zntg/colors.go

package zntg

import (
	"errors"
	"image/color"
	"regexp"
)

var hexColorRE = regexp.MustCompile(`^#?([0-9A-Fa-f]{2})-?([0-9A-Fa-f]{2})-?([0-9A-Fa-f]{2})-?([0-9A-Fa-f]{2})?$`)

// HexColor parses hexadecimal string s and returns the RGBA color. Accepted are 3 or 4 components (R, G, B, A) following an optional hash character '#'.
func HexColor(s string) (color.RGBA, error) {
	match := hexColorRE.FindStringSubmatch(s)
	if match == nil {
		return color.RGBA{}, errors.New("invalid color format")
	}
	values, err := HexToInts(match[1:]...)
	if err != nil {
		return color.RGBA{}, err
	}
	a := 255
	if len(match[4]) > 0 {
		a = values[3]
	}
	return RGBA(uint8(values[0]), uint8(values[1]), uint8(values[2]), uint8(a)), nil
}

// HexToInt tries to convert a string with hexadecimal characters to an integer.
func HexToInt(s string) (int, error) {
	var i int
	for _, c := range s {
		i *= 16
		if c >= '0' && c <= '9' {
			i += int(c - '0')
		} else if c >= 'A' && c <= 'F' {
			i += int(c - 'A' + 10)
		} else if c >= 'a' && c <= 'f' {
			i += int(c - 'a' + 10)
		} else {
			return 0, errors.New("hex digit not supported")
		}
	}
	return i, nil
}

// HexToInts tries to convert multiple strings with hexadecimal characters to a slice of integers.
func HexToInts(s ...string) ([]int, error) {
	ints := make([]int, len(s))
	for i, s := range s {
		value, err := HexToInt(s)
		if err != nil {
			return nil, err
		}
		ints[i] = value
	}
	return ints, nil
}

// MustHexColor parses hexadecimal string s and returns the RGBA color. If the conversion fails it panics.
func MustHexColor(s string) color.RGBA {
	color, err := HexColor(s)
	if err != nil {
		panic(err)
	}
	return color
}

// RGB creates an opaque color with the specified red, green and red values.
func RGB(r, g, b byte) color.RGBA { return RGBA(r, g, b, 0xff) }

// RGB creates a color with the specified red, green, red and alpha values.
func RGBA(r, g, b, a byte) color.RGBA {
	return color.RGBA{R: r, G: g, B: b, A: a}
}