color.go

package main

import (
	"errors"
	"image/color"
)

func ParseHexColor(s string) (c color.RGBA, err error) {
	c.A = 0xff

	if s[0] != '#' {
		return c, errors.New("hex string must be start at '#'")
	}

	hexToByte := func(b byte) byte {
		switch {
		case b >= '0' && b <= '9':
			return b - '0'
		case b >= 'a' && b <= 'f':
			return b - 'a' + 10
		case b >= 'A' && b <= 'F':
			return b - 'A' + 10
		}
		err = errors.New("invalid hex format")
		return 0
	}

	switch len(s) {
	case 7:
		c.R = hexToByte(s[1])<<4 + hexToByte(s[2])
		c.G = hexToByte(s[3])<<4 + hexToByte(s[4])
		c.B = hexToByte(s[5])<<4 + hexToByte(s[6])
	case 4:
		c.R = hexToByte(s[1]) * 17
		c.G = hexToByte(s[2]) * 17
		c.B = hexToByte(s[3]) * 17
	default:
		err = errors.New("invalid hex format")
	}

	return
}

func IsNoBackgroundColor(c color.RGBA) bool {
	return c.R == 0 && c.G == 0 && c.B == 0 && (c.A == 255 || c.A == 0)
}