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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 com.gemstone.gemfire.internal;

/**
 * A timer class that reports current or elapsed time in nanonseconds.
 * The static method {@link #getTime} reports the current time.
 * The instance methods support basic
 * stop-watch-style functions that are convenient for simple performance
 * measurements. For example:
 *   class Example {
     void example() {
       NanoTimer timer = new NanoTimer();
       for (int i = 0; i < n; ++i) {
	  someComputationThatYouAreMeasuring();
	  long duration = timer.reset();
	  System.out.println("Duration: " + duration);
	  // To avoid contaminating timing with printing times,
	  // you could call reset again here.
       }
       long average = timer.getTimeSinceConstruction() / n;
       System.out.println("Average: " + average);
     }
   }
 * 
 * 
 * @author Darrel Schneider
 * @author Kirk Lund
 */
public final class NanoTimer {

  public static final long NANOS_PER_MILLISECOND = 1000000;
  
  /**
   * The timestamp taken when this timer was constructed.
   */
  private final long constructionTime;

  /**
   * The timestamp taken when this timer was last reset or constructed.
   */
  private long lastResetTime;
  
  private final TimeService timeService;
  
  private final static TimeService systemTimeService = new TimeService() {
    @Override
    public long getTime() {
      return java.lang.System.nanoTime();
    }
  };
  
  /**
   * Create a NanoTimer.
   */
  public NanoTimer() {
    this.timeService = systemTimeService;
    this.lastResetTime = systemTimeService.getTime();
    this.constructionTime = this.lastResetTime;
  }
  
  /**
   * For unit testing
   */
  NanoTimer(TimeService ts) {
    this.timeService = ts;
    this.lastResetTime = ts.getTime();
    this.constructionTime = this.lastResetTime;
  }

  /**
   * Converts nanoseconds to milliseconds by dividing nanos by 
   * {@link #NANOS_PER_MILLISECOND}.
   * 
   * @param nanos value in nanoseconds
   * @return value converted to milliseconds
   */
  public static long nanosToMillis(long nanos) {
    return nanos / NANOS_PER_MILLISECOND;
  }
  
  /**
   * Converts milliseconds to nanoseconds by multiplying millis by 
   * {@link #NANOS_PER_MILLISECOND}.
   * 
   * @param millis value in milliseconds
   * @return value converted to nanoseconds
   */
  public static long millisToNanos(long millis) {
    return millis * NANOS_PER_MILLISECOND;
  }

  /**
   * Return the time in nanoseconds since some arbitrary time in the past.
   * The time rolls over to zero every 2^64 nanosecs (approx 584 years).
   * Interval computations spanning periods longer than this will be wrong. 
   */
  public static long getTime() {
    return java.lang.System.nanoTime();
  }

  /**
   * Return the construction time in nanoseconds since some arbitrary time
   * in the past.
   * 
   * @return timestamp in nanoseconds since construction.
   */
  public long getConstructionTime() {
    return this.constructionTime;
  }
  
  /**
   * Return the last reset time in naonseconds since some arbitrary time
   * in the past.
   * 
   * The time rolls over to zero every 2^64 nanosecs (approx 584 years).
   * Interval computations spanning periods longer than this will be wrong.
   * If the timer has not yet been reset then the construction time
   * is returned.
   * 
   * @return timestamp in nanoseconds of construction or the last reset.
   */
  public long getLastResetTime() {
    return this.lastResetTime;
  }

  /**
   * Compute and return the time in nanoseconds since the last reset or
   * construction of this timer, and reset the timer to the current
   * {@link #getTime}.
   * 
   * @return time in nanoseconds since construction or last reset.
   */
  public long reset() {
    long save = this.lastResetTime;
    this.lastResetTime = this.timeService.getTime();
    return this.lastResetTime - save;
  }

  /**
   * Compute and return the time in nanoseconds since the last reset or
   * construction of this Timer, but does not reset this timer. 
   * 
   * @return time in nanoseconds since construction or last reset.
   */
  public long getTimeSinceReset() {
    return this.timeService.getTime() - this.lastResetTime;
  }

  /**
   * Compute and return the time in nanoseconds since this timer was
   * constructed. 
   * 
   * @return time in nanoseconds since construction.
   */
  public long getTimeSinceConstruction() {
    return this.timeService.getTime() - this.constructionTime;
  }
  
  /**
   * Allows unit tests to insert a deterministic clock for testing.
   */
  interface TimeService {
    /**
     * Returns the current time.
     */
    public long getTime();
  }
}