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//! Temporal quantification.

use core::{
    fmt,
    ops::{Add, AddAssign, Sub, SubAssign},
    time::Duration,
};

/// Represents a timestamp on a monotonically nondecreasing clock relative to the
/// start of the user program.
///
/// # Precision
/// This type has a precision of 1 microsecond, and uses [`pros_sys::micros`] internally.
#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub struct Instant(u64);

impl Instant {
    /// Returns an instant corresponding to "now".
    ///
    /// # Examples
    ///
    /// ```
    /// use pros::time::Instant;
    ///
    /// let now = Instant::now();
    /// ```
    pub fn now() -> Self {
        Self(unsafe { pros_sys::rtos::micros() })
    }

    /// Returns the amount of time elapsed from another instant to this one,
    /// or zero duration if that instant is later than this one.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use core::time::Duration;
    /// use pros::{time::Instant, task::delay};
    ///
    /// let now = Instant::now();
    /// delay(Duration::new(1, 0));
    /// let new_now = Instant::now();
    /// println!("{:?}", new_now.duration_since(now));
    /// println!("{:?}", now.duration_since(new_now)); // 0ns
    /// ```
    pub fn duration_since(&self, earlier: Instant) -> Duration {
        self.checked_duration_since(earlier).unwrap_or_default()
    }

    /// Returns the amount of time elapsed from another instant to this one,
    /// or None if that instant is later than this one.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use core::time::Duration;
    /// use pros::{time::Instant, task::delay};
    ///
    /// let now = Instant::now();
    /// delay(Duration::new(1, 0));
    /// let new_now = Instant::now();
    /// println!("{:?}", new_now.checked_duration_since(now));
    /// println!("{:?}", now.checked_duration_since(new_now)); // None
    /// ```
    pub const fn checked_duration_since(&self, earlier: Instant) -> Option<Duration> {
        if earlier.0 < self.0 {
            Some(Duration::from_micros(self.0 - earlier.0))
        } else {
            None
        }
    }

    /// Returns the amount of time elapsed from another instant to this one,
    /// or zero duration if that instant is later than this one.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use core::time::Duration;
    /// use pros::{time::Instant, task::delay};
    ///
    /// let instant = Instant::now();
    /// let three_secs = Duration::from_secs(3);
    /// delay(three_secs);
    /// assert!(instant.elapsed() >= three_secs);
    /// ```
    pub fn saturating_duration_since(&self, earlier: Instant) -> Duration {
        self.checked_duration_since(earlier).unwrap_or_default()
    }

    /// Returns the amount of time elapsed since this instant.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use core::time::Duration;
    /// use pros::{time::Instant, task::delay};
    ///
    /// let instant = Instant::now();
    /// let three_secs = Duration::from_secs(3);
    /// delay(three_secs);
    /// assert!(instant.elapsed() >= three_secs);
    /// ```
    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, rhs: Duration) -> Option<Instant> {
        Some(Self(self.0.checked_add(rhs.as_micros().try_into().ok()?)?))
    }

    /// 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, rhs: Duration) -> Option<Instant> {
        Some(Self(self.0.checked_sub(rhs.as_micros().try_into().ok()?)?))
    }
}

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. See [`Instant::checked_add`] for a version without panic.
    fn add(self, rhs: Duration) -> Self::Output {
        self.checked_add(rhs)
            .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)
    }
}

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