trim prefix

runewidth.go

@@ -2,6 +2,7 @@ package runewidth
 
 import (
 	"os"
+	"strings"
 
 	"github.com/rivo/uniseg"
 )
@@ -214,6 +215,42 @@ func (c *Condition) Truncate(s string, w int, tail string) string {
 	return s[:pos] + tail
 }
 
+// TruncateLeft cuts w cells from the beginning of the `s`.
+func (c *Condition) TruncateLeft(s string, w int, prefix string) string {
+	if c.StringWidth(s) <= w {
+		return prefix
+	}
+
+	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 {
+			if width < w {
+				_, pos = g.Positions()
+				prefix += strings.Repeat(" ", width+chWidth-w)
+			} else {
+				pos, _ = g.Positions()
+			}
+
+			break
+		}
+
+		width += chWidth
+	}
+
+	return prefix + s[pos:]
+}
+
 // Wrap return string wrapped with w cells
 func (c *Condition) Wrap(s string, w int) string {
 	width := 0
@@ -291,6 +328,11 @@ func Truncate(s string, w int, tail string) string {
 	return DefaultCondition.Truncate(s, w, tail)
 }
 
+// TruncateLeft cuts w cells from the beginning of the `s`.
+func TruncateLeft(s string, w int, prefix string) string {
+	return DefaultCondition.TruncateLeft(s, w, prefix)
+}
+
 // Wrap return string wrapped with w cells
 func Wrap(s string, w int) string {
 	return DefaultCondition.Wrap(s, w)

runewidth_test.go

@@ -380,6 +380,32 @@ func TestTruncateNoNeeded(t *testing.T) {
 	}
 }
 
+var truncatelefttests = []struct {
+	s      string
+	w      int
+	prefix string
+	out    string
+}{
+	{"source", 4, "", "ce"},
+	{"source", 4, "...", "...ce"},
+	{"あいうえお", 6, "", "えお"},
+	{"あいうえお", 6, "...", "...えお"},
+	{"あいうえお", 10, "", ""},
+	{"あいうえお", 10, "...", "..."},
+	{"あいうえお", 5, "", " えお"},
+	{"Aあいうえお", 5, "", "うえお"},
+}
+
+func TestTruncateLeft(t *testing.T) {
+	t.Parallel()
+
+	for _, tt := range truncatelefttests {
+		if out := TruncateLeft(tt.s, tt.w, tt.prefix); out != tt.out {
+			t.Errorf("TruncateLeft(%q) = %q, want %q", tt.s, out, tt.out)
+		}
+	}
+}
+
 var isneutralwidthtests = []struct {
 	in  rune
 	out bool