gio/text/gotext.go

// SPDX-License-Identifier: Unlicense OR MIT

package text

import (
	"io"
	"sort"

	"github.com/benoitkugler/textlayout/fonts"
	"github.com/benoitkugler/textlayout/language"
	"github.com/go-text/typesetting/di"
	"github.com/go-text/typesetting/font"
	"github.com/go-text/typesetting/shaping"
	"golang.org/x/exp/slices"
	"golang.org/x/image/math/fixed"
	"golang.org/x/text/unicode/bidi"

	"gioui.org/f32"
	"gioui.org/io/system"
	"gioui.org/op"
	"gioui.org/op/clip"
)

// document holds a collection of shaped lines and alignment information for
// those lines.
type document struct {
	lines     []line
	alignment Alignment
	// alignWidth is the width used when aligning text.
	alignWidth int
}

// append adds the lines of other to the end of l and ensures they
// are aligned to the same width.
func (l *document) append(other document) {
	l.lines = append(l.lines, other.lines...)
	l.alignWidth = max(l.alignWidth, other.alignWidth)
	calculateYOffsets(l.lines)
}

// reset empties the document in preparation to reuse its memory.
func (l *document) reset() {
	l.lines = l.lines[:0]
	l.alignment = Start
	l.alignWidth = 0
}

func max(a, b int) int {
	if a > b {
		return a
	}
	return b
}

// A line contains the measurements of a line of text.
type line struct {
	// runs contains sequences of shaped glyphs with common attributes. The order
	// of runs is logical, meaning that the first run will contain the glyphs
	// corresponding to the first runes of data in the original text.
	runs []runLayout
	// visualOrder is a slice of indices into Runs that describes the visual positions
	// of each run of text. Iterating this slice and accessing Runs at each
	// of the values stored in this slice traverses the runs in proper visual
	// order from left to right.
	visualOrder []int
	// width is the width of the line.
	width fixed.Int26_6
	// ascent is the height above the baseline.
	ascent fixed.Int26_6
	// descent is the height below the baseline, including
	// the line gap.
	descent fixed.Int26_6
	// bounds is the visible bounds of the line.
	bounds fixed.Rectangle26_6
	// direction is the dominant direction of the line. This direction will be
	// used to align the text content of the line, but may not match the actual
	// direction of the runs of text within the line (such as an RTL sentence
	// within an LTR paragraph).
	direction system.TextDirection
	// runeCount is the number of text runes represented by this line's runs.
	runeCount int

	yOffset int
}

// Range describes the position and quantity of a range of text elements
// within a larger slice. The unit is usually runes of unicode data or
// glyphs of shaped font data.
type Range struct {
	// Count describes the number of items represented by the Range.
	Count int
	// Offset describes the start position of the represented
	// items within a larger list.
	Offset int
}

// glyph contains the metadata needed to render a glyph.
type glyph struct {
	// id is this glyph's identifier within the font it was shaped with.
	id GlyphID
	// clusterIndex is the identifier for the text shaping cluster that
	// this glyph is part of.
	clusterIndex int
	// glyphCount is the number of glyphs in the same cluster as this glyph.
	glyphCount int
	// runeCount is the quantity of runes in the source text that this glyph
	// corresponds to.
	runeCount int
	// xAdvance and yAdvance describe the distance the dot moves when
	// laying out the glyph on the X or Y axis.
	xAdvance, yAdvance fixed.Int26_6
	// xOffset and yOffset describe offsets from the dot that should be
	// applied when rendering the glyph.
	xOffset, yOffset fixed.Int26_6
	// bounds describes the visual bounding box of the glyph relative to
	// its dot.
	bounds fixed.Rectangle26_6
}

type runLayout struct {
	// VisualPosition describes the relative position of this run of text within
	// its line. It should be a valid index into the containing line's VisualOrder
	// slice.
	VisualPosition int
	// X is the visual offset of the dot for the first glyph in this run
	// relative to the beginning of the line.
	X fixed.Int26_6
	// Glyphs are the actual font characters for the text. They are ordered
	// from left to right regardless of the text direction of the underlying
	// text.
	Glyphs []glyph
	// Runes describes the position of the text data this layout represents
	// within the containing text.Line.
	Runes Range
	// Advance is the sum of the advances of all clusters in the Layout.
	Advance fixed.Int26_6
	// PPEM is the pixels-per-em scale used to shape this run.
	PPEM fixed.Int26_6
	// Direction is the layout direction of the glyphs.
	Direction system.TextDirection
	// face is the font face that the ID of each Glyph in the Layout refers to.
	face font.Face
}

// faceOrderer chooses the order in which faces should be applied to text.
type faceOrderer struct {
	def                 Font
	faceScratch         []font.Face
	fontDefaultOrder    map[Font]int
	defaultOrderedFonts []Font
	faces               map[Font]font.Face
	faceToIndex         map[font.Face]int
	fonts               []Font
}

func (f *faceOrderer) insert(fnt Font, face font.Face) {
	if len(f.fonts) == 0 {
		f.def = fnt
	}
	if f.fontDefaultOrder == nil {
		f.fontDefaultOrder = make(map[Font]int)
	}
	if f.faces == nil {
		f.faces = make(map[Font]font.Face)
		f.faceToIndex = make(map[font.Face]int)
	}
	f.fontDefaultOrder[fnt] = len(f.faceScratch)
	f.defaultOrderedFonts = append(f.defaultOrderedFonts, fnt)
	f.faceScratch = append(f.faceScratch, face)
	f.fonts = append(f.fonts, fnt)
	f.faces[fnt] = face
	f.faceToIndex[face] = f.fontDefaultOrder[fnt]
}

// resetFontOrder restores the fonts to a predictable order. It should be invoked
// before any operation searching the fonts.
func (c *faceOrderer) resetFontOrder() {
	copy(c.fonts, c.defaultOrderedFonts)
}

func (c *faceOrderer) indexFor(face font.Face) int {
	return c.faceToIndex[face]
}

func (c *faceOrderer) faceFor(idx int) font.Face {
	if idx < len(c.defaultOrderedFonts) {
		return c.faces[c.defaultOrderedFonts[idx]]
	}
	panic("face index not found")
}

// TODO(whereswaldon): this function could sort all faces by appropriateness for the
// given font characteristics. This would ensure that (if possible) text using a
// fallback font would select similar weights and emphases to the primary font.
func (c *faceOrderer) sortedFacesForStyle(font Font) []font.Face {
	c.resetFontOrder()
	primary, ok := c.fontForStyle(font)
	if !ok {
		font.Typeface = c.def.Typeface
		primary, ok = c.fontForStyle(font)
		if !ok {
			primary = c.def
		}
	}
	return c.sorted(primary)
}

// fontForStyle returns the closest existing font to the requested font within the
// same typeface.
func (c *faceOrderer) fontForStyle(font Font) (Font, bool) {
	if closest, ok := closestFont(font, c.fonts); ok {
		return closest, true
	}
	font.Style = Regular
	if closest, ok := closestFont(font, c.fonts); ok {
		return closest, true
	}
	return font, false
}

// faces returns a slice of faces with primary as the first element and
// the remaining faces ordered by insertion order.
func (f *faceOrderer) sorted(primary Font) []font.Face {
	// If we find primary, put it first, and omit it from the below sort.
	lowest := 0
	for i := range f.fonts {
		if f.fonts[i] == primary {
			if i != 0 {
				f.fonts[0], f.fonts[i] = f.fonts[i], f.fonts[0]
			}
			lowest = 1
			break
		}
	}
	sorting := f.fonts[lowest:]
	sort.Slice(sorting, func(i, j int) bool {
		a := sorting[i]
		b := sorting[j]
		return f.fontDefaultOrder[a] < f.fontDefaultOrder[b]
	})
	for i, font := range f.fonts {
		f.faceScratch[i] = f.faces[font]
	}
	return f.faceScratch
}

// shaperImpl implements the shaping and line-wrapping of opentype fonts.
type shaperImpl struct {
	// Fields for tracking fonts/faces.
	orderer faceOrderer

	// Shaping and wrapping state.
	shaper        shaping.HarfbuzzShaper
	wrapper       shaping.LineWrapper
	bidiParagraph bidi.Paragraph

	// Scratch buffers used to avoid re-allocating slices during routine internal
	// shaping operations.
	splitScratch1, splitScratch2 []shaping.Input
	outScratchBuf                []shaping.Output
	scratchRunes                 []rune
}

// Load registers the provided FontFace with the shaper, if it is compatible.
// It returns whether the face is now available for use. FontFaces are prioritized
// in the order in which they are loaded, with the first face being the default.
func (s *shaperImpl) Load(f FontFace) {
	s.orderer.insert(f.Font, f.Face.Face())
}

// splitByScript divides the inputs into new, smaller inputs on script boundaries
// and correctly sets the text direction per-script. It will
// use buf as the backing memory for the returned slice if buf is non-nil.
func splitByScript(inputs []shaping.Input, documentDir di.Direction, buf []shaping.Input) []shaping.Input {
	var splitInputs []shaping.Input
	if buf == nil {
		splitInputs = make([]shaping.Input, 0, len(inputs))
	} else {
		splitInputs = buf
	}
	for _, input := range inputs {
		currentInput := input
		if input.RunStart == input.RunEnd {
			return []shaping.Input{input}
		}
		firstNonCommonRune := input.RunStart
		for i := firstNonCommonRune; i < input.RunEnd; i++ {
			if language.LookupScript(input.Text[i]) != language.Common {
				firstNonCommonRune = i
				break
			}
		}
		currentInput.Script = language.LookupScript(input.Text[firstNonCommonRune])
		for i := firstNonCommonRune + 1; i < input.RunEnd; i++ {
			r := input.Text[i]
			runeScript := language.LookupScript(r)

			if runeScript == language.Common || runeScript == currentInput.Script {
				continue
			}

			if i != input.RunStart {
				currentInput.RunEnd = i
				splitInputs = append(splitInputs, currentInput)
			}

			currentInput = input
			currentInput.RunStart = i
			currentInput.Script = runeScript
			// In the future, it may make sense to try to guess the language of the text here as well,
			// but this is a complex process.
		}
		// close and add the last input
		currentInput.RunEnd = input.RunEnd
		splitInputs = append(splitInputs, currentInput)
	}

	return splitInputs
}

func (s *shaperImpl) splitBidi(input shaping.Input) []shaping.Input {
	var splitInputs []shaping.Input
	if input.Direction.Axis() != di.Horizontal || input.RunStart == input.RunEnd {
		return []shaping.Input{input}
	}
	def := bidi.LeftToRight
	if input.Direction.Progression() == di.TowardTopLeft {
		def = bidi.RightToLeft
	}
	s.bidiParagraph.SetString(string(input.Text), bidi.DefaultDirection(def))
	out, err := s.bidiParagraph.Order()
	if err != nil {
		return []shaping.Input{input}
	}
	for i := 0; i < out.NumRuns(); i++ {
		currentInput := input
		run := out.Run(i)
		dir := run.Direction()
		_, endRune := run.Pos()
		currentInput.RunEnd = endRune + 1
		if dir == bidi.RightToLeft {
			currentInput.Direction = di.DirectionRTL
		} else {
			currentInput.Direction = di.DirectionLTR
		}
		splitInputs = append(splitInputs, currentInput)
		input.RunStart = currentInput.RunEnd
	}
	return splitInputs
}

// splitByFaces divides the inputs by font coverage in the provided faces. It will use the slice provided in buf
// as the backing storage of the returned slice if buf is non-nil.
func (s *shaperImpl) splitByFaces(inputs []shaping.Input, faces []font.Face, buf []shaping.Input) []shaping.Input {
	var split []shaping.Input
	if buf == nil {
		split = make([]shaping.Input, 0, len(inputs))
	} else {
		split = buf
	}
	for _, input := range inputs {
		split = append(split, shaping.SplitByFontGlyphs(input, faces)...)
	}
	return split
}

// shapeText invokes the text shaper and returns the raw text data in the shaper's native
// format. It does not wrap lines.
func (s *shaperImpl) shapeText(faces []font.Face, ppem fixed.Int26_6, lc system.Locale, txt []rune) []shaping.Output {
	if len(faces) < 1 {
		return nil
	}
	lcfg := langConfig{
		Language:  language.NewLanguage(lc.Language),
		Direction: mapDirection(lc.Direction),
	}
	// Create an initial input.
	input := toInput(faces[0], ppem, lcfg, txt)
	// Break input on font glyph coverage.
	inputs := s.splitBidi(input)
	inputs = s.splitByFaces(inputs, faces, s.splitScratch1[:0])
	inputs = splitByScript(inputs, lcfg.Direction, s.splitScratch2[:0])
	// Shape all inputs.
	if needed := len(inputs) - len(s.outScratchBuf); needed > 0 {
		s.outScratchBuf = slices.Grow(s.outScratchBuf, needed)
	}
	s.outScratchBuf = s.outScratchBuf[:len(inputs)]
	for i := range inputs {
		s.outScratchBuf[i] = s.shaper.Shape(inputs[i])
	}
	return s.outScratchBuf
}

// shapeAndWrapText invokes the text shaper and returns wrapped lines in the shaper's native format.
func (s *shaperImpl) shapeAndWrapText(faces []font.Face, params Parameters, maxWidth int, lc system.Locale, txt []rune) []shaping.Line {
	// Wrap outputs into lines.
	return s.wrapper.WrapParagraph(shaping.WrapConfig{
		TruncateAfterLines: params.MaxLines,
	}, maxWidth, txt, s.shapeText(faces, params.PxPerEm, lc, txt)...)
}

// replaceControlCharacters replaces problematic unicode
// code points with spaces to ensure proper rune accounting.
func replaceControlCharacters(in []rune) []rune {
	for i, r := range in {
		switch r {
		// ASCII File separator.
		case '\u001C':
		// ASCII Group separator.
		case '\u001D':
		// ASCII Record separator.
		case '\u001E':
		case '\r':
		case '\n':
		// Unicode "next line" character.
		case '\u0085':
		// Unicode "paragraph separator".
		case '\u2029':
		default:
			continue
		}
		in[i] = ' '
	}
	return in
}

// Layout shapes and wraps the text, and returns the result in Gio's shaped text format.
func (s *shaperImpl) LayoutString(params Parameters, minWidth, maxWidth int, lc system.Locale, txt string) document {
	return s.LayoutRunes(params, minWidth, maxWidth, lc, []rune(txt))
}

// Layout shapes and wraps the text, and returns the result in Gio's shaped text format.
func (s *shaperImpl) Layout(params Parameters, minWidth, maxWidth int, lc system.Locale, txt io.RuneReader) document {
	s.scratchRunes = s.scratchRunes[:0]
	for r, _, err := txt.ReadRune(); err != nil; r, _, err = txt.ReadRune() {
		s.scratchRunes = append(s.scratchRunes, r)
	}
	return s.LayoutRunes(params, minWidth, maxWidth, lc, s.scratchRunes)
}

func calculateYOffsets(lines []line) {
	currentY := 0
	prevDesc := fixed.I(0)
	for i := range lines {
		ascent, descent := lines[i].ascent, lines[i].descent
		currentY += (prevDesc + ascent).Ceil()
		lines[i].yOffset = currentY
		prevDesc = descent
	}
}

// LayoutRunes shapes and wraps the text, and returns the result in Gio's shaped text format.
func (s *shaperImpl) LayoutRunes(params Parameters, minWidth, maxWidth int, lc system.Locale, txt []rune) document {
	hasNewline := len(txt) > 0 && txt[len(txt)-1] == '\n'
	if hasNewline {
		txt = txt[:len(txt)-1]
	}
	ls := s.shapeAndWrapText(s.orderer.sortedFacesForStyle(params.Font), params, maxWidth, lc, replaceControlCharacters(txt))
	// Convert to Lines.
	textLines := make([]line, len(ls))
	for i := range ls {
		otLine := toLine(&s.orderer, ls[i], lc.Direction)
		if i == len(ls)-1 && hasNewline {
			// If there was a trailing newline update the rune counts to include
			// it on the last line of the paragraph.
			finalRunIdx := len(otLine.runs) - 1
			otLine.runeCount += 1
			otLine.runs[finalRunIdx].Runes.Count += 1

			syntheticGlyph := glyph{
				id:           0,
				clusterIndex: len(txt),
				glyphCount:   0,
				runeCount:    1,
				xAdvance:     0,
				yAdvance:     0,
				xOffset:      0,
				yOffset:      0,
			}
			// Inset the synthetic newline glyph on the proper end of the run.
			if otLine.runs[finalRunIdx].Direction.Progression() == system.FromOrigin {
				otLine.runs[finalRunIdx].Glyphs = append(otLine.runs[finalRunIdx].Glyphs, syntheticGlyph)
			} else {
				// Ensure capacity.
				otLine.runs[finalRunIdx].Glyphs = append(otLine.runs[finalRunIdx].Glyphs, glyph{})
				copy(otLine.runs[finalRunIdx].Glyphs[1:], otLine.runs[finalRunIdx].Glyphs)
				otLine.runs[finalRunIdx].Glyphs[0] = syntheticGlyph
			}
		}
		textLines[i] = otLine
	}
	calculateYOffsets(textLines)
	return document{
		lines:      textLines,
		alignment:  params.Alignment,
		alignWidth: alignWidth(minWidth, textLines),
	}
}

func alignWidth(minWidth int, lines []line) int {
	for _, l := range lines {
		minWidth = max(minWidth, l.width.Ceil())
	}
	return minWidth
}

// Shape converts the provided glyphs into a path.
func (s *shaperImpl) Shape(ops *op.Ops, gs []Glyph) clip.PathSpec {
	var lastPos f32.Point
	var x fixed.Int26_6
	var builder clip.Path
	builder.Begin(ops)
	for i, g := range gs {
		if i == 0 {
			x = g.X
		}
		ppem, faceIdx, gid := splitGlyphID(g.ID)
		face := s.orderer.faceFor(faceIdx)
		ppemInt := ppem.Round()
		ppem16 := uint16(ppemInt)
		scaleFactor := float32(ppemInt) / float32(face.Upem())
		outline, ok := face.GlyphData(gid, ppem16, ppem16).(fonts.GlyphOutline)
		if !ok {
			continue
		}
		// Move to glyph position.
		pos := f32.Point{
			X: float32(g.X-x)/64 - float32(g.Offset.X)/64,
			Y: -float32(g.Offset.Y) / 64,
		}
		builder.Move(pos.Sub(lastPos))
		lastPos = pos
		var lastArg f32.Point

		// Convert fonts.Segments to relative segments.
		for _, fseg := range outline.Segments {
			nargs := 1
			switch fseg.Op {
			case fonts.SegmentOpQuadTo:
				nargs = 2
			case fonts.SegmentOpCubeTo:
				nargs = 3
			}
			var args [3]f32.Point
			for i := 0; i < nargs; i++ {
				a := f32.Point{
					X: fseg.Args[i].X * scaleFactor,
					Y: -fseg.Args[i].Y * scaleFactor,
				}
				args[i] = a.Sub(lastArg)
				if i == nargs-1 {
					lastArg = a
				}
			}
			switch fseg.Op {
			case fonts.SegmentOpMoveTo:
				builder.Move(args[0])
			case fonts.SegmentOpLineTo:
				builder.Line(args[0])
			case fonts.SegmentOpQuadTo:
				builder.Quad(args[0], args[1])
			case fonts.SegmentOpCubeTo:
				builder.Cube(args[0], args[1], args[2])
			default:
				panic("unsupported segment op")
			}
		}
		lastPos = lastPos.Add(lastArg)
	}
	return builder.End()
}

// langConfig describes the language and writing system of a body of text.
type langConfig struct {
	// Language the text is written in.
	language.Language
	// Writing system used to represent the text.
	language.Script
	// Direction of the text, usually driven by the writing system.
	di.Direction
}

// toInput converts its parameters into a shaping.Input.
func toInput(face font.Face, ppem fixed.Int26_6, lc langConfig, runes []rune) shaping.Input {
	var input shaping.Input
	input.Direction = lc.Direction
	input.Text = runes
	input.Size = ppem
	input.Face = face
	input.Language = lc.Language
	input.Script = lc.Script
	input.RunStart = 0
	input.RunEnd = len(runes)
	return input
}

func mapDirection(d system.TextDirection) di.Direction {
	switch d {
	case system.LTR:
		return di.DirectionLTR
	case system.RTL:
		return di.DirectionRTL
	}
	return di.DirectionLTR
}

func unmapDirection(d di.Direction) system.TextDirection {
	switch d {
	case di.DirectionLTR:
		return system.LTR
	case di.DirectionRTL:
		return system.RTL
	}
	return system.LTR
}

// toGioGlyphs converts text shaper glyphs into the minimal representation
// that Gio needs.
func toGioGlyphs(in []shaping.Glyph, ppem fixed.Int26_6, faceIdx int) []glyph {
	out := make([]glyph, 0, len(in))
	for _, g := range in {
		// To better understand how to calculate the bounding box, see here:
		// https://freetype.org/freetype2/docs/glyphs/glyph-metrics-3.svg
		var bounds fixed.Rectangle26_6
		bounds.Min.X = g.XBearing
		bounds.Min.Y = -g.YBearing
		bounds.Max = bounds.Min.Add(fixed.Point26_6{X: g.Width, Y: -g.Height})
		out = append(out, glyph{
			id:           newGlyphID(ppem, faceIdx, g.GlyphID),
			clusterIndex: g.ClusterIndex,
			runeCount:    g.RuneCount,
			glyphCount:   g.GlyphCount,
			xAdvance:     g.XAdvance,
			yAdvance:     g.YAdvance,
			xOffset:      g.XOffset,
			yOffset:      g.YOffset,
			bounds:       bounds,
		})
	}
	return out
}

// toLine converts the output into a Line with the provided dominant text direction.
func toLine(orderer *faceOrderer, o shaping.Line, dir system.TextDirection) line {
	if len(o) < 1 {
		return line{}
	}
	line := line{
		runs:      make([]runLayout, len(o)),
		direction: dir,
	}
	for i := range o {
		run := o[i]
		line.runs[i] = runLayout{
			Glyphs: toGioGlyphs(run.Glyphs, run.Size, orderer.indexFor(run.Face)),
			Runes: Range{
				Count:  run.Runes.Count,
				Offset: line.runeCount,
			},
			Direction: unmapDirection(run.Direction),
			face:      run.Face,
			Advance:   run.Advance,
			PPEM:      run.Size,
		}
		line.runeCount += run.Runes.Count
		if line.bounds.Min.Y > -run.LineBounds.Ascent {
			line.bounds.Min.Y = -run.LineBounds.Ascent
		}
		if line.bounds.Max.Y < -run.LineBounds.Ascent+run.LineBounds.LineHeight() {
			line.bounds.Max.Y = -run.LineBounds.Ascent + run.LineBounds.LineHeight()
		}
		line.bounds.Max.X += run.Advance
		line.width += run.Advance
		if line.ascent < run.LineBounds.Ascent {
			line.ascent = run.LineBounds.Ascent
		}
		if line.descent < -run.LineBounds.Descent+run.LineBounds.Gap {
			line.descent = -run.LineBounds.Descent + run.LineBounds.Gap
		}
	}
	computeVisualOrder(&line)
	// Account for glyphs hanging off of either side in the bounds.
	if len(line.visualOrder) > 0 {
		runIdx := line.visualOrder[0]
		run := o[runIdx]
		if len(run.Glyphs) > 0 {
			line.bounds.Min.X = run.Glyphs[0].LeftSideBearing()
		}
		runIdx = line.visualOrder[len(line.visualOrder)-1]
		run = o[runIdx]
		if len(run.Glyphs) > 0 {
			lastGlyphIdx := len(run.Glyphs) - 1
			line.bounds.Max.X += run.Glyphs[lastGlyphIdx].RightSideBearing()
		}
	}
	return line
}

// computeVisualOrder will populate the Line's VisualOrder field and the
// VisualPosition field of each element in Runs.
func computeVisualOrder(l *line) {
	l.visualOrder = make([]int, len(l.runs))
	const none = -1
	bidiRangeStart := none

	// visPos returns the visual position for an individual logically-indexed
	// run in this line, taking only the line's overall text direction into
	// account.
	visPos := func(logicalIndex int) int {
		if l.direction.Progression() == system.TowardOrigin {
			return len(l.runs) - 1 - logicalIndex
		}
		return logicalIndex
	}

	// resolveBidi populated the line's VisualOrder fields for the elements in the
	// half-open range [bidiRangeStart:bidiRangeEnd) indicating that those elements
	// should be displayed in reverse-visual order.
	resolveBidi := func(bidiRangeStart, bidiRangeEnd int) {
		firstVisual := bidiRangeEnd - 1
		// Just found the end of a bidi range.
		for startIdx := bidiRangeStart; startIdx < bidiRangeEnd; startIdx++ {
			pos := visPos(firstVisual)
			l.runs[startIdx].VisualPosition = pos
			l.visualOrder[pos] = startIdx
			firstVisual--
		}
		bidiRangeStart = none
	}
	for runIdx, run := range l.runs {
		if run.Direction.Progression() != l.direction.Progression() {
			if bidiRangeStart == none {
				bidiRangeStart = runIdx
			}
			continue
		} else if bidiRangeStart != none {
			// Just found the end of a bidi range.
			resolveBidi(bidiRangeStart, runIdx)
			bidiRangeStart = none
		}
		pos := visPos(runIdx)
		l.runs[runIdx].VisualPosition = pos
		l.visualOrder[pos] = runIdx
	}
	if bidiRangeStart != none {
		// We ended iteration within a bidi segment, resolve it.
		resolveBidi(bidiRangeStart, len(l.runs))
	}
	// Iterate and resolve the X of each run.
	x := fixed.Int26_6(0)
	for _, runIdx := range l.visualOrder {
		l.runs[runIdx].X = x
		x += l.runs[runIdx].Advance
	}
}

// closestFont returns the closest Font in available by weight.
// In case of equality the lighter weight will be returned.
func closestFont(lookup Font, available []Font) (Font, bool) {
	found := false
	var match Font
	for _, cf := range available {
		if cf == lookup {
			return lookup, true
		}
		if cf.Typeface != lookup.Typeface || cf.Variant != lookup.Variant || cf.Style != lookup.Style {
			continue
		}
		if !found {
			found = true
			match = cf
			continue
		}
		cDist := weightDistance(lookup.Weight, cf.Weight)
		mDist := weightDistance(lookup.Weight, match.Weight)
		if cDist < mDist {
			match = cf
		} else if cDist == mDist && cf.Weight < match.Weight {
			match = cf
		}
	}
	return match, found
}

// weightDistance returns the distance value between two font weights.
func weightDistance(wa Weight, wb Weight) int {
	// Avoid dealing with negative Weight values.
	a := int(wa) + 400
	b := int(wb) + 400
	diff := a - b
	if diff < 0 {
		return -diff
	}
	return diff
}