package theme

import (
	"math"

	"github.com/egoist/mygo/ui"
)

// Contrast returns the WCAG contrast ratio of two opaque colors.
func Contrast(a, b ui.Color) float64 {
	la, lb := luminance(a), luminance(b)
	if la < lb {
		la, lb = lb, la
	}
	return (la + 0.05) / (lb + 0.05)
}

func luminance(c ui.Color) float64 {
	return 0.2126*channel(c.R) + 0.7152*channel(c.G) + 0.0722*channel(c.B)
}

func channel(v uint8) float64 {
	s := float64(v) / 255
	if s <= 0.04045 {
		return s / 12.92
	}
	return math.Pow((s+0.055)/1.055, 2.4)
}