format: add utf-8 aware truncate head function

The runewidth module only allows truncating at the end of strings. Add
a function to truncate at the beginning. It will be used for the table
widget in the next commit.

Signed-off-by: Robin Jarry <robin@jarry.cc>
Acked-by: Tim Culverhouse <tim@timculverhouse.com>

1 files changed, 26 insertions, 0 deletions

diff --git a/lib/format/format.go b/lib/format/format.go
index 3f00fe18..ad0b778c 100644
--- a/lib/format/format.go
+++ b/lib/format/format.go
@@ -11,6 +11,7 @@ import (
 	"git.sr.ht/~rjarry/aerc/models"
 	"github.com/emersion/go-message/mail"
 	"github.com/mattn/go-runewidth"
+	"github.com/rivo/uniseg"
 )
 
 // AddressForHumans formats the address. If the address's name
@@ -67,6 +68,31 @@ func CompactPath(name string, sep rune) (compact string) {
 	return
 }
 
+func TruncateHead(s string, w int, head string) string {
+	width := runewidth.StringWidth(s)
+	if width <= w {
+		return s
+	}
+	w -= runewidth.StringWidth(head)
+	pos := 0
+	g := uniseg.NewGraphemes(s)
+	for g.Next() {
+		var chWidth int
+		for _, r := range g.Runes() {
+			chWidth = runewidth.RuneWidth(r)
+			if chWidth > 0 {
+				break
+			}
+		}
+		if width-chWidth <= w {
+			pos, _ = g.Positions()
+			break
+		}
+		width -= chWidth
+	}
+	return head + s[pos:]
+}
+
 type Ctx struct {
 	FromAddress string
 	AccountName string