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//! Temporal quantification.

use arceos_api::time::AxTimeValue;
use core::ops::{Add, AddAssign, Sub, SubAssign};

pub use core::time::Duration;

/// A measurement of a monotonically nondecreasing clock.
/// Opaque and useful only with [`Duration`].
#[derive(Clone, Copy)]
pub struct Instant(AxTimeValue);

impl Instant {
    /// Returns an instant corresponding to "now".
    pub fn now() -> Instant {
        Instant(arceos_api::time::ax_wall_time())
    }

    /// Returns the amount of time elapsed from another instant to this one,
    /// or zero duration if that instant is later than this one.
    ///
    /// # Panics
    ///
    /// Previous rust versions panicked when `earlier` was later than `self`. Currently this
    /// method saturates. Future versions may reintroduce the panic in some circumstances.
    pub fn duration_since(&self, earlier: Instant) -> Duration {
        self.0.checked_sub(earlier.0).unwrap_or_default()
    }

    /// Returns the amount of time elapsed since this instant was created.
    ///
    /// # Panics
    ///
    /// Previous rust versions panicked when the current time was earlier than self. Currently this
    /// method returns a Duration of zero in that case. Future versions may reintroduce the panic.
    pub fn elapsed(&self) -> Duration {
        Instant::now() - *self
    }

    /// Returns `Some(t)` where `t` is the time `self + duration` if `t` can be represented as
    /// `Instant` (which means it's inside the bounds of the underlying data structure), `None`
    /// otherwise.
    pub fn checked_add(&self, duration: Duration) -> Option<Instant> {
        self.0.checked_add(duration).map(Instant)
    }

    /// Returns `Some(t)` where `t` is the time `self - duration` if `t` can be represented as
    /// `Instant` (which means it's inside the bounds of the underlying data structure), `None`
    /// otherwise.
    pub fn checked_sub(&self, duration: Duration) -> Option<Instant> {
        self.0.checked_sub(duration).map(Instant)
    }
}

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

    /// # Panics
    ///
    /// This function may panic if the resulting point in time cannot be represented by the
    /// underlying data structure.
    fn add(self, other: Duration) -> Instant {
        self.checked_add(other)
            .expect("overflow when adding duration to instant")
    }
}

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 {
        self.checked_sub(other)
            .expect("overflow when subtracting duration from instant")
    }
}

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

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

    /// Returns the amount of time elapsed from another instant to this one,
    /// or zero duration if that instant is later than this one.
    fn sub(self, other: Instant) -> Duration {
        self.duration_since(other)
    }
}