trim prefix

runewidth.go

@@ -214,6 +214,35 @@ func (c *Condition) Truncate(s string, w int, tail string) string {
 	return s[:pos] + tail
 }
 
+// TrimPrefix cuts w cells from the beginning of the `s`.
+func (c *Condition) TrimPrefix(s string, w int, tail string) string {
+	if c.StringWidth(s) <= w {
+		return "" + tail
+	}
+
+	var width int
+	pos := len(s)
+
+	g := uniseg.NewGraphemes(s)
+	for g.Next() {
+		var chWidth int
+		for _, r := range g.Runes() {
+			chWidth = c.RuneWidth(r)
+			if chWidth > 0 {
+				break // See StringWidth() for details.
+			}
+		}
+		if width+chWidth > w {
+			pos, _ = g.Positions()
+
+			break
+		}
+		width += chWidth
+	}
+
+	return s[pos:] + tail
+}
+
 // Wrap return string wrapped with w cells
 func (c *Condition) Wrap(s string, w int) string {
 	width := 0
@@ -291,6 +320,11 @@ func Truncate(s string, w int, tail string) string {
 	return DefaultCondition.Truncate(s, w, tail)
 }
 
+// TrimPrefix cuts w cells from the beginning of the `s`.
+func TrimPrefix(s string, w int, tail string) string {
+	return DefaultCondition.TrimPrefix(s, w, tail)
+}
+
 // Wrap return string wrapped with w cells
 func Wrap(s string, w int) string {
 	return DefaultCondition.Wrap(s, w)

runewidth_test.go

@@ -380,6 +380,76 @@ func TestTruncateNoNeeded(t *testing.T) {
 	}
 }
 
+func Test_TrimPrefix(t *testing.T) {
+	t.Parallel()
+
+	t.Run("ascii", func(t *testing.T) {
+		t.Parallel()
+		s := "source"
+		expected := "ce"
+
+		out := TrimPrefix(s, 4, "")
+		if out != expected {
+			t.Errorf("TrimPrefix(%q) = %q, want %q", s, out, expected)
+		}
+	})
+
+	t.Run("ascii: with tail", func(t *testing.T) {
+		t.Parallel()
+		s := "source"
+		expected := "ce..."
+
+		out := TrimPrefix(s, 4, "...")
+		if out != expected {
+			t.Errorf("TrimPrefix(%q) = %q, want %q", s, out, expected)
+		}
+	})
+
+	t.Run("non ascii", func(t *testing.T) {
+		t.Parallel()
+		s := "あいうえお"
+		expected := "えお"
+
+		out := TrimPrefix(s, 6, "")
+		if out != expected {
+			t.Errorf("TrimPrefix(%q) = %q, want %q", s, out, expected)
+		}
+	})
+
+	t.Run("non ascii: with tail", func(t *testing.T) {
+		t.Parallel()
+		s := "あいうえお"
+		expected := "えお..."
+
+		out := TrimPrefix(s, 6, "...")
+		if out != expected {
+			t.Errorf("TrimPrefix(%q) = %q, want %q", s, out, expected)
+		}
+	})
+
+	t.Run("trim all", func(t *testing.T) {
+		t.Parallel()
+		s := "あいうえお"
+		expected := ""
+
+		out := TrimPrefix(s, 10, "")
+		if out != expected {
+			t.Errorf("TrimPrefix(%q) = %q, want %q", s, out, expected)
+		}
+	})
+
+	t.Run("trim all: with tail", func(t *testing.T) {
+		t.Parallel()
+		s := "あいうえお"
+		expected := "..."
+
+		out := TrimPrefix(s, 10, "...")
+		if out != expected {
+			t.Errorf("TrimPrefix(%q) = %q, want %q", s, out, expected)
+		}
+	})
+}
+
 var isneutralwidthtests = []struct {
 	in  rune
 	out bool