/*
 * 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.GwtCompatible;
import com.google.common.annotations.GwtIncompatible;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import com.google.j2objc.annotations.J2ObjCIncompatible;
import java.time.Duration;
import java.util.concurrent.TimeUnit;

An object that accurately measures elapsed time: the measured duration between two
successive readings of "now" in the same process.
In contrast, wall time is a reading of "now" as given by a method like System.currentTimeMillis(), best represented as an Instant. Such values can be subtracted to obtain a Duration (such as by Duration.between), but doing so does not give a reliable measurement of elapsed time, because wall time readings are inherently approximate, routinely affected by periodic clock corrections. Because this class (by default) uses System.nanoTime, it is unaffected by these changes. 
Use this class instead of direct calls to System.nanoTime for two reasons: 

  
The raw long values returned by nanoTime are meaningless and unsafe to use in any other way than how Stopwatch uses them. 
An alternative source of nanosecond ticks can be substituted, for example for testing or
      performance reasons, without affecting most of your code.

Basic usage:

Stopwatch stopwatch = Stopwatch.createStarted();
doSomething();
stopwatch.stop(); // optional
Duration duration = stopwatch.elapsed();
log.info("time: " + stopwatch); // formatted string like "12.3 ms"

The state-changing methods are not idempotent; it is an error to start or stop a stopwatch
that is already in the desired state.
When testing code that uses this class, use createUnstarted(Ticker) or createStarted(Ticker) to supply a fake or mock ticker. This allows you to simulate any valid behavior of the stopwatch. 
Note: This class is not thread-safe.
Warning for Android users: a stopwatch with default behavior may not continue to keep
time while the device is asleep. Instead, create one like this:

Stopwatch.createStarted(
     new Ticker() {
       public long read() {
         return android.os.SystemClock.elapsedRealtimeNanos();
       }
     });

Author:
Kevin Bourrillion
Since:
10.0/**
 * An object that accurately measures <i>elapsed time</i>: the measured duration between two
 * successive readings of "now" in the same process.
 *
 * <p>In contrast, <i>wall time</i> is a reading of "now" as given by a method like
 * {@link System#currentTimeMillis()}, best represented as an {@link Instant}. Such values
 * <i>can</i> be subtracted to obtain a {@code Duration} (such as by {@code Duration.between}), but
 * doing so does <i>not</i> give a reliable measurement of elapsed time, because wall time readings
 * are inherently approximate, routinely affected by periodic clock corrections. Because this class
 * (by default) uses {@link System#nanoTime}, it is unaffected by these changes.
 *
 * <p>Use this class instead of direct calls to {@link System#nanoTime} for two reasons:
 *
 * <ul>
 *   <li>The raw {@code long} values returned by {@code nanoTime} are meaningless and unsafe to use
 *       in any other way than how {@code Stopwatch} uses them.
 *   <li>An alternative source of nanosecond ticks can be substituted, for example for testing or
 *       performance reasons, without affecting most of your code.
 * </ul>
 *
 * <p>Basic usage:
 *
 * <pre>{@code
 * Stopwatch stopwatch = Stopwatch.createStarted();
 * doSomething();
 * stopwatch.stop(); // optional
 *
 * Duration duration = stopwatch.elapsed();
 *
 * log.info("time: " + stopwatch); // formatted string like "12.3 ms"
 * }</pre>
 *
 * <p>The state-changing 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. This allows you to simulate any valid
 * behavior of the stopwatch.
 *
 * <p><b>Note:</b> This class is not thread-safe.
 *
 * <p><b>Warning for Android users:</b> a stopwatch with default behavior may not continue to keep
 * time while the device is asleep. Instead, create one like this:
 *
 * <pre>{@code
 * Stopwatch.createStarted(
 *      new Ticker() {
 *        public long read() {
 *          return android.os.SystemClock.elapsedRealtimeNanos();
 *        }
 *      });
 * }</pre>
 *
 * @author Kevin Bourrillion
 * @since 10.0
 */
@GwtCompatible(emulated = true)
@SuppressWarnings("GoodTime") // lots of violations
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 System.nanoTime as its time source. 
Since:
15.0/**
   * 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/**
   * 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 System.nanoTime as its time source. 
Since:
15.0/**
   * 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/**
   * 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();
  }

  Stopwatch() {
    this.ticker = Ticker.systemTicker();
  }

  Stopwatch(Ticker ticker) {
    this.ticker = checkNotNull(ticker, "ticker");
  }

  
Returns true if start() has been called on this stopwatch, and stop() has not been called since the last call to start(). /**
   * 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.
Throws:
IllegalStateException – if the stopwatch is already running.
Returns:
this Stopwatch instance/**
   * Starts the stopwatch.
   *
   * @return this {@code Stopwatch} instance
   * @throws IllegalStateException if the stopwatch is already running.
   */
  @CanIgnoreReturnValue
  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.
Throws:
IllegalStateException – if the stopwatch is already stopped.
Returns:
this Stopwatch instance/**
   * 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.
   */
  @CanIgnoreReturnValue
  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.
Returns:
this Stopwatch instance/**
   * Sets the elapsed time for this stopwatch to zero, and places it in a stopped state.
   *
   * @return this {@code Stopwatch} instance
   */
  @CanIgnoreReturnValue
  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.
Note: the overhead of measurement can be more than a microsecond, so it is generally not useful to specify TimeUnit.NANOSECONDS precision here. 
It is generally not a good idea to use an ambiguous, unitless long to represent elapsed time. Therefore, we recommend using elapsed() instead, which returns a strongly-typed Duration instance. 
Since:
14.0 (since 10.0 as elapsedTime())/**
   * Returns the current elapsed time shown on this stopwatch, expressed in the desired time unit,
   * with any fraction rounded down.
   *
   * <p><b>Note:</b> the overhead of measurement can be more than a microsecond, so it is generally
   * not useful to specify {@link TimeUnit#NANOSECONDS} precision here.
   *
   * <p>It is generally not a good idea to use an ambiguous, unitless {@code long} to represent
   * elapsed time. Therefore, we recommend using {@link #elapsed()} instead, which returns a
   * strongly-typed {@code Duration} instance.
   *
   * @since 14.0 (since 10.0 as {@code elapsedTime()})
   */
  public long elapsed(TimeUnit desiredUnit) {
    return desiredUnit.convert(elapsedNanos(), NANOSECONDS);
  }

  
Returns the current elapsed time shown on this stopwatch as a Duration. Unlike elapsed(TimeUnit), this method does not lose any precision due to rounding. 
Since:
22.0/**
   * Returns the current elapsed time shown on this stopwatch as a {@link Duration}. Unlike {@link
   * #elapsed(TimeUnit)}, this method does not lose any precision due to rounding.
   *
   * @since 22.0
   */
  @GwtIncompatible
  @J2ObjCIncompatible
  public Duration elapsed() {
    return Duration.ofNanos(elapsedNanos());
  }

  
Returns a string representation of the current elapsed time. /** Returns a string representation of the current elapsed time. */
  @Override
  public String toString() {
    long nanos = elapsedNanos();

    TimeUnit unit = chooseUnit(nanos);
    double value = (double) nanos / NANOSECONDS.convert(1, unit);

    // Too bad this functionality is not exposed as a regular method call
    return Platform.formatCompact4Digits(value) + " " + abbreviate(unit);
  }

  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();
    }
  }
}