feat(calc): add contrast_ratio fn

src/color_calc/contrast_ratio.rs

@@ -0,0 +1,51 @@
+use crate::Color;
+
+/// Computes the WCAG contrast ratio between two colors
+///
+/// Reference: [W3C contrast ratio](https://www.w3.org/TR/2008/REC-WCAG20-20081211/#contrast-ratiodef)
+///
+/// **Tips**: A minimum contrast of 4.5:1 is [recommended](http://www.w3.org/TR/WCAG20-TECHS/G18.html) to ensure that text is still readable against a background color.
+///
+/// # Example
+///
+/// ```
+/// use color_art::{color, contrast_ratio};
+///
+/// let color1 = color!(#fefe0e);
+/// let color2 = color!(#fff);
+/// let contrast = contrast_ratio(&color1, &color2);
+///
+/// assert_eq!(contrast, 1.0826287103122008);
+pub fn contrast_ratio(color1: &Color, color2: &Color) -> f64 {
+    let l1 = color1.luminance();
+    let l2 = color2.luminance();
+    let l_max = l1.max(l2);
+    let l_min = l1.min(l2);
+    (l_max + 0.05) / (l_min + 0.05)
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::*;
+
+    #[test]
+    fn test_distance() {
+        let color1 = color!(#000);
+        let color2 = color!(#fff);
+
+        let contrast = contrast_ratio(&color1, &color2);
+        assert_eq!(contrast, 21.0);
+
+        let color1 = color!(#fefe0e);
+        let color2 = color!(#fff);
+
+        let contrast = contrast_ratio(&color1, &color2);
+        assert_eq!(contrast, 1.0826287103122008);
+
+        let color1 = Color::from_name("purple").unwrap();
+        let color2 = Color::from_name("pink").unwrap();
+
+        let contrast = contrast_ratio(&color1, &color2);
+        assert_eq!(contrast, 6.124225406859997);
+    }
+}

src/color_calc/mod.rs

@@ -1,5 +1,6 @@
 pub mod blend;
 pub mod distance;
 pub mod delta_e;
+pub mod contrast_ratio;
 
 pub use blend::BlendMode;

src/lib.rs

@@ -21,8 +21,9 @@ mod utils;
 
 pub use color::Color;
 pub use color_calc::blend::*;
-pub use color_calc::distance::*;
+pub use color_calc::contrast_ratio::*;
 pub use color_calc::delta_e::*;
+pub use color_calc::distance::*;
 pub use color_space::ColorSpace;
 use data::alpha_hex_map::ALPHA_HEX_MAP;
 use data::chinese_color::CHINESE_COLOR_HASHMAP as CHINESE_COLOR;