/**
    *
    * 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.hadoop.hbase.io.hfile;
  
  import java.util.concurrent.atomic.AtomicLong;
  
  import org.apache.hadoop.hbase.classification.InterfaceAudience;
  
  import com.yammer.metrics.core.Histogram;
  import com.yammer.metrics.core.MetricsRegistry;
  
  /**
   * Class that implements cache metrics.
   */
  @InterfaceAudience.Private
  public class CacheStats {
    /**
     * Needed making histograms.
     */
    private static final MetricsRegistry METRICS = new MetricsRegistry();
  
    /** Sliding window statistics. The number of metric periods to include in
     * sliding window hit ratio calculations.
     */
    static final int DEFAULT_WINDOW_PERIODS = 5;
  
    /** The number of getBlock requests that were cache hits */
    private final AtomicLong hitCount = new AtomicLong(0);
  
    /**
     * The number of getBlock requests that were cache hits, but only from
     * requests that were set to use the block cache.  This is because all reads
     * attempt to read from the block cache even if they will not put new blocks
     * into the block cache.  See HBASE-2253 for more information.
     */
    private final AtomicLong hitCachingCount = new AtomicLong(0);
  
    /** The number of getBlock requests that were cache misses */
    private final AtomicLong missCount = new AtomicLong(0);
  
    /**
     * The number of getBlock requests that were cache misses, but only from
     * requests that were set to use the block cache.
     */
    private final AtomicLong missCachingCount = new AtomicLong(0);
  
    /** The number of times an eviction has occurred */
    private final AtomicLong evictionCount = new AtomicLong(0);
  
    /** The total number of blocks that have been evicted */
    private final AtomicLong evictedBlockCount = new AtomicLong(0);
  
    /** The number of metrics periods to include in window */
    private final int numPeriodsInWindow;
    /** Hit counts for each period in window */
    private final long [] hitCounts;
    /** Caching hit counts for each period in window */
    private final long [] hitCachingCounts;
    /** Access counts for each period in window */
    private final long [] requestCounts;
    /** Caching access counts for each period in window */
    private final long [] requestCachingCounts;
    /** Last hit count read */
    private long lastHitCount = 0;
    /** Last hit caching count read */
    private long lastHitCachingCount = 0;
    /** Last request count read */
    private long lastRequestCount = 0;
    /** Last request caching count read */
    private long lastRequestCachingCount = 0;
    /** Current window index (next to be updated) */
    private int windowIndex = 0;
    /**
     * Keep running age at eviction time
     */
    private Histogram ageAtEviction;
    private long startTime = System.nanoTime();
  
    public CacheStats(final String name) {
      this(name, DEFAULT_WINDOW_PERIODS);
    }
  
    public CacheStats(final String name, int numPeriodsInWindow) {
     this.numPeriodsInWindow = numPeriodsInWindow;
     this.hitCounts = initializeZeros(numPeriodsInWindow);
     this.hitCachingCounts = initializeZeros(numPeriodsInWindow);
     this.requestCounts = initializeZeros(numPeriodsInWindow);
     this.requestCachingCounts = initializeZeros(numPeriodsInWindow);
     this.ageAtEviction = METRICS.newHistogram(CacheStats.class, name + ".ageAtEviction");
   }
 
   @Override
   public String toString() {
     AgeSnapshot snapshot = getAgeAtEvictionSnapshot();
     return "hitCount=" + getHitCount() + ", hitCachingCount=" + getHitCachingCount() +
       ", missCount=" + getMissCount() + ", missCachingCount=" + getMissCachingCount() +
       ", evictionCount=" + getEvictionCount() +
       ", evictedBlockCount=" + getEvictedCount() +
       ", evictedAgeMean=" + snapshot.getMean() +
       ", evictedAgeStdDev=" + snapshot.getStdDev();
   }
 
   public void miss(boolean caching) {
     missCount.incrementAndGet();
     if (caching) missCachingCount.incrementAndGet();
   }
 
   public void hit(boolean caching) {
     hitCount.incrementAndGet();
     if (caching) hitCachingCount.incrementAndGet();
   }
 
   public void evict() {
     evictionCount.incrementAndGet();
   }
 
   public void evicted(final long t) {
     if (t > this.startTime) {
       this.ageAtEviction.update((t - this.startTime)/BlockCacheUtil.NANOS_PER_SECOND);
     }
     this.evictedBlockCount.incrementAndGet();
   }
 
   public long getRequestCount() {
     return getHitCount() + getMissCount();
   }
 
   public long getRequestCachingCount() {
     return getHitCachingCount() + getMissCachingCount();
   }
 
   public long getMissCount() {
     return missCount.get();
   }
 
   public long getMissCachingCount() {
     return missCachingCount.get();
   }
 
   public long getHitCount() {
     return hitCount.get();
   }
 
   public long getHitCachingCount() {
     return hitCachingCount.get();
   }
 
   public long getEvictionCount() {
     return evictionCount.get();
   }
 
   public long getEvictedCount() {
     return this.evictedBlockCount.get();
   }
 
   public double getHitRatio() {
     return ((float)getHitCount()/(float)getRequestCount());
   }
 
   public double getHitCachingRatio() {
     return ((float)getHitCachingCount()/(float)getRequestCachingCount());
   }
 
   public double getMissRatio() {
     return ((float)getMissCount()/(float)getRequestCount());
   }
 
   public double getMissCachingRatio() {
     return ((float)getMissCachingCount()/(float)getRequestCachingCount());
   }
 
   public double evictedPerEviction() {
     return ((float)getEvictedCount()/(float)getEvictionCount());
   }
 
   public void rollMetricsPeriod() {
     hitCounts[windowIndex] = getHitCount() - lastHitCount;
     lastHitCount = getHitCount();
     hitCachingCounts[windowIndex] =
       getHitCachingCount() - lastHitCachingCount;
     lastHitCachingCount = getHitCachingCount();
     requestCounts[windowIndex] = getRequestCount() - lastRequestCount;
     lastRequestCount = getRequestCount();
     requestCachingCounts[windowIndex] =
       getRequestCachingCount() - lastRequestCachingCount;
     lastRequestCachingCount = getRequestCachingCount();
     windowIndex = (windowIndex + 1) % numPeriodsInWindow;
   }
 
   public long getSumHitCountsPastNPeriods() {
     return sum(hitCounts);
   }
 
   public long getSumRequestCountsPastNPeriods() {
     return sum(requestCounts);
   }
 
   public long getSumHitCachingCountsPastNPeriods() {
     return sum(hitCachingCounts);
   }
 
   public long getSumRequestCachingCountsPastNPeriods() {
     return sum(requestCachingCounts);
   }
 
   public double getHitRatioPastNPeriods() {
     double ratio = ((double)getSumHitCountsPastNPeriods() /
         (double)getSumRequestCountsPastNPeriods());
     return Double.isNaN(ratio) ? 0 : ratio;
   }
 
   public double getHitCachingRatioPastNPeriods() {
     double ratio = ((double)getSumHitCachingCountsPastNPeriods() /
         (double)getSumRequestCachingCountsPastNPeriods());
     return Double.isNaN(ratio) ? 0 : ratio;
   }
 
   public AgeSnapshot getAgeAtEvictionSnapshot() {
     return new AgeSnapshot(this.ageAtEviction);
   }
 
   private static long sum(long [] counts) {
     long sum = 0;
     for (long count : counts) sum += count;
     return sum;
   }
 
   private static long [] initializeZeros(int n) {
     long [] zeros = new long [n];
     for (int i=0; i<n; i++) {
       zeros[i] = 0L;
     }
     return zeros;
   }
 }