/*
    * Copyright (C) 2008 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.base;
  
  import static com.google.common.base.Preconditions.checkNotNull;
  import static com.google.common.base.Preconditions.checkState;
  import static java.util.concurrent.TimeUnit.DAYS;
  import static java.util.concurrent.TimeUnit.HOURS;
  import static java.util.concurrent.TimeUnit.MICROSECONDS;
  import static java.util.concurrent.TimeUnit.MILLISECONDS;
  import static java.util.concurrent.TimeUnit.MINUTES;
  import static java.util.concurrent.TimeUnit.NANOSECONDS;
  import static java.util.concurrent.TimeUnit.SECONDS;
  
  import com.google.common.annotations.Beta;
  import com.google.common.annotations.GwtCompatible;
  
  import java.util.concurrent.TimeUnit;
  
  /**
   * An object that measures elapsed time in nanoseconds. It is useful to measure
   * elapsed time using this class instead of direct calls to {@link
   * System#nanoTime} for a few reasons:
   *
   * <ul>
   * <li>An alternate time source can be substituted, for testing or performance
   *     reasons.
   * <li>As documented by {@code nanoTime}, the value returned has no absolute
   *     meaning, and can only be interpreted as relative to another timestamp
   *     returned by {@code nanoTime} at a different time. {@code Stopwatch} is a
   *     more effective abstraction because it exposes only these relative values,
   *     not the absolute ones.
   * </ul>
   *
   * <p>Basic usage:
   * <pre>
   *   Stopwatch stopwatch = Stopwatch.{@link #createStarted createStarted}();
   *   doSomething();
   *   stopwatch.{@link #stop stop}(); // optional
   *
   *   long millis = stopwatch.elapsed(MILLISECONDS);
   *
   *   log.info("time: " + stopwatch); // formatted string like "12.3 ms"</pre>
   *
   * <p>Stopwatch methods are not idempotent; it is an error to start or stop a
   * stopwatch that is already in the desired state.
   *
   * <p>When testing code that uses this class, use
   * {@link #createUnstarted(Ticker)} or {@link #createStarted(Ticker)} to
   * supply a fake or mock ticker.
   * <!-- TODO(kevinb): restore the "such as" --> This allows you to
   * simulate any valid behavior of the stopwatch.
   *
   * <p><b>Note:</b> This class is not thread-safe.
   *
   * @author Kevin Bourrillion
   * @since 10.0
   */
  @Beta
  @GwtCompatible(emulated = true)
  public final class Stopwatch {
    private final Ticker ticker;
    private boolean isRunning;
    private long elapsedNanos;
    private long startTick;
  
    /**
     * Creates (but does not start) a new stopwatch using {@link System#nanoTime}
     * as its time source.
     *
     * @since 15.0
     */
    public static Stopwatch createUnstarted() {
      return new Stopwatch();
    }
  
    /**
     * Creates (but does not start) a new stopwatch, using the specified time
     * source.
     *
     * @since 15.0
     */
    public static Stopwatch createUnstarted(Ticker ticker) {
      return new Stopwatch(ticker);
    }
 
   /**
    * Creates (and starts) a new stopwatch using {@link System#nanoTime}
    * as its time source.
    *
    * @since 15.0
    */
   public static Stopwatch createStarted() {
     return new Stopwatch().start();
   }
 
   /**
    * Creates (and starts) a new stopwatch, using the specified time
    * source.
    *
    * @since 15.0
    */
   public static Stopwatch createStarted(Ticker ticker) {
     return new Stopwatch(ticker).start();
   }
 
   /**
    * Creates (but does not start) a new stopwatch using {@link System#nanoTime}
    * as its time source.
    *
    * @deprecated Use {@link Stopwatch#createUnstarted()} instead.
    */
   @Deprecated
   Stopwatch() {
     this(Ticker.systemTicker());
   }
 
   /**
    * Creates (but does not start) a new stopwatch, using the specified time
    * source.
    *
    * @deprecated Use {@link Stopwatch#createUnstarted(Ticker)} instead.
    */
   @Deprecated
   Stopwatch(Ticker ticker) {
     this.ticker = checkNotNull(ticker, "ticker");
   }
 
   /**
    * Returns {@code true} if {@link #start()} has been called on this stopwatch,
    * and {@link #stop()} has not been called since the last call to {@code
    * start()}.
    */
   public boolean isRunning() {
     return isRunning;
   }
 
   /**
    * Starts the stopwatch.
    *
    * @return this {@code Stopwatch} instance
    * @throws IllegalStateException if the stopwatch is already running.
    */
   public Stopwatch start() {
     checkState(!isRunning, "This stopwatch is already running.");
     isRunning = true;
     startTick = ticker.read();
     return this;
   }
 
   /**
    * Stops the stopwatch. Future reads will return the fixed duration that had
    * elapsed up to this point.
    *
    * @return this {@code Stopwatch} instance
    * @throws IllegalStateException if the stopwatch is already stopped.
    */
   public Stopwatch stop() {
     long tick = ticker.read();
     checkState(isRunning, "This stopwatch is already stopped.");
     isRunning = false;
     elapsedNanos += tick - startTick;
     return this;
   }
 
   /**
    * Sets the elapsed time for this stopwatch to zero,
    * and places it in a stopped state.
    *
    * @return this {@code Stopwatch} instance
    */
   public Stopwatch reset() {
     elapsedNanos = 0;
     isRunning = false;
     return this;
   }
 
   private long elapsedNanos() {
     return isRunning ? ticker.read() - startTick + elapsedNanos : elapsedNanos;
   }
 
   /**
    * Returns the current elapsed time shown on this stopwatch, expressed
    * in the desired time unit, with any fraction rounded down.
    *
    * <p>Note that the overhead of measurement can be more than a microsecond, so
    * it is generally not useful to specify {@link TimeUnit#NANOSECONDS}
    * precision here.
    *
    * @since 14.0 (since 10.0 as {@code elapsedTime()})
    */
   public long elapsed(TimeUnit desiredUnit) {
     return desiredUnit.convert(elapsedNanos(), NANOSECONDS);
   }
 
   private static TimeUnit chooseUnit(long nanos) {
     if (DAYS.convert(nanos, NANOSECONDS) > 0) {
       return DAYS;
     }
     if (HOURS.convert(nanos, NANOSECONDS) > 0) {
       return HOURS;
     }
     if (MINUTES.convert(nanos, NANOSECONDS) > 0) {
       return MINUTES;
     }
     if (SECONDS.convert(nanos, NANOSECONDS) > 0) {
       return SECONDS;
     }
     if (MILLISECONDS.convert(nanos, NANOSECONDS) > 0) {
       return MILLISECONDS;
     }
     if (MICROSECONDS.convert(nanos, NANOSECONDS) > 0) {
       return MICROSECONDS;
     }
     return NANOSECONDS;
   }
 
   private static String abbreviate(TimeUnit unit) {
     switch (unit) {
       case NANOSECONDS:
         return "ns";
       case MICROSECONDS:
         return "\u03bcs"; // μs
       case MILLISECONDS:
         return "ms";
       case SECONDS:
         return "s";
       case MINUTES:
         return "min";
       case HOURS:
         return "h";
       case DAYS:
         return "d";
       default:
         throw new AssertionError();
     }
   }
 }