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//! This library provides a very simple system clock interface with microsecond precision.
//! Unlike the [`embedded-time`](https://crates.io/crates/embedded-time),
//! the interface provided by this library is as simple as possible which is better suited
//! for writing prototypes.

#![cfg_attr(not(test), no_std)]

#[cfg(test)]
mod tests;

/// Provides an abstraction for hardware-specific clocks with the microsecond precision.
pub trait SimpleClock {
    /// Returns an instant time in the microseconds.
    fn now_us(&self) -> u64;
}

/// Provides an easy way to calculate elapsed times.
///
/// This timer is usually used to compute how much time is elapsed between two events
/// or to determine the event's deadline in case of polling.
pub struct ElapsedTimer<'a, T> {
    clock: &'a T,
    now: u64,
}

impl<'a, T: SimpleClock> ElapsedTimer<'a, T> {
    /// Creates a new elapsed timer instance backed by the specified clock implementation.
    pub fn new(clock: &'a T) -> Self {
        Self {
            clock,
            now: clock.now_us(),
        }
    }

    /// Restarts the timer and returns the number of microseconds
    /// elapsed since this timer was started.
    pub fn restart(&mut self) -> u64 {
        let elapsed = self.elapsed();
        self.now = self.clock.now_us();
        elapsed
    }

    /// Returns the number of microseconds elapsed since this timer was started.
    pub fn elapsed(&self) -> u64 {
        self.clock.now_us().saturating_sub(self.now)
    }
}

/// The deadline has been reached.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct DeadlineReached(());

/// Provides an easy way to specify operation deadline
///
/// This timer is usually used to limit polling operation by a certain period of time.
#[derive(Debug)]
pub struct Deadline<'a, C: SimpleClock> {
    clock: &'a C,
    deadline: u64,
}

impl<'a, C: SimpleClock> Deadline<'a, C> {
    /// Creates a new deadline timer instance backed by the specified clock implementation
    pub fn new(clock: &'a C, timeout: u64) -> Self {
        Self {
            clock,
            deadline: clock.now_us() + timeout,
        }
    }

    /// Returns error if the deadline has been reached.
    pub fn reached(&self) -> Result<(), DeadlineReached> {
        if self.clock.now_us() > self.deadline {
            Err(DeadlineReached(()))
        } else {
            Ok(())
        }
    }
}