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use core::time::Duration;

/// Something which records the elapsed real time.
///
/// This uses shared references because it may be shared between multiple
/// components at any one time.
pub trait SystemClock {
    /// The amount of time that has passed since a clock-specific reference
    /// point (e.g. device startup or the unix epoch).
    fn elapsed(&self) -> Duration;
}

impl<'a, C: SystemClock> SystemClock for &'a C {
    fn elapsed(&self) -> Duration { (*self).elapsed() }
}

/// A monotonically non-decreasing clock backed by the operating system.
///
/// Requires the `std` feature.
#[cfg(feature = "std")]
#[derive(Debug, Clone, PartialEq)]
pub struct OperatingSystemClock {
    created_at: std::time::Instant,
}

#[cfg(feature = "std")]
impl OperatingSystemClock {
    pub fn new() -> OperatingSystemClock { OperatingSystemClock::default() }
}

#[cfg(feature = "std")]
impl SystemClock for OperatingSystemClock {
    fn elapsed(&self) -> Duration { self.created_at.elapsed() }
}

#[cfg(feature = "std")]
impl Default for OperatingSystemClock {
    fn default() -> OperatingSystemClock {
        OperatingSystemClock {
            created_at: std::time::Instant::now(),
        }
    }
}