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	/*
	* Copyright (c) 2021, 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
	* by Oracle in the LICENSE file that accompanied this code.
	*
	* 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
	* or visit www.oracle.com if you need additional information or have any
	* questions.
	*/
	package java.time;

	import java.time.Clock.SourceClock;
	import java.time.Clock.SystemInstantSource;
	import java.util.Objects;

	/**
	* Provides access to the current instant.
	* <p>
	* Instances of this interface are used to access a pluggable representation of the current instant.
	* For example, {@code InstantSource} can be used instead of {@link System#currentTimeMillis()}.
	* <p>
	* The primary purpose of this abstraction is to allow alternate instant sources 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>
	* As such, this interface does not guarantee the result actually represents the current instant
	* on the time-line. Instead, it allows the application to provide a controlled view as to what
	* the current instant is.
	* <p>
	* Best practice for applications is to pass an {@code InstantSource} into any method
	* that requires the current instant. A dependency injection framework is one
	* way to achieve this:
	* <pre>
	* public class MyBean {
	* private InstantSource source; // dependency inject
	* ...
	* public void process(Instant endInstant) {
	* if (source.instant().isAfter(endInstant) {
	* ...
	* }
	* }
	* }
	* </pre>
	* This approach allows an alternative source, such as {@link #fixed(Instant) fixed}
	* or {@link #offset(InstantSource, Duration) offset} to be used during testing.
	* <p>
	* The {@code system} factory method provides a source based on the best available
	* system clock. This may use {@link System#currentTimeMillis()}, or a higher
	* resolution clock if one is available.
	*
	* @implSpec
	* This interface must be implemented with care to ensure other classes operate correctly.
	* All implementations must be thread-safe - a single instance must be capable of be invoked
	* from multiple threads without negative consequences such as race conditions.
	* <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 InstantSource} 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 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 implementation provided here is based on the same underlying system clock
	* as {@link System#currentTimeMillis()}, but may have a precision finer than
	* milliseconds if available.
	* However, little to no guarantee is provided about the accuracy of the
	* underlying system clock. Applications requiring a more accurate system clock must
	* implement this abstract class themselves using a different external system clock,
	* such as an NTP server.
	*
	* @since 17
	*/
	public interface InstantSource {

	/**
	* Obtains a source that returns the current instant using the best available
	* system clock.
	* <p>
	* This source is based on the best available system clock. This may use
	* {@link System#currentTimeMillis()}, or a higher resolution system clock if
	* one is available.
	* <p>
	* The returned implementation is immutable, thread-safe and
	* {@code Serializable}.
	*
	* @return a source that uses the best available system clock, not null
	*/
	static InstantSource system() {
	return SystemInstantSource.INSTANCE;
	}

	//-------------------------------------------------------------------------
	/**
	* Obtains a source that returns instants from the specified source truncated to
	* the nearest occurrence of the specified duration.
	* <p>
	* This source will only tick as per the specified duration. Thus, if the
	* duration is half a second, the source 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 source.
	* <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
	* source will be later than that observed directly via the underlying source.
	* <p>
	* The returned implementation is immutable, thread-safe and
	* {@code Serializable} providing that the base source is.
	*
	* @param baseSource the base source to base the ticking source on, not null
	* @param tickDuration the duration of each visible tick, not negative, not null
	* @return a source 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
	*/
	static InstantSource tick(InstantSource baseSource, Duration tickDuration) {
	Objects.requireNonNull(baseSource, "baseSource");
	return Clock.tick(baseSource.withZone(ZoneOffset.UTC), tickDuration);
	}

	//-----------------------------------------------------------------------
	/**
	* Obtains a source that always returns the same instant.
	* <p>
	* This source simply returns the specified instant.
	* As such, it is not a source that represents the current instant.
	* The main use case for this is in testing, where the fixed source ensures
	* tests are not dependent on the current source.
	* <p>
	* The returned implementation is immutable, thread-safe and {@code Serializable}.
	*
	* @param fixedInstant the instant to use, not null
	* @return a source that always returns the same instant, not null
	*/
	static InstantSource fixed(Instant fixedInstant) {
	return Clock.fixed(fixedInstant, ZoneOffset.UTC);
	}

	//-------------------------------------------------------------------------
	/**
	* Obtains a source that returns instants from the specified source with the
	* specified duration added.
	* <p>
	* This source wraps another source, 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 source.
	* <p>
	* The returned implementation is immutable, thread-safe and {@code Serializable}
	* providing that the base source is.
	*
	* @param baseSource the base source to add the duration to, not null
	* @param offsetDuration the duration to add, not null
	* @return a source based on the base source with the duration added, not null
	*/
	static InstantSource offset(InstantSource baseSource, Duration offsetDuration) {
	Objects.requireNonNull(baseSource, "baseSource");
	return Clock.offset(baseSource.withZone(ZoneOffset.UTC), offsetDuration);
	}

	//-----------------------------------------------------------------------
	/**
	* Gets the current instant of the source.
	* <p>
	* This returns an instant representing the current instant as defined by the source.
	*
	* @return the current instant from this source, not null
	* @throws DateTimeException if the instant cannot be obtained, not thrown by most implementations
	*/
	Instant instant();

	//-------------------------------------------------------------------------
	/**
	* Gets the current millisecond instant of the source.
	* <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 source in high performance use cases
	* where the creation of an object would be unacceptable.
	*
	* @implSpec
	* The default implementation calls {@link #instant()}.
	*
	* @return the current millisecond instant from this source, 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
	*/
	default long millis() {
	return instant().toEpochMilli();
	}

	//-----------------------------------------------------------------------
	/**
	* Returns a clock with the specified time-zone.
	* <p>
	* This returns a {@link Clock}, which is an extension of this interface
	* that combines this source and the specified time-zone.
	* <p>
	* The returned implementation is immutable, thread-safe and {@code Serializable}
	* providing that this source is.
	*
	* @implSpec
	* The default implementation returns an immutable, thread-safe and
	* {@code Serializable} subclass of {@link Clock} that combines this
	* source and the specified zone.
	*
	* @param zone the time-zone to use, not null
	* @return a clock based on this source with the specified time-zone, not null
	*/
	default Clock withZone(ZoneId zone) {
	return new SourceClock(this, zone);
	}

	}

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