game_time 0.2.0

A library for handling time in games. Game_time provides methods to track frame time in games. It allows decoupling game time from wall time as well as tracking and setting frame rate. 
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//! Types for computing game time from wall time.
use float_duration::FloatDuration;
use framerate::FrameCount;

/// Compute elapsed game time for a frame.
pub trait TimeStep {
    /// Compute the time step for the next frame.
    fn time_step(&self, wall_time: &FloatDuration) -> FloatDuration;
}

/// A time step based on elapsed wall time.
#[derive(Debug, Clone, Copy, Default, PartialEq, Eq)]
pub struct VariableStep {}
/// A fixed time step derived from a set frame rate.
#[derive(Debug)]
pub struct FixedStep<'a, C: 'a + FrameCount + ?Sized> {
    counter: &'a C,
}
/// A specific, constant time step.
#[derive(Debug, Clone, Copy, Default, PartialEq)]
pub struct ConstantStep {
    step: FloatDuration,
}

impl VariableStep {
    /// Construct a new `VariableStep` object.
    pub fn new() -> VariableStep {
        VariableStep {}
    }
}
impl<'a, C> FixedStep<'a, C>
where
    C: 'a + FrameCount + ?Sized,
{
    /// Construct a new `FixedStep` object based on the set frame rate in `counter`.
    pub fn new(counter: &'a C) -> FixedStep<'a, C> {
        FixedStep { counter: counter }
    }
}
impl ConstantStep {
    /// Construct a new `ConstantStep` object with a set time step `step`.
    pub fn new(step: FloatDuration) -> ConstantStep {
        ConstantStep { step }
    }
    /// Construct a new `ConstantStep` that does not advance game time.
    pub fn null_step() -> ConstantStep {
        ConstantStep::new(FloatDuration::zero())
    }
}

impl TimeStep for VariableStep {
    fn time_step(&self, wall_time: &FloatDuration) -> FloatDuration {
        *wall_time
    }
}
impl<'a, C> TimeStep for FixedStep<'a, C>
where
    C: 'a + FrameCount + ?Sized,
{
    fn time_step(&self, _: &FloatDuration) -> FloatDuration {
        self.counter.target_time_per_frame()
    }
}
impl TimeStep for ConstantStep {
    fn time_step(&self, _: &FloatDuration) -> FloatDuration {
        self.step
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use clock::GameClock;
    use framerate::{counter, sample};

    #[test]
    fn test_fixed_step() {
        let mut clock = GameClock::new();
        let mut count =
            counter::FrameCounter::new(20.0, sample::RunningAverageSampler::with_max_samples(20));

        for _ in 0..20 {
            let time = {
                let step = FixedStep::new(&count);
                clock.tick(&step)
            };
            count.tick(&time);
            assert_eq!(time.elapsed_game_time(), FloatDuration::seconds(1.0 / 20.0));
        }
        let time = clock.last_frame_time();
        assert_eq!(time.total_game_time(), FloatDuration::seconds(1.0));
    }

    #[test]
    fn test_null_step() {
        let mut clock = GameClock::new();
        let step = ConstantStep::null_step();

        for _ in 0..1000 {
            let time = clock.tick(&step);
            assert_eq!(time.elapsed_game_time(), FloatDuration::zero());
        }

        assert_eq!(
            clock.last_frame_time().total_game_time(),
            FloatDuration::zero()
        );

    }
}