// Copyright (C) 2017-2018 Baidu, Inc. 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 Baidu, Inc., 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.

//! Temporal quantification.

use core::fmt;
use core::ops::{Add, Sub, AddAssign, SubAssign};
use error::Error;
use sys::time;
use sys_common::FromInner;

pub use self::duration::Duration;

mod duration;

/// A measurement of a monotonically nondecreasing clock.
/// Opaque and useful only with `Duration`.
///
/// Instants are always guaranteed to be no less than any previously measured
/// instant when created, and are often useful for tasks such as measuring
/// benchmarks or timing how long an operation takes.
///
/// Note, however, that instants are not guaranteed to be **steady**.  In other
/// words, each tick of the underlying clock may not be the same length (e.g.
/// some seconds may be longer than others). An instant may jump forwards or
/// experience time dilation (slow down or speed up), but it will never go
/// backwards.
///
/// Instants are opaque types that can only be compared to one another. There is
/// no method to get "the number of seconds" from an instant. Instead, it only
/// allows measuring the duration between two instants (or comparing two
/// instants).
///
/// The size of an `Instant` struct may vary depending on the target operating
/// system.
///
#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub struct Instant(time::Instant);

/// A measurement of the system clock, useful for talking to
/// external entities like the file system or other processes.
///
/// Distinct from the [`Instant`] type, this time measurement **is not
/// monotonic**. This means that you can save a file to the file system, then
/// save another file to the file system, **and the second file has a
/// `SystemTime` measurement earlier than the first**. In other words, an
/// operation that happens after another operation in real time may have an
/// earlier `SystemTime`!
///
/// Consequently, comparing two `SystemTime` instances to learn about the
/// duration between them returns a [`Result`] instead of an infallible [`Duration`]
/// to indicate that this sort of time drift may happen and needs to be handled.
///
/// Although a `SystemTime` cannot be directly inspected, the [`UNIX_EPOCH`]
/// constant is provided in this module as an anchor in time to learn
/// information about a `SystemTime`. By calculating the duration from this
/// fixed point in time, a `SystemTime` can be converted to a human-readable time,
/// or perhaps some other string representation.
///
/// The size of a `SystemTime` struct may vary depending on the target operating
/// system.
///
#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub struct SystemTime(time::SystemTime);

/// An error returned from the `duration_since` and `elapsed` methods on
/// `SystemTime`, used to learn how far in the opposite direction a system time
/// lies.
///
#[derive(Clone, Debug)]
pub struct SystemTimeError(Duration);

impl Instant {
    /// Returns an instant corresponding to "now".
    ///
    #[cfg(feature = "untrusted_time")]
    pub fn now() -> Instant {
        Instant(time::Instant::now())
    }

    /// Returns the amount of time elapsed from another instant to this one.
    ///
    /// # Panics
    ///
    /// This function will panic if `earlier` is later than `self`.
    ///
    pub fn duration_since(&self, earlier: Instant) -> Duration {
        self.0.sub_instant(&earlier.0)
    }

    /// Returns the amount of time elapsed since this instant was created.
    ///
    /// # Panics
    ///
    /// This function may panic if the current time is earlier than this
    /// instant, which is something that can happen if an `Instant` is
    /// produced synthetically.
    ///
    #[cfg(feature = "untrusted_time")]
    pub fn elapsed(&self) -> Duration {
        Instant::now() - *self
    }
}

impl Add<Duration> for Instant {
    type Output = Instant;

    fn add(self, other: Duration) -> Instant {
        Instant(self.0.add_duration(&other))
    }
}

impl AddAssign<Duration> for Instant {
    fn add_assign(&mut self, other: Duration) {
        *self = *self + other;
    }
}

impl Sub<Duration> for Instant {
    type Output = Instant;

    fn sub(self, other: Duration) -> Instant {
        Instant(self.0.sub_duration(&other))
    }
}

impl SubAssign<Duration> for Instant {
    fn sub_assign(&mut self, other: Duration) {
        *self = *self - other;
    }
}

impl Sub<Instant> for Instant {
    type Output = Duration;

    fn sub(self, other: Instant) -> Duration {
        self.duration_since(other)
    }
}

impl fmt::Debug for Instant {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        self.0.fmt(f)
    }
}

impl SystemTime {
    /// An anchor in time which can be used to create new `SystemTime` instances or
    /// learn about where in time a `SystemTime` lies.
    ///
    /// This constant is defined to be "1970-01-01 00:00:00 UTC" on all systems with
    /// respect to the system clock. Using `duration_since` on an existing
    /// `SystemTime` instance can tell how far away from this point in time a
    /// measurement lies, and using `UNIX_EPOCH + duration` can be used to create a
    /// `SystemTime` instance to represent another fixed point in time.
    ///
    pub const UNIX_EPOCH: SystemTime = UNIX_EPOCH;
    /// Returns the system time corresponding to "now".
    ///
    #[cfg(feature = "untrusted_time")]
    pub fn now() -> SystemTime {
        SystemTime(time::SystemTime::now())
    }

    /// Returns the amount of time elapsed from an earlier point in time.
    ///
    /// This function may fail because measurements taken earlier are not
    /// guaranteed to always be before later measurements (due to anomalies such
    /// as the system clock being adjusted either forwards or backwards).
    ///
    /// If successful, [`Ok`]`(`[`Duration`]`)` is returned where the duration represents
    /// the amount of time elapsed from the specified measurement to this one.
    ///
    /// Returns an [`Err`] if `earlier` is later than `self`, and the error
    /// contains how far from `self` the time is.
    ///
    pub fn duration_since(&self, earlier: SystemTime)
                          -> Result<Duration, SystemTimeError> {
        self.0.sub_time(&earlier.0).map_err(SystemTimeError)
    }

    /// Returns the amount of time elapsed since this system time was created.
    ///
    /// This function may fail as the underlying system clock is susceptible to
    /// drift and updates (e.g. the system clock could go backwards), so this
    /// function may not always succeed. If successful, [`Ok`]`(`[`Duration`]`)` is
    /// returned where the duration represents the amount of time elapsed from
    /// this time measurement to the current time.
    ///
    /// Returns an [`Err`] if `self` is later than the current system time, and
    /// the error contains how far from the current system time `self` is.
    ///
    #[cfg(feature = "untrusted_time")]
    pub fn elapsed(&self) -> Result<Duration, SystemTimeError> {
        SystemTime::now().duration_since(*self)
    }
}

impl Add<Duration> for SystemTime {
    type Output = SystemTime;

    fn add(self, dur: Duration) -> SystemTime {
        SystemTime(self.0.add_duration(&dur))
    }
}

impl AddAssign<Duration> for SystemTime {
    fn add_assign(&mut self, other: Duration) {
        *self = *self + other;
    }
}

impl Sub<Duration> for SystemTime {
    type Output = SystemTime;

    fn sub(self, dur: Duration) -> SystemTime {
        SystemTime(self.0.sub_duration(&dur))
    }
}

impl SubAssign<Duration> for SystemTime {
    fn sub_assign(&mut self, other: Duration) {
        *self = *self - other;
    }
}

impl fmt::Debug for SystemTime {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        self.0.fmt(f)
    }
}

/// An anchor in time which can be used to create new `SystemTime` instances or
/// learn about where in time a `SystemTime` lies.
///
/// This constant is defined to be "1970-01-01 00:00:00 UTC" on all systems with
/// respect to the system clock. Using `duration_since` on an existing
/// [`SystemTime`] instance can tell how far away from this point in time a
/// measurement lies, and using `UNIX_EPOCH + duration` can be used to create a
/// [`SystemTime`] instance to represent another fixed point in time.
///
pub const UNIX_EPOCH: SystemTime = SystemTime(time::UNIX_EPOCH);

impl SystemTimeError {
    /// Returns the positive duration which represents how far forward the
    /// second system time was from the first.
    ///
    /// A `SystemTimeError` is returned from the [`duration_since`] and [`elapsed`]
    /// methods of [`SystemTime`] whenever the second system time represents a point later
    /// in time than the `self` of the method call.
    ///
    pub fn duration(&self) -> Duration {
        self.0
    }
}

impl Error for SystemTimeError {
    fn description(&self) -> &str { "other time was not earlier than self" }
}

impl fmt::Display for SystemTimeError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "second time provided was later than self")
    }
}

impl FromInner<time::SystemTime> for SystemTime {
    fn from_inner(time: time::SystemTime) -> SystemTime {
        SystemTime(time)
    }
}