/**
    * 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.quotas;
  
  import java.util.concurrent.TimeUnit;
  
  import org.apache.hadoop.hbase.classification.InterfaceAudience;
  import org.apache.hadoop.hbase.classification.InterfaceStability;
  
  import com.google.common.annotations.VisibleForTesting;
  
  /**
   * Simple rate limiter.
   *
   * Usage Example:
   *    // At this point you have a unlimited resource limiter
   *   RateLimiter limiter = new AverageIntervalRateLimiter();
   *                         or new FixedIntervalRateLimiter();
   *   limiter.set(10, TimeUnit.SECONDS);       // set 10 resources/sec
   *
   *   while (true) {
   *     // call canExecute before performing resource consuming operation
   *     bool canExecute = limiter.canExecute();
   *     // If there are no available resources, wait until one is available
   *     if (!canExecute) Thread.sleep(limiter.waitInterval());
   *     // ...execute the work and consume the resource...
   *     limiter.consume();
   *   }
   */
  @InterfaceAudience.Private
  @InterfaceStability.Evolving
  public abstract class RateLimiter {
    public static final String QUOTA_RATE_LIMITER_CONF_KEY = "hbase.quota.rate.limiter";
    private long tunit = 1000;           // Timeunit factor for translating to ms.
    private long limit = Long.MAX_VALUE; // The max value available resource units can be refilled to.
    private long avail = Long.MAX_VALUE; // Currently available resource units
  
    /**
     * Refill the available units w.r.t the elapsed time.
     * @param limit Maximum available resource units that can be refilled to.
     * @return how many resource units may be refilled ?
     */
    abstract long refill(long limit);
  
    /**
     * Time in milliseconds to wait for before requesting to consume 'amount' resource.
     * @param limit Maximum available resource units that can be refilled to.
     * @param available Currently available resource units
     * @param amount Resources for which time interval to calculate for
     * @return estimate of the ms required to wait before being able to provide 'amount' resources.
     */
    abstract long getWaitInterval(long limit, long available, long amount);
  
  
    /**
     * Set the RateLimiter max available resources and refill period.
     * @param limit The max value available resource units can be refilled to.
     * @param timeUnit Timeunit factor for translating to ms.
     */
    public void set(final long limit, final TimeUnit timeUnit) {
      switch (timeUnit) {
      case MILLISECONDS:
        tunit = 1;
        break;
      case SECONDS:
        tunit = 1000;
        break;
      case MINUTES:
        tunit = 60 * 1000;
        break;
      case HOURS:
        tunit = 60 * 60 * 1000;
        break;
      case DAYS:
        tunit = 24 * 60 * 60 * 1000;
        break;
      default:
        throw new RuntimeException("Unsupported " + timeUnit.name() + " TimeUnit.");
      }
      this.limit = limit;
      this.avail = limit;
    }
  
   public String toString() {
     String rateLimiter = this.getClass().getSimpleName();
     if (limit == Long.MAX_VALUE) {
       return rateLimiter + "(Bypass)";
     }
     return rateLimiter + "(avail=" + avail + " limit=" + limit + " tunit=" + tunit + ")";
   }
 
   /**
    * Sets the current instance of RateLimiter to a new values.
    *
    * if current limit is smaller than the new limit, bump up the available resources.
    * Otherwise allow clients to use up the previously available resources.
    */
   public synchronized void update(final RateLimiter other) {
     this.tunit = other.tunit;
     if (this.limit < other.limit) {
       this.avail += (other.limit - this.limit);
     }
     this.limit = other.limit;
   }
 
   public synchronized boolean isBypass() {
     return limit == Long.MAX_VALUE;
   }
 
   public synchronized long getLimit() {
     return limit;
   }
 
   public synchronized long getAvailable() {
     return avail;
   }
 
   protected long getTimeUnitInMillis() {
     return tunit;
   }
 
   /**
    * Is there at least one resource available to allow execution?
    * @return true if there is at least one resource available, otherwise false
    */
   public boolean canExecute() {
     return canExecute(1);
   }
 
   /**
    * Are there enough available resources to allow execution?
    * @param amount the number of required resources
    * @return true if there are enough available resources, otherwise false
    */
   public synchronized boolean canExecute(final long amount) {
     long refillAmount = refill(limit);
     if (refillAmount == 0 && avail < amount) {
       return false;
     }
     // check for positive overflow
     if (avail <= Long.MAX_VALUE - refillAmount) {
       avail = Math.max(0, Math.min(avail + refillAmount, limit));
     } else {
       avail = Math.max(0, limit);
     }
     if (avail >= amount) {
       return true;
     }
     return false;
   }
 
   /**
    * consume one available unit.
    */
   public void consume() {
     consume(1);
   }
 
   /**
    * consume amount available units.
    * @param amount the number of units to consume
    */
   public synchronized void consume(final long amount) {
     this.avail -= amount;
     if (this.avail < 0) {
       this.avail = 0;
     }
   }
 
   /**
    * @return estimate of the ms required to wait before being able to provide 1 resource.
    */
   public long waitInterval() {
     return waitInterval(1);
   }
 
   /**
    * @return estimate of the ms required to wait before being able to provide "amount" resources.
    */
   public synchronized long waitInterval(final long amount) {
     // TODO Handle over quota?
     return (amount <= avail) ? 0 : getWaitInterval(limit, avail, amount);
   }
 
   // These two method are for strictly testing purpose only
   @VisibleForTesting
   public abstract void setNextRefillTime(long nextRefillTime);
 
   @VisibleForTesting
   public abstract long getNextRefillTime();
 }