delay.rs

pub const SPEED_OF_SOUND: f32 = 343.0;

const GLIDE: f32 = 0.0005;

pub struct DelayLine {
    buffer: Vec<f32>,
    write: usize,
    delay: f32,
}

impl DelayLine {
    pub fn new(capacity: usize) -> Self {
        Self {
            buffer: vec![0.0; capacity.max(2)],
            write: 0,
            delay: 0.0,
        }
    }

    pub fn process(&mut self, input: f32, target: f32) -> f32 {
        let ceiling = (self.buffer.len() - 2) as f32;
        self.delay += (target.clamp(0.0, ceiling) - self.delay) * GLIDE;

        self.buffer[self.write] = input;
        let output = self.read(self.delay);
        self.write = (self.write + 1) % self.buffer.len();
        output
    }

    fn read(&self, delay: f32) -> f32 {
        let length = self.buffer.len();
        let position = self.write as f32 - delay;
        let wrapped = position.rem_euclid(length as f32);
        let index = wrapped.floor() as usize;
        let fraction = wrapped - index as f32;
        let next = (index + 1) % length;
        self.buffer[index] * (1.0 - fraction) + self.buffer[next] * fraction
    }
}

pub fn delay_samples(distance: f32, sample_rate: u32) -> f32 {
    distance / SPEED_OF_SOUND * sample_rate as f32
}

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

    #[test]
    fn zero_delay_passes_input_through() {
        let mut line = DelayLine::new(64);
        assert!((line.process(0.7, 0.0) - 0.7).abs() < 1e-6);
        assert!((line.process(-0.3, 0.0) + 0.3).abs() < 1e-6);
    }

    #[test]
    fn distance_maps_to_time_of_flight_in_samples() {
        assert!((delay_samples(343.0, 48_000) - 48_000.0).abs() < 1e-3);
        assert!(delay_samples(0.0, 48_000).abs() < 1e-6);
    }

    #[test]
    fn delay_glides_toward_target_without_overshooting() {
        let mut line = DelayLine::new(48_000);
        let mut last = 0.0;
        for _ in 0..1_000 {
            line.process(0.0, 4_800.0);
            assert!(line.delay >= last);
            assert!(line.delay <= 4_800.0);
            last = line.delay;
        }
    }
}