add a hex format color string parser

common/color.go

@@ -1,6 +1,10 @@
 package common
 
-import "image/color"
+import (
+	"fmt"
+	"image/color"
+	"strings"
+)
 
 var (
 	MistyRose        = color.RGBA{R: 0xFF, G: 0xE4, B: 0xE1, A: 0xFF}
@@ -114,3 +118,56 @@ var (
 		{R: 0xD2, G: 0xFD, B: 0xFF, A: 0xFF},
 	}
 )
+
+// ParseHexColor parses color string likes #FFFFFF or #2398EFFF
+func ParseHexColor(s string) (color.RGBA, error) {
+	s = strings.TrimSpace(strings.ToLower(s))
+
+	if !strings.HasPrefix(s, "#") {
+		return Black, fmt.Errorf("invalid hex color string %v", s)
+	}
+
+	if c, ok := parseHex(s); ok {
+		return c, nil
+	}
+	return Black, fmt.Errorf("invalid hex color string %v", s)
+}
+
+// parseHex returns a color.RGBA by parsing a hex string
+// Reference: https://stackoverflow.com/questions/54197913/parse-hex-string-to-image-color
+func parseHex(s string) (color.RGBA, bool) {
+	c := color.RGBA{}
+	c.A = 1
+	ok := true
+
+	hexToByte := func(b byte) byte {
+		switch {
+		case b >= '0' && b <= '9':
+			return b - '0'
+		case b >= 'a' && b <= 'f':
+			return b - 'a' + 10
+		}
+		ok = false
+		return 0
+	}
+
+	n := len(s)
+	if n == 6 || n == 8 {
+		c.R = hexToByte(s[0])<<4+hexToByte(s[1])
+		c.G = hexToByte(s[2])<<4+hexToByte(s[3])
+		c.B = hexToByte(s[4])<<4+hexToByte(s[5])
+		if n == 8 {
+			c.A = hexToByte(s[6])<<4+hexToByte(s[7])
+		}
+	} else if n == 3 || n == 4 {
+		c.R = hexToByte(s[0])*17
+		c.G = hexToByte(s[1])*17
+		c.B = hexToByte(s[2])*17
+		if n == 4 {
+			c.A = hexToByte(s[3])*17
+		}
+	} else {
+		ok = false
+	}
+	return c, ok
+}