Added hexadecimal color string conversions.

colors.go

@@ -0,0 +1,66 @@
+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 color.RGBA{R: uint8(values[0]), G: uint8(values[1]), B: uint8(values[2]), A: 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
+}