Inform ansiString() about terminal color count

So that it can downsample colors when needed.

moar.go

@@ -500,7 +500,7 @@ func main() {
 		panic("Invariant broken: stdout is not a terminal")
 	}
 
-	screen, err := twin.NewScreenWithMouseMode(*mouseMode)
+	screen, err := twin.NewScreenWithMouseModeAndColorType(*mouseMode, *terminalColorsCount)
 	if err != nil {
 		// Ref: https://github.com/walles/moar/issues/149
 		log.Debug("Failed to set up screen for paging, pumping to stdout instead: ", err)

twin/colors.go

@@ -1,6 +1,8 @@
 package twin
 
-import "fmt"
+import (
+	"fmt"
+)
 
 // Create using NewColor16(), NewColor256 or NewColor24Bit(), or use
 // ColorDefault.
@@ -83,7 +85,8 @@ func (color Color) colorValue() uint32 {
 // Render color into an ANSI string.
 //
 // Ref: https://en.wikipedia.org/wiki/ANSI_escape_code#SGR_(Select_Graphic_Rendition)_parameters
-func (color Color) ansiString(foreground bool) string {
+func (color Color) ansiString(foreground bool, terminalColorCount ColorType) string {
+	// FIXME: Downsample colors to at most terminalColorCount colors as needed.
 	value := color.colorValue()
 
 	fgBgMarker := "3"
@@ -124,14 +127,14 @@ func (color Color) ansiString(foreground bool) string {
 	panic(fmt.Errorf("unhandled color type=%d value=%#x", color.colorType(), value))
 }
 
-func (color Color) ForegroundAnsiString() string {
+func (color Color) ForegroundAnsiString(terminalColorCount ColorType) string {
 	// FIXME: Test this function with all different color types.
-	return color.ansiString(true)
+	return color.ansiString(true, terminalColorCount)
 }
 
-func (color Color) BackgroundAnsiString() string {
+func (color Color) BackgroundAnsiString(terminalColorCount ColorType) string {
 	// FIXME: Test this function with all different color types.
-	return color.ansiString(false)
+	return color.ansiString(false, terminalColorCount)
 }
 
 func (color Color) String() string {

twin/screen.go

@@ -77,6 +77,8 @@ type UnixScreen struct {
 
 	ttyOut        *os.File
 	oldTtyOutMode uint32 //nolint Windows only
+
+	terminalColorCount ColorType
 }
 
 // Example event: "\x1b[<65;127;41M"
@@ -94,18 +96,29 @@ type UnixScreen struct {
 // * "M" marks the end of the mouse event.
 var MOUSE_EVENT_REGEX = regexp.MustCompile("^\x1b\\[<([0-9]+);([0-9]+);([0-9]+)M")
 
+// NewScreen() requires Close() to be called after you are done with your new
+// screen, most likely somewhere in your shutdown code.
 func NewScreen() (Screen, error) {
 	return NewScreenWithMouseMode(MouseModeAuto)
 }
 
-// NewScreen() requires Close() to be called after you are done with your new
-// screen, most likely somewhere in your shutdown code.
 func NewScreenWithMouseMode(mouseMode MouseMode) (Screen, error) {
+	terminalColorCount := ColorType24bit
+	if strings.Contains(os.Getenv("TERM"), "256") {
+		// Covers "xterm-256color" as used by the macOS Terminal
+		terminalColorCount = ColorType256
+	}
+	return NewScreenWithMouseModeAndColorType(mouseMode, terminalColorCount)
+}
+
+func NewScreenWithMouseModeAndColorType(mouseMode MouseMode, terminalColorCount ColorType) (Screen, error) {
 	if !term.IsTerminal(int(os.Stdout.Fd())) {
 		return nil, fmt.Errorf("stdout (fd=%d) must be a terminal for paging to work", os.Stdout.Fd())
 	}
 
-	screen := UnixScreen{}
+	screen := UnixScreen{
+		terminalColorCount: terminalColorCount,
+	}
 
 	// The number "80" here is from manual testing on my MacBook:
 	//
@@ -505,7 +518,7 @@ func (screen *UnixScreen) Clear() {
 
 // Returns the rendered line, plus how many information carrying cells went into
 // it
-func renderLine(row []Cell) (string, int) {
+func renderLine(row []Cell, terminalColorCount ColorType) (string, int) {
 	// Strip trailing whitespace
 	lastSignificantCellIndex := len(row) - 1
 	for ; lastSignificantCellIndex >= 0; lastSignificantCellIndex-- {
@@ -538,7 +551,7 @@ func renderLine(row []Cell) (string, int) {
 		}
 
 		if style != lastStyle {
-			builder.WriteString(style.RenderUpdateFrom(lastStyle))
+			builder.WriteString(style.RenderUpdateFrom(lastStyle, terminalColorCount))
 			lastStyle = style
 		}
 
@@ -547,7 +560,7 @@ func renderLine(row []Cell) (string, int) {
 
 	// Clear to end of line
 	// https://en.wikipedia.org/wiki/ANSI_escape_code#CSI_(Control_Sequence_Introducer)_sequences
-	builder.WriteString(StyleDefault.RenderUpdateFrom(lastStyle))
+	builder.WriteString(StyleDefault.RenderUpdateFrom(lastStyle, terminalColorCount))
 	builder.WriteString("\x1b[K")
 
 	return builder.String(), len(row)
@@ -572,7 +585,7 @@ func (screen *UnixScreen) showNLines(height int, clearFirst bool) {
 	}
 
 	for row := 0; row < height; row++ {
-		rendered, lineLength := renderLine(screen.cells[row])
+		rendered, lineLength := renderLine(screen.cells[row], screen.terminalColorCount)
 		builder.WriteString(rendered)
 
 		wasLastLine := row == (height - 1)

twin/screen_test.go

@@ -61,7 +61,7 @@ func TestRenderLine(t *testing.T) {
 		},
 	}
 
-	rendered, count := renderLine(row)
+	rendered, count := renderLine(row, ColorType16)
 	assert.Equal(t, count, 2)
 	reset := ""
 	reversed := ""
@@ -76,7 +76,7 @@ func TestRenderLine(t *testing.T) {
 func TestRenderLineEmpty(t *testing.T) {
 	row := []Cell{}
 
-	rendered, count := renderLine(row)
+	rendered, count := renderLine(row, ColorType16)
 	assert.Equal(t, count, 0)
 
 	// All lines are expected to stand on their own, so we always need to clear
@@ -92,7 +92,7 @@ func TestRenderLineLastReversed(t *testing.T) {
 		},
 	}
 
-	rendered, count := renderLine(row)
+	rendered, count := renderLine(row, ColorType16)
 	assert.Equal(t, count, 1)
 	reset := ""
 	reversed := ""
@@ -110,7 +110,7 @@ func TestRenderLineLastNonSpace(t *testing.T) {
 		},
 	}
 
-	rendered, count := renderLine(row)
+	rendered, count := renderLine(row, ColorType16)
 	assert.Equal(t, count, 1)
 	reset := ""
 	clearToEol := ""
@@ -131,7 +131,7 @@ func TestRenderLineLastReversedPlusTrailingSpace(t *testing.T) {
 		},
 	}
 
-	rendered, count := renderLine(row)
+	rendered, count := renderLine(row, ColorType16)
 	assert.Equal(t, count, 1)
 	reset := ""
 	reversed := ""
@@ -153,7 +153,7 @@ func TestRenderLineOnlyTrailingSpaces(t *testing.T) {
 		},
 	}
 
-	rendered, count := renderLine(row)
+	rendered, count := renderLine(row, ColorType16)
 	assert.Equal(t, count, 0)
 
 	// All lines are expected to stand on their own, so we always need to clear
@@ -169,7 +169,7 @@ func TestRenderLineLastReversedSpaces(t *testing.T) {
 		},
 	}
 
-	rendered, count := renderLine(row)
+	rendered, count := renderLine(row, ColorType16)
 	assert.Equal(t, count, 1)
 	reset := ""
 	reversed := ""
@@ -186,7 +186,7 @@ func TestRenderLineNonPrintable(t *testing.T) {
 		},
 	}
 
-	rendered, count := renderLine(row)
+	rendered, count := renderLine(row, ColorType16)
 	assert.Equal(t, count, 1)
 	reset := ""
 	white := ""
@@ -207,7 +207,7 @@ func TestRenderHyperlinkAtEndOfLine(t *testing.T) {
 		},
 	}
 
-	rendered, count := renderLine(row)
+	rendered, count := renderLine(row, ColorType16)
 	assert.Equal(t, count, 1)
 
 	assert.Equal(t,
@@ -232,7 +232,7 @@ func TestMultiCharHyperlink(t *testing.T) {
 		},
 	}
 
-	rendered, count := renderLine(row)
+	rendered, count := renderLine(row, ColorType16)
 	assert.Equal(t, count, 3)
 
 	assert.Equal(t,

twin/styles.go

@@ -137,7 +137,7 @@ func (style Style) Foreground(color Color) Style {
 
 // Emit an ANSI escape sequence switching from a previous style to the current
 // one.
-func (current Style) RenderUpdateFrom(previous Style) string {
+func (current Style) RenderUpdateFrom(previous Style, terminalColorCount ColorType) string {
 	if current == previous {
 		// Shortcut for the common case
 		return ""
@@ -150,11 +150,11 @@ func (current Style) RenderUpdateFrom(previous Style) string {
 
 	var builder strings.Builder
 	if current.fg != previous.fg {
-		builder.WriteString(current.fg.ForegroundAnsiString())
+		builder.WriteString(current.fg.ForegroundAnsiString(terminalColorCount))
 	}
 
 	if current.bg != previous.bg {
-		builder.WriteString(current.bg.BackgroundAnsiString())
+		builder.WriteString(current.bg.BackgroundAnsiString(terminalColorCount))
 	}
 
 	// Handle AttrDim / AttrBold changes

twin/styles_test.go

@@ -12,6 +12,6 @@ func TestHyperlinkToNormal(t *testing.T) {
 
 	style := StyleDefault.WithHyperlink(&url)
 	assert.Equal(t,
-		strings.ReplaceAll(StyleDefault.RenderUpdateFrom(style), "", "ESC"),
+		strings.ReplaceAll(StyleDefault.RenderUpdateFrom(style, ColorType16), "", "ESC"),
 		"ESC]8;;ESC\\")
 }