// from https://www.musicdsp.org/en/latest/Analysis/97-envelope-detector.html
// with some modifications
pub struct EnvelopeFollower {
    x1: f32,
    x2: f32,

    ga: f32,
    gr: f32,
}

impl EnvelopeFollower {
    pub fn new() -> Self {
        Self {
            x1: 0.0,
            x2: 0.0,
            ga: 0.0,
            gr: 0.0,
        }
    }

    /// attack and release are in seconds
    pub fn set_attack_release(&mut self, sample_rate: f32, attack: f32, release: f32) {
        self.ga = (-1.0 / (sample_rate * attack)).exp();
        self.gr = (-1.0 / (sample_rate * release)).exp();
    }

    pub fn process(&mut self, input: f32) -> f32 {
        let in_abs = input.abs();

        // 2nd order lowpass
        if self.x1 < in_abs {
            self.x1 = in_abs + self.ga * (self.x1 - in_abs);
            self.x2 = self.x1 + self.ga * (self.x2 - self.x1);
        } else {
            self.x1 = in_abs + self.gr * (self.x1 - in_abs);
            self.x2 = self.x1 + self.gr * (self.x2 - self.x1);
        }

        self.x2
    }
}