Add functions for parsing/formatting NRGBA values

internal/color.go

@@ -1,5 +1,6 @@
 package internal
 
+import "fmt"
 import "image/color"
 
 // HSV represents a color with hue, saturation, and value components. Each
@@ -173,3 +174,58 @@ func rgbToHSV (r, g, b uint32) HSV {
 	
 	return hsv
 }
+
+// FormatNRGBA formats an NRGBA value into a hex string.
+func FormatNRGBA (nrgba color.NRGBA) string {
+	return fmt.Sprintf("%02X%02X%02X%02X", nrgba.R, nrgba.G, nrgba.B, nrgba.A)
+}
+
+// ParseNRGBA parses an NRGBA value from a hex string. It can be of the format:
+//   - RGB
+//   - RGBA
+//   - RRGGBB
+//   - RRGGBBAA
+// If none of these are specified, this function will return an opaque black
+// color. Hex digits may either be upper case or lower case.
+func ParseNRGBA (str string) color.NRGBA {
+	runes := []rune(str)
+	c := color.NRGBA { A: 255 }
+	switch len(runes) {
+	case 3:
+		c.R = fillOctet(hexDigit(runes[0]))
+		c.G = fillOctet(hexDigit(runes[1]))
+		c.B = fillOctet(hexDigit(runes[2]))
+	case 4:
+		c.R = fillOctet(hexDigit(runes[0]))
+		c.G = fillOctet(hexDigit(runes[1]))
+		c.B = fillOctet(hexDigit(runes[2]))
+		c.A = fillOctet(hexDigit(runes[3]))
+	case 6:
+		c.R = hexOctet(runes[0], runes[1])
+		c.G = hexOctet(runes[2], runes[3])
+		c.B = hexOctet(runes[4], runes[5])
+	case 8:
+		c.R = hexOctet(runes[0], runes[1])
+		c.G = hexOctet(runes[2], runes[3])
+		c.B = hexOctet(runes[4], runes[5])
+		c.A = hexOctet(runes[6], runes[7])
+	}
+	return c
+}
+
+func hexDigit (r rune) uint8 {
+	switch {
+	case r >= '0' && r <= '9': return uint8(r - '0')
+	case r >= 'A' && r <= 'F': return uint8(r - 'A') + 10
+	case r >= 'a' && r <= 'f': return uint8(r - 'a') + 10
+	default: return 0
+	}
+}
+
+func fillOctet (low uint8) uint8 {
+	return low << 4 | low
+}
+
+func hexOctet (high, low rune) uint8 {
+	return hexDigit(high) << 4 | hexDigit(low)
+}