hexmap/server/state/hexcolor.go

package state

import "fmt"

// HexColor is an internal representation of a hexadecimal color string.
// It can take one of these formats:
// #RRGGBBAA
// #RRGGBB
// #RGBA
// #RGB
// When marshaling, it will always choose the most efficient format, but any format can be used when unmarshaling.
type HexColor struct {
	// R is the red component of the color.
	R uint8
	// G is the green component of the color.
	G uint8
	// B is the blue component of the color.
	B uint8
	// A is the alpha component of the color.
	A uint8
}

// HexColorFromString decodes a hexadecimal string into a hex color.
func HexColorFromString(text string) (HexColor, error) {
	var hex HexColor
	if err := (&hex).UnmarshalText([]byte(text)); err != nil {
		return hex, err
	}
	return hex, nil
}

func (h *HexColor) UnmarshalText(text []byte) error {
	var count, expected int
	var short bool
	var scanErr error
	switch len(text) {
	case 9:
		// Long form with alpha: #RRGGBBAA
		expected = 4
		short = false
		count, scanErr = fmt.Sscanf(string(text), "#%02X%02X%02X%02X", &h.R, &h.G, &h.B, &h.A)
	case 7:
		// Long form: #RRGGBB
		expected = 3
		h.A = 0xFF
		count, scanErr = fmt.Sscanf(string(text), "#%02X%02X%02X", &h.R, &h.G, &h.B)
	case 5:
		// Short form with alpha: #RGBA
		expected = 4
		short = true
		count, scanErr = fmt.Sscanf(string(text), "#%01X%01X%01X%01X", &h.R, &h.G, &h.B, &h.A)
	case 4:
		// Short form: #RGB
		expected = 3
		h.A = 0xF
		short = true
		count, scanErr = fmt.Sscanf(string(text), "#%01X%01X%01X", &h.R, &h.G, &h.B)
	default:
		return fmt.Errorf("can't decode %s as HexColor: wrong length", text)
	}
	if scanErr != nil {
		return scanErr
	}
	if count != expected {
		return fmt.Errorf("can't decode %s as HexColor: missing components", text)
	}
	if short {
		h.R *= 0x11
		h.G *= 0x11
		h.B *= 0x11
		h.A *= 0x11
	}
	return nil
}

// MarshalText marshals the HexColor into a small string.
func (h HexColor) MarshalText() (text []byte, err error) {
	return []byte(h.String()), nil
}

// String prints the HexColor as an abbreviated notation.
// Specifically, short form is used when possible (i.e., when all components are evenly divisible by 0x11).
// The alpha component is left out if it's 0xFF.
func (h HexColor) String() string {
	if h.R%0x11 == 0 && h.G%0x11 == 0 && h.B%0x11 == 0 && h.A%0x11 == 0 {
		// Short form works.
		if h.A == 0xFF {
			// It's great when it's easy!
			return fmt.Sprintf("#%01X%01X%01X", h.R/0x11, h.G/0x11, h.B/0x11)
		} else {
			// Just need to add the alpha.
			return fmt.Sprintf("#%01X%01X%01X%01X", h.R/0x11, h.G/0x11, h.B/0x11, h.A/0x11)
		}
	} else {
		// Gotta use long form.
		if h.A == 0xFF {
			// Can skip the alpha channel, though.
			return fmt.Sprintf("#%02X%02X%02X", h.R, h.G, h.B)
		} else {
			// Doing things the hard way.
			return fmt.Sprintf("#%02X%02X%02X%02X", h.R, h.G, h.B, h.A)
		}
	}
}