Add a truncation function that puts the ellipsis in the middle

TruncateWithEllipsis cuts off the end of a string, which is the wrong
end for a path: what distinguishes two paths is often the last segment,
and it is the one thing the reader wants to see. Keep both ends and put
the ellipsis between them.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

pkg/utils/formatting.go

@@ -199,6 +199,49 @@ func TruncateWithEllipsis(str string, limit int) string {
 	return truncatedStr + "…"
 }
 
+// TruncateWithEllipsisInMiddle returns a string, truncated to a certain width,
+// with an ellipsis in the middle. Use it where the end of the string is as
+// informative as its beginning, e.g. for paths.
+func TruncateWithEllipsisInMiddle(str string, limit int) string {
+	if StringWidth(str) <= limit {
+		return str
+	}
+	if limit <= 2 {
+		return strings.Repeat(".", limit)
+	}
+
+	clusters := []string{}
+	widths := []int{}
+	graphemes := uniseg.NewGraphemes(str)
+	for graphemes.Next() {
+		clusters = append(clusters, graphemes.Str())
+		widths = append(widths, graphemes.Width())
+	}
+
+	// One column goes to the ellipsis; the rest is split between the two ends,
+	// with the odd one going to the front.
+	remaining := limit - 1
+	frontLimit := (remaining + 1) / 2
+
+	front := 0
+	frontWidth := 0
+	for front < len(clusters) && frontWidth+widths[front] <= frontLimit {
+		frontWidth += widths[front]
+		front++
+	}
+
+	// Whatever the front didn't use, e.g. because a wide grapheme didn't fit
+	// into it, is available to the back.
+	back := len(clusters)
+	backWidth := 0
+	for back > front && backWidth+widths[back-1] <= remaining-frontWidth {
+		backWidth += widths[back-1]
+		back--
+	}
+
+	return strings.Join(clusters[:front], "") + "…" + strings.Join(clusters[back:], "")
+}
+
 func SafeTruncate(str string, limit int) string {
 	if len(str) > limit {
 		return str[0:limit]

pkg/utils/formatting_test.go

@@ -162,6 +162,38 @@ func TestTruncateWithEllipsis(t *testing.T) {
 	}
 }
 
+func TestTruncateWithEllipsisInMiddle(t *testing.T) {
+	type scenario struct {
+		str      string
+		limit    int
+		expected string
+	}
+
+	scenarios := []scenario{
+		{"hello world !", 0, ""},
+		{"hello world !", 1, "."},
+		{"hello world !", 2, ".."},
+		{"hello world !", 3, "h…!"},
+		{"hello world !", 4, "he…!"},
+		{"hello world !", 5, "he… !"},
+		{"hello world !", 12, "hello …rld !"},
+		{"hello world !", 13, "hello world !"},
+		{"hello world !", 14, "hello world !"},
+		// A wide grapheme that doesn't fit into the front leaves its column to
+		// the back
+		{"大大大大", 5, "大…大"},
+		{"大大大大", 7, "大…大大"},
+		{"大大大大", 8, "大大大大"},
+		{"大大大大", 2, ".."},
+		{"大大大大", 1, "."},
+		{"大大大大", 0, ""},
+	}
+
+	for _, s := range scenarios {
+		assert.EqualValues(t, s.expected, TruncateWithEllipsisInMiddle(s.str, s.limit))
+	}
+}
+
 func TestRenderDisplayStrings(t *testing.T) {
 	type scenario struct {
 		input                   [][]string