text: remove String, Layout and add Glyph

In preparation for using Shaper with an io.Reader, rework the API to not refer
to strings. In particular, introduce Glyph for holding the rune in addition to
the advance. For fast traversing of the underlying text, add Len to Line with
the UTF8 length.

Layout is a useless wrapper around []Line; remove it while we're
here.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

widget/label.go

@@ -32,11 +32,12 @@ type lineIterator struct {
 	Offset    image.Point
 
 	y, prevDesc fixed.Int26_6
+	txtOff      int
 }
 
 const inf = 1e6
 
-func (l *lineIterator) Next() (text.String, f32.Point, bool) {
+func (l *lineIterator) Next() (int, int, []text.Glyph, f32.Point, bool) {
 	for len(l.Lines) > 0 {
 		line := l.Lines[0]
 		l.Lines = l.Lines[1:]
@@ -50,42 +51,41 @@ func (l *lineIterator) Next() (text.String, f32.Point, bool) {
 		if (off.Y + line.Bounds.Min.Y).Floor() > l.Clip.Max.Y {
 			break
 		}
+		layout := line.Layout
+		start := l.txtOff
+		l.txtOff += line.Len
 		if (off.Y + line.Bounds.Max.Y).Ceil() < l.Clip.Min.Y {
 			continue
 		}
-		str := line.Text
-		for len(str.Advances) > 0 {
-			adv := str.Advances[0]
+		for len(layout) > 0 {
+			g := layout[0]
+			adv := g.Advance
 			if (off.X + adv + line.Bounds.Max.X - line.Width).Ceil() >= l.Clip.Min.X {
 				break
 			}
 			off.X += adv
-			_, s := utf8.DecodeRuneInString(str.String)
-			str.String = str.String[s:]
-			str.Advances = str.Advances[1:]
+			layout = layout[1:]
+			start += utf8.RuneLen(g.Rune)
 		}
-		n := 0
+		end := start
 		endx := off.X
-		for i, adv := range str.Advances {
+		for i, g := range layout {
 			if (endx + line.Bounds.Min.X).Floor() > l.Clip.Max.X {
-				str.String = str.String[:n]
-				str.Advances = str.Advances[:i]
+				layout = layout[:i]
 				break
 			}
-			_, s := utf8.DecodeRuneInString(str.String[n:])
-			n += s
-			endx += adv
+			end += utf8.RuneLen(g.Rune)
+			endx += g.Advance
 		}
 		offf := f32.Point{X: float32(off.X) / 64, Y: float32(off.Y) / 64}
-		return str, offf, true
+		return start, end, layout, offf, true
 	}
-	return text.String{}, f32.Point{}, false
+	return 0, 0, nil, f32.Point{}, false
 }
 
 func (l Label) Layout(gtx *layout.Context, s text.Shaper, font text.Font, txt string) {
 	cs := gtx.Constraints
-	textLayout := s.Layout(gtx, font, txt, text.LayoutOptions{MaxWidth: cs.Width.Max})
-	lines := textLayout.Lines
+	lines := s.Layout(gtx, font, txt, text.LayoutOptions{MaxWidth: cs.Width.Max})
 	if max := l.MaxLines; max > 0 && len(lines) > max {
 		lines = lines[:max]
 	}
@@ -100,7 +100,7 @@ func (l Label) Layout(gtx *layout.Context, s text.Shaper, font text.Font, txt st
 		Width:     dims.Size.X,
 	}
 	for {
-		str, off, ok := it.Next()
+		start, end, layout, off, ok := it.Next()
 		if !ok {
 			break
 		}
@@ -108,7 +108,8 @@ func (l Label) Layout(gtx *layout.Context, s text.Shaper, font text.Font, txt st
 		var stack op.StackOp
 		stack.Push(gtx.Ops)
 		op.TransformOp{}.Offset(off).Add(gtx.Ops)
-		s.Shape(gtx, font, str).Add(gtx.Ops)
+		str := txt[start:end]
+		s.Shape(gtx, font, str, layout).Add(gtx.Ops)
 		paint.PaintOp{Rect: lclip}.Add(gtx.Ops)
 		stack.Pop()
 	}