  @Override
  public void invalidate(Object key) {
    throw new UnsupportedOperationException();
  }

  /**
   * @since 11.0
   */
  @Override
  // For discussion of <? extends Object>, see getAllPresent.
  public void invalidateAll(Iterable<? extends Object> keys) {
    for (Object key : keys) {
      invalidate(key);
    }
  }

  @Override
  public void invalidateAll() {
    throw new UnsupportedOperationException();
  }

  @Override
  public CacheStats stats() {
    throw new UnsupportedOperationException();
  }

  @Override
  @J2ktIncompatible // Thread safety complexity
  public ConcurrentMap<K, V> asMap() {
    throw new UnsupportedOperationException();
  }

  /**
   * Accumulates statistics during the operation of a {@link Cache} for presentation by {@link
   * Cache#stats}. This is solely intended for consumption by {@code Cache} implementors.
   *
   * @since 10.0
   */
  public interface StatsCounter {
    /**
     * Records cache hits. This should be called when a cache request returns a cached value.
     *
     * @param count the number of hits to record
     * @since 11.0
     */
    void recordHits(int count);

    /**
     * Records cache misses. This should be called when a cache request returns a value that was not
     * found in the cache. This method should be called by the loading thread, as well as by threads
     * blocking on the load. Multiple concurrent calls to {@link Cache} lookup methods with the same
     * key on an absent value should result in a single call to either {@code recordLoadSuccess} or
     * {@code recordLoadException} and multiple calls to this method, despite all being served by
     * the results of a single load operation.
     *
     * @param count the number of misses to record
     * @since 11.0
     */
    void recordMisses(int count);

    /**
     * Records the successful load of a new entry. This should be called when a cache request causes
     * an entry to be loaded, and the loading completes successfully. In contrast to {@link
     * #recordMisses}, this method should only be called by the loading thread.
     *
     * @param loadTime the number of nanoseconds the cache spent computing or retrieving the new
     *     value
     */
    @SuppressWarnings("GoodTime") // should accept a java.time.Duration
    void recordLoadSuccess(long loadTime);

    /**
     * Records the failed load of a new entry. This should be called when a cache request causes an
     * entry to be loaded, but an exception is thrown while loading the entry. In contrast to {@link
     * #recordMisses}, this method should only be called by the loading thread.
     *
     * @param loadTime the number of nanoseconds the cache spent computing or retrieving the new
     *     value prior to an exception being thrown
     */
    @SuppressWarnings("GoodTime") // should accept a java.time.Duration
    void recordLoadException(long loadTime);

    /**
     * Records the eviction of an entry from the cache. This should only been called when an entry
     * is evicted due to the cache's eviction strategy, and not as a result of manual {@linkplain
     * Cache#invalidate invalidations}.
     */
    void recordEviction();

    /**
     * Returns a snapshot of this counter's values. Note that this may be an inconsistent view, as
     * it may be interleaved with update operations.
     */
    CacheStats snapshot();
  }

  /**
   * A thread-safe {@link StatsCounter} implementation for use by {@link Cache} implementors.
   *
   * <p>Each counter is updated atomically, but the counters are not updated together: For
   * performance, this class does not lock, so a {@link #snapshot} taken concurrently with updates
   * may reflect only part of an update. For example, it may include the increment of the
   * load-success count from a call to {@link #recordLoadSuccess} but not the corresponding load
   * time. As a result, values derived from a snapshot, like {@link CacheStats#averageLoadPenalty},
   * may be briefly inconsistent.
   *
   * @since 10.0
   */
  public static final class SimpleStatsCounter implements StatsCounter {
    private final LongAddable hitCount = LongAddables.create();
    private final LongAddable missCount = LongAddables.create();
    private final LongAddable loadSuccessCount = LongAddables.create();
    private final LongAddable loadExceptionCount = LongAddables.create();
    private final LongAddable totalLoadTime = LongAddables.create();
    private final LongAddable evictionCount = LongAddables.create();

    /** Constructs an instance with all counts initialized to zero. */
    public SimpleStatsCounter() {}

    /**
     * @since 11.0
     */
    @Override
    public void recordHits(int count) {
      hitCount.add(count);
    }

    /**
     * @since 11.0
     */
    @Override
    public void recordMisses(int count) {
      missCount.add(count);
    }

    @SuppressWarnings("GoodTime") // b/122668874
    @Override
    public void recordLoadSuccess(long loadTime) {
      loadSuccessCount.increment();
      totalLoadTime.add(loadTime);
    }

    @SuppressWarnings("GoodTime") // b/122668874
    @Override
    public void recordLoadException(long loadTime) {
      loadExceptionCount.increment();
      totalLoadTime.add(loadTime);
    }

    @Override
    public void recordEviction() {
      evictionCount.increment();
    }

    @Override
    public CacheStats snapshot() {
      return new CacheStats(
          negativeToMaxValue(hitCount.sum()),
          negativeToMaxValue(missCount.sum()),
          negativeToMaxValue(loadSuccessCount.sum()),
          negativeToMaxValue(loadExceptionCount.sum()),
          negativeToMaxValue(totalLoadTime.sum()),
          negativeToMaxValue(evictionCount.sum()));
    }

    /** Returns {@code value}, if non-negative. Otherwise, returns {@link Long#MAX_VALUE}. */
    private static long negativeToMaxValue(long value) {
      return (value >= 0) ? value : Long.MAX_VALUE;
    }

    /** Increments all counters by the values in {@code other}. */
    public void incrementBy(StatsCounter other) {
      CacheStats otherStats = other.snapshot();
      hitCount.add(otherStats.hitCount());
      missCount.add(otherStats.missCount());
      loadSuccessCount.add(otherStats.loadSuccessCount());
      loadExceptionCount.add(otherStats.loadExceptionCount());
      totalLoadTime.add(otherStats.totalLoadTime());
      evictionCount.add(otherStats.evictionCount());
    }
  }
}