feat: add underline style and color

This adds support for setting the underline style and color. The
underline style can be set to single, double, curly, dotted, or dashed.
The color of the underline can be set to any color.

See https://sw.kovidgoyal.net/kitty/underlines/

Author: aymanbagabas

get.go

@@ -21,7 +21,7 @@ func (s Style) GetItalic() bool {
 // GetUnderline returns the style's underline value. If no value is set false is
 // returned.
 func (s Style) GetUnderline() bool {
-	return s.getAsBool(underlineKey, false)
+	return s.ul != NoUnderline
 }
 
 // GetStrikethrough returns the style's strikethrough value. If no value is set false

set.go

@@ -13,6 +13,10 @@ func (s *Style) set(key propKey, value interface{}) {
 		s.fgColor = colorOrNil(value)
 	case backgroundKey:
 		s.bgColor = colorOrNil(value)
+	case underlineColorKey:
+		s.ulColor = colorOrNil(value)
+	case underlineKey:
+		s.ul = value.(Underline)
 	case widthKey:
 		s.width = max(0, value.(int))
 	case heightKey:
@@ -95,6 +99,10 @@ func (s *Style) setFrom(key propKey, i Style) {
 		s.set(foregroundKey, i.fgColor)
 	case backgroundKey:
 		s.set(backgroundKey, i.bgColor)
+	case underlineColorKey:
+		s.set(underlineColorKey, i.ulColor)
+	case underlineKey:
+		s.set(underlineKey, i.ul)
 	case widthKey:
 		s.set(widthKey, i.width)
 	case heightKey:
@@ -177,7 +185,23 @@ func (s Style) Italic(v bool) Style {
 // whitespace like margins and padding. To change this behavior set
 // [Style.UnderlineSpaces].
 func (s Style) Underline(v bool) Style {
-	s.set(underlineKey, v)
+	if v {
+		return s.UnderlineStyle(SingleUnderline)
+	}
+	return s.UnderlineStyle(NoUnderline)
+}
+
+// UnderlineStyle sets the underline style. This can be used to set the underline
+// to be a single, double, curly, dotted, or dashed line.
+func (s Style) UnderlineStyle(u Underline) Style {
+	s.set(underlineKey, u)
+	return s
+}
+
+// UnderlineColor sets the color of the underline. By default, the underline
+// will be the same color as the foreground.
+func (s Style) UnderlineColor(c color.Color) Style {
+	s.set(underlineColorKey, c)
 	return s
 }
 

style.go

@@ -19,7 +19,6 @@ const (
 	// Boolean props come first.
 	boldKey propKey = 1 << iota
 	italicKey
-	underlineKey
 	strikethroughKey
 	reverseKey
 	blinkKey
@@ -29,8 +28,10 @@ const (
 	colorWhitespaceKey
 
 	// Non-boolean props.
+	underlineKey
 	foregroundKey
 	backgroundKey
+	underlineColorKey
 	widthKey
 	heightKey
 	alignHorizontalKey
@@ -96,6 +97,24 @@ func (p props) has(k propKey) bool {
 	return p&props(k) != 0
 }
 
+// Underline is the style of the underline.
+type Underline uint8
+
+const (
+	// NoUnderline is no underline.
+	NoUnderline = Underline(ansi.NoUnderlineStyle)
+	// SingleUnderline is a single underline. This is the default when underline is enabled.
+	SingleUnderline = Underline(ansi.SingleUnderlineStyle)
+	// DoubleUnderline is a double underline.
+	DoubleUnderline = Underline(ansi.DoubleUnderlineStyle)
+	// CurlyUnderline is a curly underline.
+	CurlyUnderline = Underline(ansi.CurlyUnderlineStyle)
+	// DottedUnderline is a dotted underline.
+	DottedUnderline = Underline(ansi.DottedUnderlineStyle)
+	// DashedUnderline is a dashed underline.
+	DashedUnderline = Underline(ansi.DashedUnderlineStyle)
+)
+
 // NewStyle returns a new, empty Style. While it's syntactic sugar for the
 // [Style]{} primitive, it's recommended to use this function for creating styles
 // in case the underlying implementation changes.
@@ -114,6 +133,9 @@ type Style struct {
 	// props that have values
 	fgColor color.Color
 	bgColor color.Color
+	ulColor color.Color
+
+	ul Underline
 
 	width  int
 	height int
@@ -234,15 +256,16 @@ func (s Style) Render(strs ...string) string {
 
 		bold          = s.getAsBool(boldKey, false)
 		italic        = s.getAsBool(italicKey, false)
-		underline     = s.getAsBool(underlineKey, false)
 		strikethrough = s.getAsBool(strikethroughKey, false)
 		reverse       = s.getAsBool(reverseKey, false)
 		blink         = s.getAsBool(blinkKey, false)
 		faint         = s.getAsBool(faintKey, false)
 
 		fg = s.getAsColor(foregroundKey)
 		bg = s.getAsColor(backgroundKey)
+		ul = s.getAsColor(underlineColorKey)
 
+		underline       = s.ul != NoUnderline
 		width           = s.getAsInt(widthKey)
 		height          = s.getAsInt(heightKey)
 		horizontalAlign = s.getAsPosition(alignHorizontalKey)
@@ -322,8 +345,18 @@ func (s Style) Render(strs ...string) string {
 		}
 	}
 
+	if ul != noColor {
+		te = te.UnderlineColor(ul)
+		if colorWhitespace {
+			teWhitespace = teWhitespace.UnderlineColor(ul)
+		}
+		if useSpaceStyler {
+			teSpace = teSpace.UnderlineColor(ul)
+		}
+	}
+
 	if underline {
-		te = te.Underline()
+		te = te.UnderlineStyle(ansi.UnderlineStyle(s.ul))
 	}
 	if strikethrough {
 		te = te.Strikethrough()

style_test.go

@@ -30,6 +30,10 @@ func TestUnderline(t *testing.T) {
 			NewStyle().UnderlineSpaces(true),
 			"ab\x1b[4m \x1b[mc",
 		},
+		{
+			NewStyle().UnderlineStyle(CurlyUnderline),
+			"\x1b[4;4:3ma\x1b[m\x1b[4;4:3mb\x1b[m\x1b[4m \x1b[m\x1b[4;4:3mc\x1b[m",
+		},
 	}
 
 	for i, tc := range tt {

unset.go

@@ -19,8 +19,7 @@ func (s Style) UnsetItalic() Style {
 
 // UnsetUnderline removes the underline style rule, if set.
 func (s Style) UnsetUnderline() Style {
-	s.unset(underlineKey)
-	return s
+	return s.Underline(false)
 }
 
 // UnsetStrikethrough removes the strikethrough style rule, if set.