feat: query and set terminal background, foreground, and cursor colors

This adds the ability to read and set terminal background, foreground,
and cursor color.

color.go

@@ -3,10 +3,65 @@ package tea
 import (
 	"fmt"
 	"image/color"
+	"math"
 	"strconv"
 	"strings"
 )
 
+// backgroundColorMsg is a message that requests the terminal background color.
+type backgroundColorMsg struct{}
+
+// BackgroundColor is a command that requests the terminal background color.
+func BackgroundColor() Msg {
+	return backgroundColorMsg{}
+}
+
+// foregroundColorMsg is a message that requests the terminal foreground color.
+type foregroundColorMsg struct{}
+
+// ForegroundColor is a command that requests the terminal foreground color.
+func ForegroundColor() Msg {
+	return foregroundColorMsg{}
+}
+
+// cursorColorMsg is a message that requests the terminal cursor color.
+type cursorColorMsg struct{}
+
+// CursorColor is a command that requests the terminal cursor color.
+func CursorColor() Msg {
+	return cursorColorMsg{}
+}
+
+// setBackgroundColorMsg is a message that sets the terminal background color.
+type setBackgroundColorMsg struct{ color.Color }
+
+// SetBackgroundColor is a command that sets the terminal background color.
+func SetBackgroundColor(c color.Color) Cmd {
+	return func() Msg {
+		return setBackgroundColorMsg{c}
+	}
+}
+
+// setForegroundColorMsg is a message that sets the terminal foreground color.
+type setForegroundColorMsg struct{ color.Color }
+
+// SetForegroundColor is a command that sets the terminal foreground color.
+func SetForegroundColor(c color.Color) Cmd {
+	return func() Msg {
+		return setForegroundColorMsg{c}
+	}
+}
+
+// setCursorColorMsg is a message that sets the terminal cursor color.
+type setCursorColorMsg struct{ color.Color }
+
+// SetCursorColor is a command that sets the terminal cursor color.
+func SetCursorColor(c color.Color) Cmd {
+	return func() Msg {
+		return setCursorColorMsg{c}
+	}
+}
+
 // ForegroundColorMsg represents a foreground color message.
 // This message is emitted when the program requests the terminal foreground
 // color.
@@ -17,6 +72,11 @@ func (e ForegroundColorMsg) String() string {
 	return colorToHex(e)
 }
 
+// IsDark returns whether the color is dark.
+func (e ForegroundColorMsg) IsDark() bool {
+	return isDarkColor(e)
+}
+
 // BackgroundColorMsg represents a background color message.
 // This message is emitted when the program requests the terminal background
 // color.
@@ -27,6 +87,11 @@ func (e BackgroundColorMsg) String() string {
 	return colorToHex(e)
 }
 
+// IsDark returns whether the color is dark.
+func (e BackgroundColorMsg) IsDark() bool {
+	return isDarkColor(e)
+}
+
 // CursorColorMsg represents a cursor color change message.
 // This message is emitted when the program requests the terminal cursor color.
 type CursorColorMsg struct{ color.Color }
@@ -36,6 +101,11 @@ func (e CursorColorMsg) String() string {
 	return colorToHex(e)
 }
 
+// IsDark returns whether the color is dark.
+func (e CursorColorMsg) IsDark() bool {
+	return isDarkColor(e)
+}
+
 type shiftable interface {
 	~uint | ~uint16 | ~uint32 | ~uint64
 }
@@ -80,3 +150,68 @@ func xParseColor(s string) color.Color {
 	}
 	return color.Black
 }
+
+func getMaxMin(a, b, c float64) (max, min float64) {
+	if a > b {
+		max = a
+		min = b
+	} else {
+		max = b
+		min = a
+	}
+	if c > max {
+		max = c
+	} else if c < min {
+		min = c
+	}
+	return max, min
+}
+
+func round(x float64) float64 {
+	return math.Round(x*1000) / 1000
+}
+
+// rgbToHSL converts an RGB triple to an HSL triple.
+func rgbToHSL(r, g, b uint8) (h, s, l float64) {
+	// convert uint32 pre-multiplied value to uint8
+	// The r,g,b values are divided by 255 to change the range from 0..255 to 0..1:
+	Rnot := float64(r) / 255
+	Gnot := float64(g) / 255
+	Bnot := float64(b) / 255
+	Cmax, Cmin := getMaxMin(Rnot, Gnot, Bnot)
+	Δ := Cmax - Cmin
+	// Lightness calculation:
+	l = (Cmax + Cmin) / 2
+	// Hue and Saturation Calculation:
+	if Δ == 0 {
+		h = 0
+		s = 0
+	} else {
+		switch Cmax {
+		case Rnot:
+			h = 60 * (math.Mod((Gnot-Bnot)/Δ, 6))
+		case Gnot:
+			h = 60 * (((Bnot - Rnot) / Δ) + 2)
+		case Bnot:
+			h = 60 * (((Rnot - Gnot) / Δ) + 4)
+		}
+		if h < 0 {
+			h += 360
+		}
+
+		s = Δ / (1 - math.Abs((2*l)-1))
+	}
+
+	return h, round(s), round(l)
+}
+
+// isDarkColor returns whether the given color is dark.
+func isDarkColor(c color.Color) bool {
+	if c == nil {
+		return true
+	}
+
+	r, g, b, _ := c.RGBA()
+	_, _, l := rgbToHSL(uint8(r>>8), uint8(g>>8), uint8(b>>8))
+	return l < 0.5
+}

tea.go

@@ -392,6 +392,30 @@ func (p *Program) eventLoop(model Model, cmds chan Cmd) (Model, error) {
 				p.renderer.execute(ansi.DisableBracketedPaste)
 				p.bpActive = false
 
+			case setBackgroundColorMsg:
+				if msg.Color != nil {
+					p.renderer.execute(ansi.SetBackgroundColor(msg.Color))
+				}
+
+			case setForegroundColorMsg:
+				if msg.Color != nil {
+					p.renderer.execute(ansi.SetForegroundColor(msg.Color))
+				}
+
+			case setCursorColorMsg:
+				if msg.Color != nil {
+					p.renderer.execute(ansi.SetCursorColor(msg.Color))
+				}
+
+			case backgroundColorMsg:
+				p.renderer.execute(ansi.RequestBackgroundColor)
+
+			case foregroundColorMsg:
+				p.renderer.execute(ansi.RequestForegroundColor)
+
+			case cursorColorMsg:
+				p.renderer.execute(ansi.RequestCursorColor)
+
 			case execMsg:
 				// NB: this blocks.
 				p.exec(msg.cmd, msg.fn)