viewport-lib 0.18.2

//! Fixed timestep accumulator for physics and simulation.
//!
//! Call [`FixedTimestep::advance`] each frame with the wall-clock delta. Iterate
//! the returned [`FixedStepIter`] to consume as many fixed steps as the
//! accumulator has filled. After iteration, [`FixedTimestep::alpha`] gives the
//! blend factor for transform interpolation when rendering between steps.
//!
//! # Example
//!
//! ```rust
//! use viewport_lib::runtime::FixedTimestep;
//!
//! let mut ts = FixedTimestep::new(60.0);
//!
//! let wall_dt = 0.016_f32;
//! for step_dt in ts.advance(wall_dt) {
//!     // run physics for step_dt seconds
//!     let _ = step_dt;
//! }
//! let alpha = ts.alpha(); // use this to lerp between previous and current transforms
//! ```

/// Accumulates wall-clock time and yields fixed simulation steps.
///
/// The step size is `1.0 / hz` seconds. The accumulator drains by `step_dt`
/// for each step yielded. A long frame yields multiple steps; a short frame
/// yields zero.
#[derive(Debug, Clone)]
#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
pub struct FixedTimestep {
    /// Duration of each fixed step in seconds (`1.0 / hz`).
    pub step_dt: f32,
    accumulator: f32,
}

impl FixedTimestep {
    /// Create a new accumulator that yields steps at `hz` Hz.
    ///
    /// Panics if `hz` is not positive and finite.
    pub fn new(hz: f32) -> Self {
        assert!(hz > 0.0 && hz.is_finite(), "hz must be positive and finite");
        Self {
            step_dt: 1.0 / hz,
            accumulator: 0.0,
        }
    }

    /// Add `dt` seconds to the accumulator and return an iterator over ready steps.
    ///
    /// Each call to `next()` on the returned iterator drains one step from the
    /// accumulator and yields `self.step_dt`. Exhaust the iterator before calling
    /// [`alpha`](Self::alpha) so the blend factor reflects the remaining leftover.
    pub fn advance(&mut self, dt: f32) -> FixedStepIter<'_> {
        self.accumulator += dt;
        FixedStepIter { ts: self }
    }

    /// Blend factor for rendering interpolation: `accumulator / step_dt`.
    ///
    /// Ranges from `0.0` (step just completed) to just under `1.0` (about to
    /// step). Use this to lerp between the previous and current physics transform
    /// when rendering between fixed steps. Clamped to `[0.0, 1.0]`.
    ///
    /// Call this after exhausting the iterator from [`advance`](Self::advance).
    pub fn alpha(&self) -> f32 {
        (self.accumulator / self.step_dt).clamp(0.0, 1.0)
    }

    /// Reset the accumulator to zero without changing the step rate.
    pub fn reset(&mut self) {
        self.accumulator = 0.0;
    }
}

/// Iterator over pending fixed simulation steps.
///
/// Returned by [`FixedTimestep::advance`]. Each `next()` drains one step from
/// the accumulator and yields the step duration in seconds.
pub struct FixedStepIter<'a> {
    ts: &'a mut FixedTimestep,
}

impl<'a> Iterator for FixedStepIter<'a> {
    type Item = f32;

    fn next(&mut self) -> Option<f32> {
        if self.ts.accumulator >= self.ts.step_dt {
            self.ts.accumulator -= self.ts.step_dt;
            Some(self.ts.step_dt)
        } else {
            None
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_no_steps_on_short_frame() {
        let mut ts = FixedTimestep::new(60.0);
        let steps: Vec<_> = ts.advance(0.005).collect();
        assert!(steps.is_empty());
    }

    #[test]
    fn test_one_step_on_normal_frame() {
        let mut ts = FixedTimestep::new(60.0);
        let steps: Vec<_> = ts.advance(1.0 / 60.0).collect();
        assert_eq!(steps.len(), 1);
        assert!((steps[0] - 1.0 / 60.0).abs() < 1e-6);
    }

    #[test]
    fn test_multiple_steps_on_long_frame() {
        let mut ts = FixedTimestep::new(60.0);
        // Use an exact multiple of step_dt to avoid f32 rounding.
        let steps: Vec<_> = ts.advance(ts.step_dt * 3.0 + 0.0001).collect();
        assert_eq!(steps.len(), 3);
    }

    #[test]
    fn test_alpha_between_zero_and_one_after_step() {
        let mut ts = FixedTimestep::new(60.0);
        let step_dt = 1.0 / 60.0;
        // Advance slightly more than one step so there is leftover.
        ts.advance(step_dt + 0.004).for_each(drop);
        let alpha = ts.alpha();
        assert!(alpha > 0.0 && alpha < 1.0, "alpha was {alpha}");
    }

    #[test]
    fn test_accumulator_carries_over_across_frames() {
        let mut ts = FixedTimestep::new(60.0);
        let step_dt = 1.0 / 60.0;
        // First frame: half a step.
        ts.advance(step_dt * 0.5).for_each(drop);
        // Second frame: another half step. Should now yield one full step.
        let steps: Vec<_> = ts.advance(step_dt * 0.5).collect();
        assert_eq!(steps.len(), 1);
    }

    #[test]
    fn test_reset_clears_accumulator() {
        let mut ts = FixedTimestep::new(60.0);
        ts.advance(0.5).for_each(drop);
        ts.reset();
        assert_eq!(ts.alpha(), 0.0);
        let steps: Vec<_> = ts.advance(0.0).collect();
        assert!(steps.is_empty());
    }

    #[test]
    #[should_panic]
    fn test_new_panics_on_zero_hz() {
        FixedTimestep::new(0.0);
    }

    #[test]
    #[should_panic]
    fn test_new_panics_on_negative_hz() {
        FixedTimestep::new(-60.0);
    }
}