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//! # Band Pass

use super::*;

/// -12dB per octave BandPass filter.
pub struct BandPass {
    central_f: MathT,
    quality: MathT,
    a0: MathT,
    b1: MathT,
    b2: MathT,
    x1: SampleT,
    x2: SampleT,
    y1: SampleT,
    y2: SampleT,
}

impl BandPass {
    /// Creates a new BandPass object from the given central frequency and q
    /// value.
    /// 
    /// The filter's quality is set to the central frequency divided by the
    /// difference between the corner frequencies.
    pub fn new(f: MathT, q: MathT) -> Self {
        let mut bp = BandPass {
            central_f: f,
            quality: q,
            a0: 0.0,
            b1: 0.0,
            b2: 0.0,
            x1: SampleT::default(),
            x2: SampleT::default(),
            y1: SampleT::default(),
            y2: SampleT::default(),
        };

        bp.reset();

        bp
    }

    /// Creates a new BandPass object from given corner frequencies.
    pub fn from_corners(f: (MathT,MathT)) -> Self {
        let mut bp = BandPass {
            central_f: (f.0*f.1).abs().sqrt(),
            quality: (f.0*f.1).abs().sqrt()/(f.1-f.0).abs(),
            a0: 0.0,
            b1: 0.0,
            b2: 0.0,
            x1: SampleT::default(),
            x2: SampleT::default(),
            y1: SampleT::default(),
            y2: SampleT::default(),
        };

        bp.reset();

        bp
    }

    /// Returns the central frequency of the filter.
    pub fn get_central_frequency(&self) -> MathT {
        self.central_f
    }

    /// Sets a new central frequency.
    pub fn set_central_frequency(&mut self, f: MathT) {
        self.central_f = f;

        self.reset();
    }

    /// Returns the quality of the filter.
    pub fn get_quality(&self) -> MathT {
        self.quality
    }

    /// Sets the quality of the filter.
    /// 
    /// The filter's quality is set to the central frequency divided by the
    /// difference between the corner frequencies.
    pub fn set_quality(&mut self, q: MathT) {
        self.quality = q;

        self.reset();
    }

    /// Returns the corner frequencies of the filter.
    pub fn get_corner_frequencies(&self) -> (MathT,MathT) {
        let b = -self.central_f/self.quality;

        let (p,n) = quadratic(1.0, b, -self.central_f*self.central_f);
        let fl = if p > 0.0 {
            p
        } else {
            n
        };
        let fh = fl + b;

        if fl < fh {
            (fl, fh)
        } else {
            (fh, fl)
        }
    }

    /// Sets the corner frequencies of the filter.
    pub fn set_corner_frequencies(&mut self, f: (MathT,MathT)) {
        self.central_f = (f.0 * f.1).sqrt();
        self.quality = self.central_f/(f.0-f.1).abs();

        self.reset();
    }

    fn reset(&mut self) {
        let (fh, fl) = self.get_corner_frequencies();

        let theta_l = (std::f64::consts::PI * fl * INV_SAMPLE_RATE).tan();
        let theta_h = (std::f64::consts::PI * fh * INV_SAMPLE_RATE).tan();

        let al = 1.0 / (1.0+theta_l);
        let ah = 1.0 / (1.0+theta_h);

        let bl = (1.0-theta_l) / (1.0+theta_l);
        let bh = (1.0-theta_h) / (1.0+theta_h);

        self.a0 = (1.0-al) * ah;
        self.b1 = bl + bh;
        self.b2 = bl * bh;
    }
}

impl Modifier for BandPass {
    fn process(&mut self, x: SampleT) -> SampleT {
        let y = (self.a0 * (x - self.x2) as MathT +
            self.b1 * self.y1 as MathT - 
            self.b2 * self.y2 as MathT) as SampleT;

        self.y2 = self.y1;
        self.y1 = y;
        self.x2 = self.x1;
        self.x1 = x;

        y
    }
}

fn quadratic(a: MathT, b: MathT, c: MathT) -> (MathT,MathT) {
    (
        (-b + (b*b - 4.0*a*c).sqrt())/(2.0*a),
        (-b - (b*b - 4.0*a*c).sqrt())/(2.0*a)
    )
}

impl Clone for BandPass {
    fn clone(&self) -> Self {
        BandPass {
            central_f: self.central_f,
            quality: self.quality,
            a0: self.a0,
            b1: self.b1,
            b2: self.b2,
            x1: SampleT::default(),
            x2: SampleT::default(),
            y1: SampleT::default(),
            y2: SampleT::default(),
        }
    }
}