Measuring a font's x-height/cap-height ratio using rusttype

xheight_ratio.rs

extern crate num_rational; // 0.1
extern crate rusttype; // 0.6

use num_rational::Ratio;
use rusttype::{Font, FontCollection, Glyph, Rect};

fn main() {
    let filename = "C:\\WINDOWS\\Fonts\\Tahoma.ttf";

    let font = read_font_from_filename(filename);
    let ratio = x_height_ratio(&font);

    println!("x-height ratio: {} (~{:.3})",
             ratio, ratio_into_f32(ratio).unwrap()
    );
}

fn x_height_ratio(font: &Font) -> Ratio<i32> {
    let x_height_glyphs = font.glyphs_for("vxz".chars());
    let cap_height_glyphs = font.glyphs_for("HIT".chars());

    let x_heights_sum = x_height_glyphs.map(glyph_height).sum::<i32>();
    let cap_heights_sum = cap_height_glyphs.map(glyph_height).sum::<i32>();

    Ratio::new(x_heights_sum, cap_heights_sum)
}

fn glyph_height(glyph: Glyph) -> i32 {
    let glyph = glyph.standalone();

    let extents: Rect<i32> = glyph
        .get_data().unwrap()
        .extents.unwrap();

    extents.max.y - extents.min.y
}

fn read_font_from_filename(filename: &str) -> Font {
    let data = std::fs::read(filename).unwrap();

    FontCollection::from_bytes(data).unwrap()
        .into_font().unwrap()
}

fn ratio_into_f32(ratio: Ratio<i32>) -> Option<f32> {
    match ratio.denom() {
        0 => None,
        _ => Some(((*ratio.numer() as f64) / (*ratio.denom() as f64)) as f32)
    }
}