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/editor.go

@@ -273,11 +273,13 @@ func (e *Editor) layout(gtx *layout.Context, sh text.Shaper) {
 	}
 	e.shapes = e.shapes[:0]
 	for {
-		str, off, ok := it.Next()
+		start, end, layout, off, ok := it.Next()
 		if !ok {
 			break
 		}
-		path := sh.Shape(gtx, e.font, str)
+		// TODO: remove
+		str := e.rr.String()[start:end]
+		path := sh.Shape(gtx, e.font, str, layout)
 		e.shapes = append(e.shapes, line{off, path})
 	}
 
@@ -433,15 +435,13 @@ func (e *Editor) moveCoord(c unit.Converter, pos image.Point) {
 func (e *Editor) layoutText(c unit.Converter, s text.Shaper, font text.Font) ([]text.Line, layout.Dimensions) {
 	txt := e.rr.String()
 	opts := text.LayoutOptions{MaxWidth: e.maxWidth}
-	textLayout := s.Layout(c, font, txt, opts)
-	lines := textLayout.Lines
+	lines := s.Layout(c, font, txt, opts)
 	dims := linesDimens(lines)
 	for i := 0; i < len(lines)-1; i++ {
-		s := lines[i].Text.String
 		// To avoid layout flickering while editing, assume a soft newline takes
 		// up all available space.
-		if len(s) > 0 {
-			r, _ := utf8.DecodeLastRuneInString(s)
+		if layout := lines[i].Layout; len(layout) > 0 {
+			r := layout[len(layout)-1].Rune
 			if r != '\n' {
 				dims.Size.X = e.maxWidth
 				break
@@ -459,21 +459,18 @@ loop:
 		l := e.lines[carLine]
 		y += (prevDesc + l.Ascent).Ceil()
 		prevDesc = l.Descent
-		if carLine == len(e.lines)-1 || idx+len(l.Text.String) > e.rr.caret {
-			str := l.Text.String
-			for _, adv := range l.Text.Advances {
+		if carLine == len(e.lines)-1 || idx+len(l.Layout) > e.rr.caret {
+			for _, g := range l.Layout {
 				if idx == e.rr.caret {
 					break loop
 				}
-				x += adv
-				_, s := utf8.DecodeRuneInString(str)
-				idx += s
-				str = str[s:]
+				x += g.Advance
+				idx += utf8.RuneLen(g.Rune)
 				carCol++
 			}
 			break
 		}
-		idx += len(l.Text.String)
+		idx += l.Len
 	}
 	x += align(e.Alignment, e.lines[carLine].Width, e.viewSize.X)
 	return
@@ -553,11 +550,11 @@ func (e *Editor) moveToLine(carX fixed.Int26_6, carLine2 int) fixed.Int26_6 {
 		// Move to start of line2.
 		if carLine2 > carLine {
 			for i := carLine; i < carLine2; i++ {
-				e.rr.caret += len(e.lines[i].Text.String)
+				e.rr.caret += e.lines[i].Len
 			}
 		} else {
 			for i := carLine - 1; i >= carLine2; i-- {
-				e.rr.caret -= len(e.lines[i].Text.String)
+				e.rr.caret -= e.lines[i].Len
 			}
 		}
 	}
@@ -569,15 +566,15 @@ func (e *Editor) moveToLine(carX fixed.Int26_6, carLine2 int) fixed.Int26_6 {
 		end = 1
 	}
 	// Move to rune closest to previous horizontal position.
-	for i := 0; i < len(l2.Text.Advances)-end; i++ {
-		adv := l2.Text.Advances[i]
+	for i := 0; i < len(l2.Layout)-end; i++ {
+		g := l2.Layout[i]
 		if carX2 >= carX {
 			break
 		}
-		if carX2+adv-carX >= carX-carX2 {
+		if carX2+g.Advance-carX >= carX-carX2 {
 			break
 		}
-		carX2 += adv
+		carX2 += g.Advance
 		_, s := e.rr.runeAt(e.rr.caret)
 		e.rr.caret += s
 	}
@@ -593,11 +590,11 @@ func (e *Editor) Move(distance int) {
 
 func (e *Editor) moveStart() {
 	carLine, carCol, x, _ := e.layoutCaret()
-	advances := e.lines[carLine].Text.Advances
+	layout := e.lines[carLine].Layout
 	for i := carCol - 1; i >= 0; i-- {
 		_, s := e.rr.runeBefore(e.rr.caret)
 		e.rr.caret -= s
-		x -= advances[i]
+		x -= layout[i].Advance
 	}
 	e.carXOff = -x
 }
@@ -610,8 +607,9 @@ func (e *Editor) moveEnd() {
 	if carLine < len(e.lines)-1 {
 		end = 1
 	}
-	for i := carCol; i < len(l.Text.Advances)-end; i++ {
-		adv := l.Text.Advances[i]
+	layout := l.Layout
+	for i := carCol; i < len(layout)-end; i++ {
+		adv := layout[i].Advance
 		_, s := e.rr.runeAt(e.rr.caret)
 		e.rr.caret += s
 		x += adv