fix(progress): last gradient color off by one (#338)

* fix(progress): add failing test

The progress bar component does not render the gradient 100% correctly:
The very last color that appears in the rendered progress bar should be
the second color defined in the gradient but it is not, due to an
off-by-one one error.

This test case shows this. The next commit will contain the fix.

* fix(progress): use the second gradient color as the last char of the bar

Due to an off-by-one error, the very last rendered color+char was never
exactly the second specified gradient color. The fixed code looks less
elegant unfortunately, but now the last color is the desired one.

progress/progress.go

@@ -299,10 +299,15 @@ func (m Model) barView(b *strings.Builder, percent float64, textWidth int) {
 	if m.useRamp {
 		// Gradient fill
 		for i := 0; i < fw; i++ {
-			if m.scaleRamp {
-				p = float64(i) / float64(fw)
+			if fw == 1 {
+				// this is up for debate: in a gradient of width=1, should the
+				// single character rendered be the first color, the last color
+				// or exactly 50% inbetween? I opted for 50%
+				p = 0.5
+			} else if m.scaleRamp {
+				p = float64(i) / float64(fw-1)
 			} else {
-				p = float64(i) / float64(tw)
+				p = float64(i) / float64(tw-1)
 			}
 			c := m.rampColorA.BlendLuv(m.rampColorB, p).Hex()
 			b.WriteString(termenv.

progress/progress_test.go

@@ -0,0 +1,66 @@
+package progress
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/muesli/termenv"
+)
+
+const (
+	AnsiReset = "\x1b[0m"
+)
+
+func TestGradient(t *testing.T) {
+
+	colA := "#FF0000"
+	colB := "#00FF00"
+
+	var p Model
+	var descr string
+
+	for _, scale := range []bool{false, true} {
+		opts := []Option{
+			WithColorProfile(termenv.TrueColor), WithoutPercentage(),
+		}
+		if scale {
+			descr = "progress bar with scaled gradient"
+			opts = append(opts, WithScaledGradient(colA, colB))
+		} else {
+			descr = "progress bar with gradient"
+			opts = append(opts, WithGradient(colA, colB))
+		}
+
+		t.Run(descr, func(t *testing.T) {
+			p = New(opts...)
+
+			// build the expected colors by colorizing an empty string and then cutting off the following reset sequence
+			sb := strings.Builder{}
+			sb.WriteString(termenv.String("").Foreground(p.color(colA)).String())
+			expFirst := strings.Split(sb.String(), AnsiReset)[0]
+			sb.Reset()
+			sb.WriteString(termenv.String("").Foreground(p.color(colB)).String())
+			expLast := strings.Split(sb.String(), AnsiReset)[0]
+
+			for _, width := range []int{3, 5, 50} {
+				p.Width = width
+				res := p.ViewAs(1.0)
+
+				// extract colors from the progrss bar by splitting at p.Full+AnsiReset, leaving us with just the color sequences
+				colors := strings.Split(res, string(p.Full)+AnsiReset)
+
+				// discard the last color, because it is empty (no new color comes after the last char of the bar)
+				colors = colors[0 : len(colors)-1]
+
+				if expFirst != colors[0] {
+					t.Errorf("expected first color of bar to be first gradient color %q, instead got %q", expFirst, colors[0])
+				}
+
+				if expLast != colors[len(colors)-1] {
+					t.Errorf("expected last color of bar to be second gradient color %q, instead got %q", expLast, colors[len(colors)-1])
+				}
+			}
+		})
+	}
+
+}