fretboard_layout 0.8.0

Generate an svg template of a musical instrument fretboard
Documentation 
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//! Mathematical factors used in laying out the frets in 2d space

use crate::Variant;

#[cfg(feature = "serde")]
use serde::{Deserialize, Serialize};

#[cfg_attr(feature = "serde", derive(Deserialize, Serialize))]
/// This struct contains multiplication factors used to convert the raw lengths
/// from bridge to fret into x,y coordinates. It also contains an offset distance
/// used to correctly orient the two scales in a multiscale design so that the
/// desired fret is perpendicular to the centerline.
pub struct Factors {
    pub x_ratio: f64,
    /// This ratio is half the difference between the nut and bridge divided
    /// by the scale and is used in determining the X ratio
    pub y_ratio: f64,
    /// How far forward the treble side of the bridge should start with respect
    /// to the trable side
    pub treble_offset: f64,
}

impl Default for Factors {
    fn default() -> Self {
        Self::init(655.0, &Variant::default(), 43.0, 56.0)
    }
}

impl Factors {
    /// Uses trigonometry to place the fret ends, based on visualizing their
    /// locations as a triangle where the hypotenuse is the string, and the
    /// opposite is the distance from the bridge parallel to the centerline.
    pub fn init(scale: f64, variant: &Variant, nut: f64, bridge: f64) -> Self {
        let height = (bridge - nut) / 2.0;
        let y_ratio = height / scale;
        let x_ratio = y_ratio.acos().sin();
        let pfret = variant.pfret().unwrap_or(8.0);
        let factor = 2.0_f64.powf(pfret / 12.0);
        let length_bass = scale / factor;
        let length_treble = match variant {
            Variant::Monoscale => length_bass,
            Variant::Multiscale { scale: s, .. } => s / factor,
        };
        let bass_pfret = x_ratio * length_bass;
        let treble_pfret = x_ratio * length_treble;
        let treble_offset = bass_pfret - treble_pfret;
        Self {
            x_ratio,
            y_ratio,
            treble_offset,
        }
    }
}

#[cfg(test)]
mod tests {
    use crate::Specs;

    #[test]
    fn factors_default() {
        let specs = Specs::default();
        assert_eq!(specs.factors.x_ratio, 0.9999507592328689);
        assert_eq!(specs.factors.y_ratio, 0.009923664122137405);
        assert_eq!(specs.factors.treble_offset, 0.0);
    }

    #[test]
    fn factors_multi() {
        let specs = Specs::multi();
        assert_eq!(specs.factors.x_ratio, 0.9999507592328689);
        assert_eq!(specs.factors.y_ratio, 0.009923664122137405);
        assert_eq!(specs.factors.treble_offset, 28.346827734356623);
    }
}