text: represent laid out text as strings to facilitate caching of layouts

Commit https://gioui.org/commit/b331407e81456 added text layout and shaping
based on io.Reader and changed Editor to use it. Unfortunately, as ~inkeliz
discovered, caching of shapes were also lost.

~inkeliz suggested fix,

https://lists.sr.ht/~eliasnaur/gio-patches/patches/15059

adds caching of shapes to Editor to regain lost performance.

This change repairs the cache to work on io.Reader API, in hope that the
already complicated Editor won't need additional caching.

Before this change, text layouts were represented as a slice of (rune, advance)
pairs. Unfortunately, this representation doesn't lend itself to caching of
shaping results, so change the representation of a line of text to be a pair
of text and advances:

	package text

	type Layout {
		Text string
		Advances []fixed.Int26_6
	}

The Text field can then be used in a cache key, assuming Advances is
consistent with it.

The end result is that the two shaper variants of text.Shaper is reduced to
just one, and the Len field field of text.Line is no longer needed.

The changed representation adds a bit of extra work to package opentype.
Cleaning that up is left as a future TODO.

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

font/opentype/opentype.go

@@ -5,6 +5,7 @@
 package opentype
 
 import (
+	"bytes"
 	"io"
 	"unicode"
 	"unicode/utf8"
@@ -36,6 +37,13 @@ type opentype struct {
 	Hinting font.Hinting
 }
 
+// a glyph represents a rune and its advance according to a Font.
+// TODO: remove this type and work on io.Readers directly.
+type glyph struct {
+	Rune    rune
+	Advance fixed.Int26_6
+}
+
 // NewFont parses an SFNT font, such as TTF or OTF data, from a []byte
 // data source.
 func Parse(src []byte) (*Font, error) {
@@ -110,7 +118,7 @@ func (f *Font) Layout(ppem fixed.Int26_6, maxWidth int, txt io.Reader) ([]text.L
 	return layoutText(&buf, ppem, maxWidth, fonts, glyphs)
 }
 
-func (f *Font) Shape(ppem fixed.Int26_6, str []text.Glyph) op.CallOp {
+func (f *Font) Shape(ppem fixed.Int26_6, str text.Layout) op.CallOp {
 	var buf sfnt.Buffer
 	return textPath(&buf, ppem, []*opentype{{Font: f.font, Hinting: font.HintingFull}}, str)
 }
@@ -130,7 +138,7 @@ func (c *Collection) Layout(ppem fixed.Int26_6, maxWidth int, txt io.Reader) ([]
 	return layoutText(&buf, ppem, maxWidth, c.fonts, glyphs)
 }
 
-func (c *Collection) Shape(ppem fixed.Int26_6, str []text.Glyph) op.CallOp {
+func (c *Collection) Shape(ppem fixed.Int26_6, str text.Layout) op.CallOp {
 	var buf sfnt.Buffer
 	return textPath(&buf, ppem, c.fonts, str)
 }
@@ -147,7 +155,7 @@ func fontForGlyph(buf *sfnt.Buffer, fonts []*opentype, r rune) *opentype {
 	return fonts[0] // Use replacement character from the first font if necessary
 }
 
-func layoutText(sbuf *sfnt.Buffer, ppem fixed.Int26_6, maxWidth int, fonts []*opentype, glyphs []text.Glyph) ([]text.Line, error) {
+func layoutText(sbuf *sfnt.Buffer, ppem fixed.Int26_6, maxWidth int, fonts []*opentype, glyphs []glyph) ([]text.Line, error) {
 	var lines []text.Line
 	var nextLine text.Line
 	updateBounds := func(f *opentype) {
@@ -180,8 +188,7 @@ func layoutText(sbuf *sfnt.Buffer, ppem fixed.Int26_6, maxWidth int, fonts []*op
 			prev.f = fonts[0]
 		}
 		updateBounds(prev.f)
-		nextLine.Layout = glyphs[:prev.idx:prev.idx]
-		nextLine.Len = prev.len
+		nextLine.Layout = toLayout(glyphs[:prev.idx:prev.idx])
 		nextLine.Width = prev.x + prev.adv
 		nextLine.Bounds.Max.X += prev.x
 		lines = append(lines, nextLine)
@@ -242,20 +249,32 @@ func layoutText(sbuf *sfnt.Buffer, ppem fixed.Int26_6, maxWidth int, fonts []*op
 	return lines, nil
 }
 
-func textPath(buf *sfnt.Buffer, ppem fixed.Int26_6, fonts []*opentype, str []text.Glyph) op.CallOp {
+// toLayout converts a slice of glyphs to a text.Layout.
+func toLayout(glyphs []glyph) text.Layout {
+	var buf bytes.Buffer
+	advs := make([]fixed.Int26_6, len(glyphs))
+	for i, g := range glyphs {
+		buf.WriteRune(g.Rune)
+		advs[i] = glyphs[i].Advance
+	}
+	return text.Layout{Text: buf.String(), Advances: advs}
+}
+
+func textPath(buf *sfnt.Buffer, ppem fixed.Int26_6, fonts []*opentype, str text.Layout) op.CallOp {
 	var lastPos f32.Point
 	var builder clip.Path
 	ops := new(op.Ops)
 	m := op.Record(ops)
 	var x fixed.Int26_6
 	builder.Begin(ops)
-	for _, g := range str {
-		if !unicode.IsSpace(g.Rune) {
-			f := fontForGlyph(buf, fonts, g.Rune)
+	rune := 0
+	for _, r := range str.Text {
+		if !unicode.IsSpace(r) {
+			f := fontForGlyph(buf, fonts, r)
 			if f == nil {
 				continue
 			}
-			segs, ok := f.LoadGlyph(buf, ppem, g.Rune)
+			segs, ok := f.LoadGlyph(buf, ppem, r)
 			if !ok {
 				continue
 			}
@@ -301,14 +320,15 @@ func textPath(buf *sfnt.Buffer, ppem fixed.Int26_6, fonts []*opentype, str []tex
 			}
 			lastPos = lastPos.Add(lastArg)
 		}
-		x += g.Advance
+		x += str.Advances[rune]
+		rune++
 	}
 	builder.Outline().Add(ops)
 	return m.Stop()
 }
 
-func readGlyphs(r io.Reader) ([]text.Glyph, error) {
-	var glyphs []text.Glyph
+func readGlyphs(r io.Reader) ([]glyph, error) {
+	var glyphs []glyph
 	buf := make([]byte, 0, 1024)
 	for {
 		n, err := r.Read(buf[len(buf):cap(buf)])
@@ -322,7 +342,7 @@ func readGlyphs(r io.Reader) ([]text.Glyph, error) {
 		for i < lim {
 			c, s := utf8.DecodeRune(buf[i:])
 			i += s
-			glyphs = append(glyphs, text.Glyph{Rune: c})
+			glyphs = append(glyphs, glyph{Rune: c})
 		}
 		n = copy(buf, buf[i:])
 		buf = buf[:n]