feat(ansi): implement DecodeColor function (#327)

* feat(ansi): implement ReadColor function This implements reading SGR colors according to the ITU T.416 and ECMA standards. It supports reading colors in the following formats: RGB, CMY, CMYK, and indexed colors. The function also supports reading color values separated by semicolons (;) and colons (:). * feat(ansi): color: support wezterm RGBA extension See https://wezfurlong.org/wezterm/escape-sequences.html#graphic-rendition-sgr * refactor(ansi): color: rename ReadColor to DecodeColor * Update color.go * Update color.go * fix: tests
charmbracelet · Jan 15, 2025 · 62b1fae · 62b1fae
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diff --git a/ansi/color.go b/ansi/color.go
@@ -194,3 +194,173 @@ func toRGBA(r, g, b uint32) (uint32, uint32, uint32, uint32) {
 	b |= b << 8
 	return r, g, b, 0xffff
 }
+
+// DecodeColor decodes a color from a slice of parameters. It returns the
+// number of parameters read and the color. This function is used to read SGR
+// color parameters following the ITU T.416 standard.
+//
+// It supports reading the following color types:
+//   - 0: implementation defined
+//   - 1: transparent
+//   - 2: RGB direct color
+//   - 3: CMY direct color
+//   - 4: CMYK direct color
+//   - 5: indexed color
+//   - 6: RGBA direct color (WezTerm extension)
+//
+// The parameters can be separated by semicolons (;) or colons (:). Mixing
+// separators is not allowed.
+//
+// The specs supports defining a color space id, a color tolerance value, and a
+// tolerance color space id. However, these values have no effect on the
+// returned color and will be ignored.
+//
+// This implementation includes a few modifications to the specs:
+//  1. Support for legacy color values separated by semicolons (;) with respect to RGB, and indexed colors
+//  2. Support ignoring and omitting the color space id (second parameter) with respect to RGB colors
+//  3. Support ignoring and omitting the 6th parameter with respect to RGB and CMY colors
+//  4. Support reading RGBA colors
+func DecodeColor(params []Parameter) (n int, co Color) {
+	if len(params) < 2 { // Need at least SGR type and color type
+		return 0, nil
+	}
+
+	// First parameter indicates one of 38, 48, or 58 (foreground, background, or underline)
+	s := params[0]
+	p := params[1]
+	colorType := p.Param(0)
+	n = 2
+
+	paramsfn := func() (p1, p2, p3, p4 int) {
+		// Where should we start reading the color?
+		switch {
+		case s.HasMore() && p.HasMore() && len(params) > 8 && params[2].HasMore() && params[3].HasMore() && params[4].HasMore() && params[5].HasMore() && params[6].HasMore() && params[7].HasMore():
+			// We have color space id, a 6th parameter, a tolerance value, and a tolerance color space
+			n += 7
+			return params[3].Param(0), params[4].Param(0), params[5].Param(0), params[6].Param(0)
+		case s.HasMore() && p.HasMore() && len(params) > 7 && params[2].HasMore() && params[3].HasMore() && params[4].HasMore() && params[5].HasMore() && params[6].HasMore():
+			// We have color space id, a 6th parameter, and a tolerance value
+			n += 6
+			return params[3].Param(0), params[4].Param(0), params[5].Param(0), params[6].Param(0)
+		case s.HasMore() && p.HasMore() && len(params) > 6 && params[2].HasMore() && params[3].HasMore() && params[4].HasMore() && params[5].HasMore():
+			// We have color space id and a 6th parameter
+			// 48 : 4 : : 1 : 2 : 3 :4
+			n += 5
+			return params[3].Param(0), params[4].Param(0), params[5].Param(0), params[6].Param(0)
+		case s.HasMore() && p.HasMore() && len(params) > 5 && params[2].HasMore() && params[3].HasMore() && params[4].HasMore() && !params[5].HasMore():
+			// We have color space
+			// 48 : 3 : : 1 : 2 : 3
+			n += 4
+			return params[3].Param(0), params[4].Param(0), params[5].Param(0), -1
+		case s.HasMore() && p.HasMore() && p.Param(0) == 2 && params[2].HasMore() && params[3].HasMore() && !params[4].HasMore():
+			// We have color values separated by colons (:)
+			// 48 : 2 : 1 : 2 : 3
+			fallthrough
+		case !s.HasMore() && !p.HasMore() && p.Param(0) == 2 && !params[2].HasMore() && !params[3].HasMore() && !params[4].HasMore():
+			// Support legacy color values separated by semicolons (;)
+			// 48 ; 2 ; 1 ; 2 ; 3
+			n += 3
+			return params[2].Param(0), params[3].Param(0), params[4].Param(0), -1
+		}
+		// Ambiguous SGR color
+		return -1, -1, -1, -1
+	}
+
+	switch colorType {
+	case 0: // implementation defined
+		return 2, nil
+	case 1: // transparent
+		return 2, color.Transparent
+	case 2: // RGB direct color
+		if len(params) < 5 {
+			return 0, nil
+		}
+
+		r, g, b, _ := paramsfn()
+		if r == -1 || g == -1 || b == -1 {
+			return 0, nil
+		}
+
+		co = color.RGBA{
+			R: uint8(r), //nolint:gosec
+			G: uint8(g), //nolint:gosec
+			B: uint8(b), //nolint:gosec
+			A: 0xff,
+		}
+		return
+
+	case 3: // CMY direct color
+		if len(params) < 5 {
+			return 0, nil
+		}
+
+		c, m, y, _ := paramsfn()
+		if c == -1 || m == -1 || y == -1 {
+			return 0, nil
+		}
+
+		co = color.CMYK{
+			C: uint8(c), //nolint:gosec
+			M: uint8(m), //nolint:gosec
+			Y: uint8(y), //nolint:gosec
+			K: 0,
+		}
+		return
+
+	case 4: // CMYK direct color
+		if len(params) < 6 {
+			return 0, nil
+		}
+
+		c, m, y, k := paramsfn()
+		if c == -1 || m == -1 || y == -1 || k == -1 {
+			return 0, nil
+		}
+
+		co = color.CMYK{
+			C: uint8(c), //nolint:gosec
+			M: uint8(m), //nolint:gosec
+			Y: uint8(y), //nolint:gosec
+			K: uint8(k), //nolint:gosec
+		}
+		return
+
+	case 5: // indexed color
+		if len(params) < 3 {
+			return 0, nil
+		}
+		switch {
+		case s.HasMore() && p.HasMore() && !params[2].HasMore():
+			// Colon separated indexed color
+			// 38 : 5 : 234
+		case !s.HasMore() && !p.HasMore() && !params[2].HasMore():
+			// Legacy semicolon indexed color
+			// 38 ; 5 ; 234
+		default:
+			return 0, nil
+		}
+		co = ExtendedColor(params[2].Param(0)) //nolint:gosec
+		return 3, co
+
+	case 6: // RGBA direct color
+		if len(params) < 6 {
+			return 0, nil
+		}
+
+		r, g, b, a := paramsfn()
+		if r == -1 || g == -1 || b == -1 || a == -1 {
+			return 0, nil
+		}
+
+		co = color.RGBA{
+			R: uint8(r), //nolint:gosec
+			G: uint8(g), //nolint:gosec
+			B: uint8(b), //nolint:gosec
+			A: uint8(a), //nolint:gosec
+		}
+		return
+
+	default:
+		return 0, nil
+	}
+}