// https://www.musicdsp.org/en/latest/Filters/236-3-band-equaliser.html

/// Very small amount
const VSA: f32 = 1.0 / 4294967295.0;

/// Splits the input into three bands
#[derive(Default)]
pub struct ThreeBandSplitter {
    // Filter #1 (Low band)
    f1p0: f32, // Poles ...
    f1p1: f32,
    f1p2: f32,
    f1p3: f32,

    // Filter #2 (High band)
    f2p0: f32, // Poles ...
    f2p1: f32,
    f2p2: f32,
    f2p3: f32,

    // Sample history buffer
    sdm1: f32, // Sample data minus 1
    sdm2: f32, //                   2
    sdm3: f32, //                   3
}
impl ThreeBandSplitter {
    /// Returns (low, mid, high) bands
    pub fn process(&mut self, input: f32, lf: f32, hf: f32, sample_rate: f32) -> (f32, f32, f32) {
        let lf = 2.0 * (std::f32::consts::PI * (lf / sample_rate)).sin();
        let hf = 2.0 * (std::f32::consts::PI * (hf / sample_rate)).sin();

        // Filter #1 (low pass)
        self.f1p0 += lf * (input - self.f1p0) + VSA;
        self.f1p1 += lf * (self.f1p0 - self.f1p1);
        self.f1p2 += lf * (self.f1p1 - self.f1p2);
        self.f1p3 += lf * (self.f1p2 - self.f1p3);

        let l = self.f1p3;

        // Filter #2 (high pass)
        self.f2p0 += hf * (input - self.f2p0) + VSA;
        self.f2p1 += hf * (self.f2p0 - self.f2p1);
        self.f2p2 += hf * (self.f2p1 - self.f2p2);
        self.f2p3 += hf * (self.f2p2 - self.f2p3);

        // we subtract from sdm3 because the filter has a three sample delay
        let h = self.sdm3 - self.f2p3;

        // Calculate midrange (signal - (low + high))
        let m = self.sdm3 - (h + l);

        // Shuffle history buffer
        self.sdm3 = self.sdm2;
        self.sdm2 = self.sdm1;
        self.sdm1 = input;

        (l, m, h)
    }
}