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   * Copyright (c) 2012, 2013, Oracle and/or its affiliates. All rights reserved.
   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   *
   * This code is free software; you can redistribute it and/or modify it
   * under the terms of the GNU General Public License version 2 only, as
   * published by the Free Software Foundation.  Oracle designates this
   * particular file as subject to the "Classpath" exception as provided
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   *
   * This code is distributed in the hope that it will be useful, but WITHOUT
   * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
   * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
   * version 2 for more details (a copy is included in the LICENSE file that
   * accompanied this code).
   *
   * You should have received a copy of the GNU General Public License version
   * 2 along with this work; if not, write to the Free Software Foundation,
   * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
   *
   * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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   */
  
  /*
   * This file is available under and governed by the GNU General Public
   * License version 2 only, as published by the Free Software Foundation.
   * However, the following notice accompanied the original version of this
   * file:
   *
   * Copyright (c) 2007-2012, Stephen Colebourne & Michael Nascimento Santos
   *
   * All rights reserved.
   *
   * Redistribution and use in source and binary forms, with or without
   * modification, are permitted provided that the following conditions are met:
   *
   *  * Redistributions of source code must retain the above copyright notice,
   *    this list of conditions and the following disclaimer.
   *
   *  * Redistributions in binary form must reproduce the above copyright notice,
   *    this list of conditions and the following disclaimer in the documentation
   *    and/or other materials provided with the distribution.
   *
   *  * Neither the name of JSR-310 nor the names of its contributors
   *    may be used to endorse or promote products derived from this software
   *    without specific prior written permission.
   *
   * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
   * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
   * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
   * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
   * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
   * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
   * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
   * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
   * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
   * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
   */
  package java.time;
  
  import static java.time.LocalTime.NANOS_PER_MINUTE;
  import static java.time.LocalTime.NANOS_PER_SECOND;
  
  import java.io.Serializable;
  import java.util.Objects;
  import java.util.TimeZone;
  
  /**
   * A clock providing access to the current instant, date and time using a time-zone.
   * <p>
   * Instances of this class are used to find the current instant, which can be
   * interpreted using the stored time-zone to find the current date and time.
   * As such, a clock can be used instead of {@link System#currentTimeMillis()}
   * and {@link TimeZone#getDefault()}.
   * <p>
   * Use of a {@code Clock} is optional. All key date-time classes also have a
   * {@code now()} factory method that uses the system clock in the default time zone.
   * The primary purpose of this abstraction is to allow alternate clocks to be
   * plugged in as and when required. Applications use an object to obtain the
   * current time rather than a static method. This can simplify testing.
   * <p>
   * Best practice for applications is to pass a {@code Clock} into any method
   * that requires the current instant. A dependency injection framework is one
   * way to achieve this:
   * <pre>
   *  public class MyBean {
   *    private Clock clock;  // dependency inject
   *    ...
   *    public void process(LocalDate eventDate) {
   *      if (eventDate.isBefore(LocalDate.now(clock)) {
   *        ...
   *      }
   *    }
   *  }
   * </pre>
   * This approach allows an alternate clock, such as {@link #fixed(Instant, ZoneId) fixed}
   * or {@link #offset(Clock, Duration) offset} to be used during testing.
   * <p>
   * The {@code system} factory methods provide clocks based on the best available
   * system clock This may use {@link System#currentTimeMillis()}, or a higher
   * resolution clock if one is available.
   *
   * @implSpec
   * This abstract class must be implemented with care to ensure other classes operate correctly.
   * All implementations that can be instantiated must be final, immutable and thread-safe.
   * <p>
   * The principal methods are defined to allow the throwing of an exception.
   * In normal use, no exceptions will be thrown, however one possible implementation would be to
   * obtain the time from a central time server across the network. Obviously, in this case the
   * lookup could fail, and so the method is permitted to throw an exception.
   * <p>
   * The returned instants from {@code Clock} work on a time-scale that ignores leap seconds,
   * as described in {@link Instant}. If the implementation wraps a source that provides leap
   * second information, then a mechanism should be used to "smooth" the leap second.
   * The Java Time-Scale mandates the use of UTC-SLS, however clock implementations may choose
   * how accurate they are with the time-scale so long as they document how they work.
   * Implementations are therefore not required to actually perform the UTC-SLS slew or to
   * otherwise be aware of leap seconds.
   * <p>
   * Implementations should implement {@code Serializable} wherever possible and must
   * document whether or not they do support serialization.
   *
   * @implNote
   * The clock implementation provided here is based on {@link System#currentTimeMillis()}.
   * That method provides little to no guarantee about the accuracy of the clock.
   * Applications requiring a more accurate clock must implement this abstract class
   * themselves using a different external clock, such as an NTP server.
   *
   * @since 1.8
   */
  public abstract class Clock {
  
      /**
       * Obtains a clock that returns the current instant using the best available
       * system clock, converting to date and time using the UTC time-zone.
       * <p>
       * This clock, rather than {@link #systemDefaultZone()}, should be used when
       * you need the current instant without the date or time.
       * <p>
       * This clock is based on the best available system clock.
       * This may use {@link System#currentTimeMillis()}, or a higher resolution
       * clock if one is available.
       * <p>
       * Conversion from instant to date or time uses the {@linkplain ZoneOffset#UTC UTC time-zone}.
       * <p>
       * The returned implementation is immutable, thread-safe and {@code Serializable}.
       * It is equivalent to {@code system(ZoneOffset.UTC)}.
       *
       * @return a clock that uses the best available system clock in the UTC zone, not null
       */
      public static Clock systemUTC() {
          return new SystemClock(ZoneOffset.UTC);
      }
  
      /**
       * Obtains a clock that returns the current instant using the best available
       * system clock, converting to date and time using the default time-zone.
       * <p>
       * This clock is based on the best available system clock.
       * This may use {@link System#currentTimeMillis()}, or a higher resolution
       * clock if one is available.
       * <p>
       * Using this method hard codes a dependency to the default time-zone into your application.
       * It is recommended to avoid this and use a specific time-zone whenever possible.
       * The {@link #systemUTC() UTC clock} should be used when you need the current instant
       * without the date or time.
       * <p>
       * The returned implementation is immutable, thread-safe and {@code Serializable}.
       * It is equivalent to {@code system(ZoneId.systemDefault())}.
       *
       * @return a clock that uses the best available system clock in the default zone, not null
       * @see ZoneId#systemDefault()
       */
      public static Clock systemDefaultZone() {
          return new SystemClock(ZoneId.systemDefault());
      }
  
      /**
       * Obtains a clock that returns the current instant using best available
       * system clock.
       * <p>
       * This clock is based on the best available system clock.
       * This may use {@link System#currentTimeMillis()}, or a higher resolution
       * clock if one is available.
       * <p>
       * Conversion from instant to date or time uses the specified time-zone.
       * <p>
       * The returned implementation is immutable, thread-safe and {@code Serializable}.
       *
       * @param zone  the time-zone to use to convert the instant to date-time, not null
       * @return a clock that uses the best available system clock in the specified zone, not null
       */
      public static Clock system(ZoneId zone) {
          Objects.requireNonNull(zone, "zone");
          return new SystemClock(zone);
      }
  
      //-------------------------------------------------------------------------
      /**
       * Obtains a clock that returns the current instant ticking in whole seconds
       * using best available system clock.
       * <p>
       * This clock will always have the nano-of-second field set to zero.
       * This ensures that the visible time ticks in whole seconds.
       * The underlying clock is the best available system clock, equivalent to
       * using {@link #system(ZoneId)}.
       * <p>
       * Implementations may use a caching strategy for performance reasons.
       * As such, it is possible that the start of the second observed via this
       * clock will be later than that observed directly via the underlying clock.
       * <p>
       * The returned implementation is immutable, thread-safe and {@code Serializable}.
       * It is equivalent to {@code tick(system(zone), Duration.ofSeconds(1))}.
       *
       * @param zone  the time-zone to use to convert the instant to date-time, not null
       * @return a clock that ticks in whole seconds using the specified zone, not null
       */
      public static Clock tickSeconds(ZoneId zone) {
          return new TickClock(system(zone), NANOS_PER_SECOND);
      }
  
      /**
       * Obtains a clock that returns the current instant ticking in whole minutes
       * using best available system clock.
       * <p>
       * This clock will always have the nano-of-second and second-of-minute fields set to zero.
       * This ensures that the visible time ticks in whole minutes.
       * The underlying clock is the best available system clock, equivalent to
       * using {@link #system(ZoneId)}.
       * <p>
       * Implementations may use a caching strategy for performance reasons.
       * As such, it is possible that the start of the minute observed via this
       * clock will be later than that observed directly via the underlying clock.
       * <p>
       * The returned implementation is immutable, thread-safe and {@code Serializable}.
       * It is equivalent to {@code tick(system(zone), Duration.ofMinutes(1))}.
       *
       * @param zone  the time-zone to use to convert the instant to date-time, not null
       * @return a clock that ticks in whole minutes using the specified zone, not null
       */
      public static Clock tickMinutes(ZoneId zone) {
          return new TickClock(system(zone), NANOS_PER_MINUTE);
      }
  
      /**
       * Obtains a clock that returns instants from the specified clock truncated
       * to the nearest occurrence of the specified duration.
       * <p>
       * This clock will only tick as per the specified duration. Thus, if the duration
       * is half a second, the clock will return instants truncated to the half second.
       * <p>
       * The tick duration must be positive. If it has a part smaller than a whole
       * millisecond, then the whole duration must divide into one second without
       * leaving a remainder. All normal tick durations will match these criteria,
       * including any multiple of hours, minutes, seconds and milliseconds, and
       * sensible nanosecond durations, such as 20ns, 250,000ns and 500,000ns.
       * <p>
       * A duration of zero or one nanosecond would have no truncation effect.
       * Passing one of these will return the underlying clock.
       * <p>
       * Implementations may use a caching strategy for performance reasons.
       * As such, it is possible that the start of the requested duration observed
       * via this clock will be later than that observed directly via the underlying clock.
       * <p>
       * The returned implementation is immutable, thread-safe and {@code Serializable}
       * providing that the base clock is.
       *
       * @param baseClock  the base clock to base the ticking clock on, not null
       * @param tickDuration  the duration of each visible tick, not negative, not null
       * @return a clock that ticks in whole units of the duration, not null
       * @throws IllegalArgumentException if the duration is negative, or has a
       *  part smaller than a whole millisecond such that the whole duration is not
       *  divisible into one second
       * @throws ArithmeticException if the duration is too large to be represented as nanos
       */
      public static Clock tick(Clock baseClock, Duration tickDuration) {
          Objects.requireNonNull(baseClock, "baseClock");
          Objects.requireNonNull(tickDuration, "tickDuration");
          if (tickDuration.isNegative()) {
              throw new IllegalArgumentException("Tick duration must not be negative");
          }
          long tickNanos = tickDuration.toNanos();
          if (tickNanos % 1000_000 == 0) {
              // ok, no fraction of millisecond
          } else if (1000_000_000 % tickNanos == 0) {
              // ok, divides into one second without remainder
          } else {
              throw new IllegalArgumentException("Invalid tick duration");
          }
          if (tickNanos <= 1) {
              return baseClock;
          }
          return new TickClock(baseClock, tickNanos);
      }
  
      //-----------------------------------------------------------------------
      /**
       * Obtains a clock that always returns the same instant.
       * <p>
       * This clock simply returns the specified instant.
       * As such, it is not a clock in the conventional sense.
       * The main use case for this is in testing, where the fixed clock ensures
       * tests are not dependent on the current clock.
       * <p>
       * The returned implementation is immutable, thread-safe and {@code Serializable}.
       *
       * @param fixedInstant  the instant to use as the clock, not null
       * @param zone  the time-zone to use to convert the instant to date-time, not null
       * @return a clock that always returns the same instant, not null
       */
      public static Clock fixed(Instant fixedInstant, ZoneId zone) {
          Objects.requireNonNull(fixedInstant, "fixedInstant");
          Objects.requireNonNull(zone, "zone");
          return new FixedClock(fixedInstant, zone);
      }
  
      //-------------------------------------------------------------------------
      /**
       * Obtains a clock that returns instants from the specified clock with the
       * specified duration added
       * <p>
       * This clock wraps another clock, returning instants that are later by the
       * specified duration. If the duration is negative, the instants will be
       * earlier than the current date and time.
       * The main use case for this is to simulate running in the future or in the past.
       * <p>
       * A duration of zero would have no offsetting effect.
       * Passing zero will return the underlying clock.
       * <p>
       * The returned implementation is immutable, thread-safe and {@code Serializable}
       * providing that the base clock is.
       *
       * @param baseClock  the base clock to add the duration to, not null
       * @param offsetDuration  the duration to add, not null
       * @return a clock based on the base clock with the duration added, not null
       */
      public static Clock offset(Clock baseClock, Duration offsetDuration) {
          Objects.requireNonNull(baseClock, "baseClock");
          Objects.requireNonNull(offsetDuration, "offsetDuration");
          if (offsetDuration.equals(Duration.ZERO)) {
              return baseClock;
          }
          return new OffsetClock(baseClock, offsetDuration);
      }
  
      //-----------------------------------------------------------------------
      /**
       * Constructor accessible by subclasses.
       */
      protected Clock() {
      }
  
      //-----------------------------------------------------------------------
      /**
       * Gets the time-zone being used to create dates and times.
       * <p>
       * A clock will typically obtain the current instant and then convert that
       * to a date or time using a time-zone. This method returns the time-zone used.
       *
       * @return the time-zone being used to interpret instants, not null
       */
      public abstract ZoneId getZone();
  
      /**
       * Returns a copy of this clock with a different time-zone.
       * <p>
       * A clock will typically obtain the current instant and then convert that
       * to a date or time using a time-zone. This method returns a clock with
       * similar properties but using a different time-zone.
       *
       * @param zone  the time-zone to change to, not null
       * @return a clock based on this clock with the specified time-zone, not null
       */
      public abstract Clock withZone(ZoneId zone);
  
      //-------------------------------------------------------------------------
      /**
       * Gets the current millisecond instant of the clock.
       * <p>
       * This returns the millisecond-based instant, measured from 1970-01-01T00:00Z (UTC).
       * This is equivalent to the definition of {@link System#currentTimeMillis()}.
       * <p>
       * Most applications should avoid this method and use {@link Instant} to represent
       * an instant on the time-line rather than a raw millisecond value.
       * This method is provided to allow the use of the clock in high performance use cases
       * where the creation of an object would be unacceptable.
       * <p>
       * The default implementation currently calls {@link #instant}.
       *
       * @return the current millisecond instant from this clock, measured from
       *  the Java epoch of 1970-01-01T00:00Z (UTC), not null
       * @throws DateTimeException if the instant cannot be obtained, not thrown by most implementations
       */
      public long millis() {
          return instant().toEpochMilli();
      }
  
      //-----------------------------------------------------------------------
      /**
       * Gets the current instant of the clock.
       * <p>
       * This returns an instant representing the current instant as defined by the clock.
       *
       * @return the current instant from this clock, not null
       * @throws DateTimeException if the instant cannot be obtained, not thrown by most implementations
       */
      public abstract Instant instant();
  
      //-----------------------------------------------------------------------
      /**
       * Checks if this clock is equal to another clock.
       * <p>
       * Clocks should override this method to compare equals based on
       * their state and to meet the contract of {@link Object#equals}.
       * If not overridden, the behavior is defined by {@link Object#equals}
       *
       * @param obj  the object to check, null returns false
       * @return true if this is equal to the other clock
       */
      @Override
      public boolean equals(Object obj) {
          return super.equals(obj);
      }
  
      /**
       * A hash code for this clock.
       * <p>
       * Clocks should override this method based on
       * their state and to meet the contract of {@link Object#hashCode}.
       * If not overridden, the behavior is defined by {@link Object#hashCode}
       *
       * @return a suitable hash code
       */
      @Override
      public  int hashCode() {
          return super.hashCode();
      }
  
      //-----------------------------------------------------------------------
      /**
       * Implementation of a clock that always returns the latest time from
       * {@link System#currentTimeMillis()}.
       */
      static final class SystemClock extends Clock implements Serializable {
          private static final long serialVersionUID = 6740630888130243051L;
          private final ZoneId zone;
  
          SystemClock(ZoneId zone) {
              this.zone = zone;
          }
          @Override
          public ZoneId getZone() {
              return zone;
          }
          @Override
          public Clock withZone(ZoneId zone) {
              if (zone.equals(this.zone)) {  // intentional NPE
                  return this;
              }
              return new SystemClock(zone);
          }
          @Override
          public long millis() {
              return System.currentTimeMillis();
          }
          @Override
          public Instant instant() {
              return Instant.ofEpochMilli(millis());
          }
          @Override
          public boolean equals(Object obj) {
              if (obj instanceof SystemClock) {
                  return zone.equals(((SystemClock) obj).zone);
              }
              return false;
          }
          @Override
          public int hashCode() {
              return zone.hashCode() + 1;
          }
          @Override
          public String toString() {
              return "SystemClock[" + zone + "]";
          }
      }
  
      //-----------------------------------------------------------------------
      /**
       * Implementation of a clock that always returns the same instant.
       * This is typically used for testing.
       */
      static final class FixedClock extends Clock implements Serializable {
         private static final long serialVersionUID = 7430389292664866958L;
          private final Instant instant;
          private final ZoneId zone;
  
          FixedClock(Instant fixedInstant, ZoneId zone) {
              this.instant = fixedInstant;
              this.zone = zone;
          }
          @Override
          public ZoneId getZone() {
              return zone;
          }
          @Override
          public Clock withZone(ZoneId zone) {
              if (zone.equals(this.zone)) {  // intentional NPE
                  return this;
              }
              return new FixedClock(instant, zone);
          }
          @Override
          public long millis() {
              return instant.toEpochMilli();
          }
          @Override
          public Instant instant() {
              return instant;
          }
          @Override
          public boolean equals(Object obj) {
              if (obj instanceof FixedClock) {
                  FixedClock other = (FixedClock) obj;
                  return instant.equals(other.instant) && zone.equals(other.zone);
              }
              return false;
          }
          @Override
          public int hashCode() {
              return instant.hashCode() ^ zone.hashCode();
          }
          @Override
          public String toString() {
              return "FixedClock[" + instant + "," + zone + "]";
          }
      }
  
      //-----------------------------------------------------------------------
      /**
       * Implementation of a clock that adds an offset to an underlying clock.
       */
      static final class OffsetClock extends Clock implements Serializable {
         private static final long serialVersionUID = 2007484719125426256L;
          private final Clock baseClock;
          private final Duration offset;
  
          OffsetClock(Clock baseClock, Duration offset) {
              this.baseClock = baseClock;
              this.offset = offset;
          }
          @Override
          public ZoneId getZone() {
              return baseClock.getZone();
          }
          @Override
          public Clock withZone(ZoneId zone) {
              if (zone.equals(baseClock.getZone())) {  // intentional NPE
                  return this;
              }
              return new OffsetClock(baseClock.withZone(zone), offset);
          }
          @Override
          public long millis() {
              return Math.addExact(baseClock.millis(), offset.toMillis());
          }
          @Override
          public Instant instant() {
              return baseClock.instant().plus(offset);
          }
          @Override
          public boolean equals(Object obj) {
              if (obj instanceof OffsetClock) {
                  OffsetClock other = (OffsetClock) obj;
                  return baseClock.equals(other.baseClock) && offset.equals(other.offset);
              }
              return false;
          }
          @Override
          public int hashCode() {
              return baseClock.hashCode() ^ offset.hashCode();
          }
          @Override
          public String toString() {
              return "OffsetClock[" + baseClock + "," + offset + "]";
          }
      }
  
      //-----------------------------------------------------------------------
      /**
       * Implementation of a clock that adds an offset to an underlying clock.
       */
      static final class TickClock extends Clock implements Serializable {
          private static final long serialVersionUID = 6504659149906368850L;
          private final Clock baseClock;
          private final long tickNanos;
  
          TickClock(Clock baseClock, long tickNanos) {
              this.baseClock = baseClock;
              this.tickNanos = tickNanos;
          }
          @Override
          public ZoneId getZone() {
              return baseClock.getZone();
          }
          @Override
          public Clock withZone(ZoneId zone) {
              if (zone.equals(baseClock.getZone())) {  // intentional NPE
                  return this;
              }
              return new TickClock(baseClock.withZone(zone), tickNanos);
          }
          @Override
          public long millis() {
              long millis = baseClock.millis();
              return millis - Math.floorMod(millis, tickNanos / 1000_000L);
          }
          @Override
          public Instant instant() {
              if ((tickNanos % 1000_000) == 0) {
                  long millis = baseClock.millis();
                  return Instant.ofEpochMilli(millis - Math.floorMod(millis, tickNanos / 1000_000L));
              }
              Instant instant = baseClock.instant();
              long nanos = instant.getNano();
              long adjust = Math.floorMod(nanos, tickNanos);
              return instant.minusNanos(adjust);
          }
          @Override
          public boolean equals(Object obj) {
              if (obj instanceof TickClock) {
                  TickClock other = (TickClock) obj;
                  return baseClock.equals(other.baseClock) && tickNanos == other.tickNanos;
              }
              return false;
          }
          @Override
          public int hashCode() {
              return baseClock.hashCode() ^ ((int) (tickNanos ^ (tickNanos >>> 32)));
          }
          @Override
          public String toString() {
              return "TickClock[" + baseClock + "," + Duration.ofNanos(tickNanos) + "]";
          }
      }
  
  }
  