canvas.go

package goat

import (
	"bufio"
	"bytes"
	"io"
	"sort"
)

// Characters where more than one line segment can come together.
var jointRunes = []rune{'.', '\'', '+', '*', 'o'}

var reservedRunes = map[rune]bool{
	'-':  true,
	'_':  true,
	'|':  true,
	'v':  true,
	'^':  true,
	'>':  true,
	'<':  true,
	'o':  true,
	'*':  true,
	'+':  true,
	'.':  true,
	'\'': true,
	'/':  true,
	'\\': true,
	')':  true,
	'(':  true,
	' ':  true,
}

func contains(in []rune, r rune) bool {
	for _, v := range in {
		if r == v {
			return true
		}
	}
	return false
}

func isJoint(r rune) bool {
	return contains(jointRunes, r)
}

func isDot(r rune) bool {
	return r == 'o' || r == '*'
}

func isTriangle(r rune) bool {
	return r == '^' || r == 'v' || r == '<' || r == '>'
}

// Canvas represents a 2D ASCII rectangle.
type Canvas struct {
	Width  int
	Height int
	data   map[Index]rune
	text   map[Index]rune
}

func (c *Canvas) String() string {
	var buffer bytes.Buffer

	for h := 0; h < c.Height; h++ {
		for w := 0; w < c.Width; w++ {
			idx := Index{w, h}

			// Grab from our text buffer and if nothing's there try the data
			// buffer.
			r := c.text[idx]
			if r == 0 {
				r = c.runeAt(idx)
			}

			_, err := buffer.WriteRune(r)
			if err != nil {
				continue
			}
		}

		err := buffer.WriteByte('\n')
		if err != nil {
			continue
		}
	}

	return buffer.String()
}

func (c *Canvas) heightScreen() int {
	return c.Height*16 + 8 + 1
}

func (c *Canvas) widthScreen() int {
	return (c.Width + 1) * 8
}

func (c *Canvas) runeAt(i Index) rune {
	if val, ok := c.data[i]; ok {
		return val
	}

	return ' '
}

// NewCanvas creates a new canvas with contents read from the given io.Reader.
// Content should be newline delimited.
func NewCanvas(in io.Reader) Canvas {
	width := 0
	height := 0

	scanner := bufio.NewScanner(in)

	data := make(map[Index]rune)

	for scanner.Scan() {
		line := scanner.Text()

		w := 0
		// Can't use index here because it corresponds to unicode offsets
		// instead of logical characters.
		for _, c := range line {
			idx := Index{x: w, y: height}
			data[idx] = rune(c)
			w++
		}

		if w > width {
			width = w
		}
		height++
	}

	text := make(map[Index]rune)

	c := Canvas{
		Width:  width,
		Height: height,
		data:   data,
		text:   text,
	}

	// Extract everything we detect as text to make diagram parsing easier.
	for idx := range leftRight(width, height) {
		if c.isText(idx) {
			c.text[idx] = c.runeAt(idx)
		}
	}
	for idx := range c.text {
		delete(c.data, idx)
	}

	return c
}

// Drawable represents anything that can Draw itself.
type Drawable interface {
	Draw(out io.Writer)
}

// Line represents a straight segment between two points.
type Line struct {
	start Index
	stop  Index
	// dashed           bool
	needsNudgingDown      bool
	needsNudgingLeft      bool
	needsNudgingRight     bool
	needsTinyNudgingLeft  bool
	needsTinyNudgingRight bool

	// This is a line segment all by itself. This centers the segment around
	// the midline.
	lonely bool
	// N or S. Only useful for half steps - chops of this half of the line.
	chop Orientation

	orientation Orientation

	state lineState
}

type lineState int

const (
	_Unstarted lineState = iota
	_Started
)

func (l *Line) started() bool {
	return l.state == _Started
}

func (l *Line) setStart(i Index) {
	if l.state == _Unstarted {
		l.start = i
		l.stop = i
		l.state = _Started
	}
}

func (l *Line) setStop(i Index) {
	if l.state == _Started {
		l.stop = i
	}
}

func (l *Line) goesSomewhere() bool {
	return l.start != l.stop
}

func (l *Line) horizontal() bool {
	return l.orientation == E || l.orientation == W
}

func (l *Line) vertical() bool {
	return l.orientation == N || l.orientation == S
}

func (l *Line) diagonal() bool {
	return l.orientation == NE || l.orientation == SE || l.orientation == SW || l.orientation == NW
}

// Triangle corresponds to "^", "v", "<" and ">" runes in the absence of
// surrounding alphanumerics.
type Triangle struct {
	start        Index
	orientation  Orientation
	needsNudging bool
}

// Circle corresponds to "o" or "*" runes in the absence of surrounding
// alphanumerics.
type Circle struct {
	start Index
	bold  bool
}

// RoundedCorner corresponds to combinations of "-." or "-'".
type RoundedCorner struct {
	start       Index
	orientation Orientation
}

// Text corresponds to any runes not reserved for diagrams, or reserved runes
// surrounded by alphanumerics.
type Text struct {
	start    Index
	contents string
}

// Bridge correspondes to combinations of "-)-" or "-(-" and is displayed as
// the vertical line "hopping over" the horizontal.
type Bridge struct {
	start       Index
	orientation Orientation
}

// Orientation represents the primary direction that a Drawable is facing.
type Orientation int

const (
	NONE Orientation = iota // No orientation; no structure present.
	N                       // North
	NE                      // Northeast
	NW                      // Northwest
	S                       // South
	SE                      // Southeast
	SW                      // Southwest
	E                       // East
	W                       // West
)

func (c *Canvas) WriteSVGBody(dst io.Writer) {
	writeBytes(dst, "<g transform='translate(8,16)'>\n")

	for _, l := range c.Lines() {
		l.Draw(dst)
	}

	for _, t := range c.Triangles() {
		t.Draw(dst)
	}

	for _, c := range c.RoundedCorners() {
		c.Draw(dst)
	}

	for _, c := range c.Circles() {
		c.Draw(dst)
	}

	for _, b := range c.Bridges() {
		b.Draw(dst)
	}

	for _, t := range c.Text() {
		t.Draw(dst)
	}

	writeBytes(dst, "</g>\n")
}

// Lines returns a slice of all Line drawables that we can detect -- in all
// possible orientations.
func (c *Canvas) Lines() []Line {
	horizontalMidlines := c.getLinesForSegment('-')

	diagUpLines := c.getLinesForSegment('/')
	for i, l := range diagUpLines {
		// /_
		if c.runeAt(l.start.east()) == '_' {
			diagUpLines[i].needsTinyNudgingLeft = true
		}

		// _
		// /
		if c.runeAt(l.stop.north()) == '_' {
			diagUpLines[i].needsTinyNudgingRight = true
		}

		//  _
		// /
		if !l.lonely && c.runeAt(l.stop.nEast()) == '_' {
			diagUpLines[i].needsTinyNudgingRight = true
		}

		// _/
		if !l.lonely && c.runeAt(l.start.west()) == '_' {
			diagUpLines[i].needsTinyNudgingLeft = true
		}

		// \
		// /
		if !l.lonely && c.runeAt(l.stop.north()) == '\\' {
			diagUpLines[i].needsTinyNudgingRight = true
		}

		// /
		// \
		if !l.lonely && c.runeAt(l.start.south()) == '\\' {
			diagUpLines[i].needsTinyNudgingLeft = true
		}
	}

	diagDownLines := c.getLinesForSegment('\\')
	for i, l := range diagDownLines {
		// _\
		if c.runeAt(l.stop.west()) == '_' {
			diagDownLines[i].needsTinyNudgingRight = true
		}

		// _
		// \
		if c.runeAt(l.start.north()) == '_' {
			diagDownLines[i].needsTinyNudgingLeft = true
		}

		//  _
		//   \
		if !l.lonely && c.runeAt(l.start.nWest()) == '_' {
			diagDownLines[i].needsTinyNudgingLeft = true
		}

		// \_
		if !l.lonely && c.runeAt(l.stop.east()) == '_' {
			diagDownLines[i].needsTinyNudgingRight = true
		}

		// \
		// /
		if !l.lonely && c.runeAt(l.stop.south()) == '/' {
			diagDownLines[i].needsTinyNudgingRight = true
		}

		// /
		// \
		if !l.lonely && c.runeAt(l.start.north()) == '/' {
			diagDownLines[i].needsTinyNudgingLeft = true
		}
	}

	horizontalBaselines := c.getLinesForSegment('_')
	for i, l := range horizontalBaselines {
		// TODO: make this nudge an orientation
		horizontalBaselines[i].needsNudgingDown = true

		//     _
		// _| |
		if c.runeAt(l.stop.sEast()) == '|' || c.runeAt(l.stop.nEast()) == '|' {
			horizontalBaselines[i].needsNudgingRight = true
		}

		// _
		//  |  _|
		if c.runeAt(l.start.sWest()) == '|' || c.runeAt(l.start.nWest()) == '|' {
			horizontalBaselines[i].needsNudgingLeft = true
		}

		//     _
		// _/   \
		if c.runeAt(l.stop.east()) == '/' || c.runeAt(l.stop.sEast()) == '\\' {
			horizontalBaselines[i].needsTinyNudgingRight = true
		}

		//       _
		// \_   /
		if c.runeAt(l.start.west()) == '\\' || c.runeAt(l.start.sWest()) == '/' {
			horizontalBaselines[i].needsTinyNudgingLeft = true
		}

		// _\
		if c.runeAt(l.stop.east()) == '\\' {
			horizontalBaselines[i].needsNudgingRight = true
			horizontalBaselines[i].needsTinyNudgingRight = true
		}

		//
		// /_
		if c.runeAt(l.start.west()) == '/' {
			horizontalBaselines[i].needsNudgingLeft = true
			horizontalBaselines[i].needsTinyNudgingLeft = true
		}
		//  _
		//  /
		if c.runeAt(l.stop.south()) == '/' {
			horizontalBaselines[i].needsTinyNudgingRight = true
		}

		//  _
		//  \
		if c.runeAt(l.start.south()) == '\\' {
			horizontalBaselines[i].needsTinyNudgingLeft = true
		}

		//  _
		// '
		if c.runeAt(l.start.sWest()) == '\'' {
			horizontalBaselines[i].needsNudgingLeft = true
		}

		// _
		//  '
		if c.runeAt(l.stop.sEast()) == '\'' {
			horizontalBaselines[i].needsNudgingRight = true
		}
	}

	verticalLines := c.getLinesForSegment('|')

	var lines []Line

	lines = append(lines, horizontalMidlines...)
	lines = append(lines, horizontalBaselines...)
	lines = append(lines, verticalLines...)
	lines = append(lines, diagUpLines...)
	lines = append(lines, diagDownLines...)
	lines = append(lines, c.HalfSteps()...)

	return lines
}

func newHalfStep(i Index, chop Orientation) Line {
	return Line{
		start:       i,
		stop:        i.south(),
		lonely:      true,
		chop:        chop,
		orientation: S,
	}
}

func (c *Canvas) HalfSteps() []Line {
	var lines []Line

	for idx := range upDown(c.Width, c.Height) {
		if o := c.partOfHalfStep(idx); o != NONE {
			lines = append(
				lines,
				newHalfStep(idx, o),
			)
		}
	}

	return lines
}

func (c *Canvas) getLinesForSegment(segment rune) []Line {
	var iter canvasIterator
	var orientation Orientation
	var passThroughs []rune

	switch segment {
	case '-':
		iter = leftRight
		orientation = E
		passThroughs = append(jointRunes, '<', '>', '(', ')')
	case '_':
		iter = leftRight
		orientation = E
		passThroughs = append(jointRunes, '|')
	case '|':
		iter = upDown
		orientation = S
		passThroughs = append(jointRunes, '^', 'v')
	case '/':
		iter = diagUp
		orientation = NE
		passThroughs = append(jointRunes, 'o', '*', '<', '>', '^', 'v', '|')
	case '\\':
		iter = diagDown
		orientation = SE
		passThroughs = append(jointRunes, 'o', '*', '<', '>', '^', 'v', '|')
	default:
		return nil
	}

	return c.getLines(iter, segment, passThroughs, orientation)
}

// ci: the order that we traverse locations on the canvas.
// segment: the primary character we're tracking for this line.
// passThroughs: characters the line segment is allowed to be drawn underneath
// (without terminating the line).
// orientation: the orientation for this line.
func (c *Canvas) getLines(
	ci canvasIterator,
	segment rune,
	passThroughs []rune,
	o Orientation,
) []Line {

	var lines []Line

	// Helper to throw the current line we're tracking on to the slice and
	// start a new one.
	snip := func(l Line) Line {
		// Only collect lines that actually go somewhere or are isolated
		// segments.
		if l.goesSomewhere() {
			lines = append(lines, l)
		}

		return Line{orientation: o}
	}

	currentLine := Line{orientation: o}
	lastSeenRune := ' '

	for idx := range ci(c.Width+1, c.Height+1) {
		r := c.runeAt(idx)

		isSegment := r == segment
		isPassThrough := contains(passThroughs, r)
		isRoundedCorner := c.isRoundedCorner(idx)
		isDot := isDot(r)
		isTriangle := isTriangle(r)

		justPassedThrough := contains(passThroughs, lastSeenRune)

		shouldKeep := (isSegment || isPassThrough) && isRoundedCorner == NONE

		// This is an edge case where we have a rounded corner... that's also a
		// joint... attached to orthogonal line, e.g.:
		//
		//  '+--
		//   |
		//
		// TODO: This also depends on the orientation of the corner and our
		// line.
		// NW / NE line can't go with EW/NS lines, vertical is OK though.
		if isRoundedCorner != NONE && o != E && (c.partOfVerticalLine(idx) || c.partOfDiagonalLine(idx)) {
			shouldKeep = true
		}

		// Don't connect | to > for diagonal lines or )) for horizontal lines.
		if isPassThrough && justPassedThrough && o != S {
			currentLine = snip(currentLine)
		}

		// Don't connect o to o, + to o, etc. This character is a new pass-through
		// so we still want to respect shouldKeep; we just don't want to draw
		// the existing line through this cell.
		if justPassedThrough && (isDot || isTriangle) {
			currentLine = snip(currentLine)
		}

		switch currentLine.state {
		case _Unstarted:
			if shouldKeep {
				currentLine.setStart(idx)
			}
		case _Started:
			if !shouldKeep {
				// Snip the existing line, don't add the current cell to it
				// *unless* its a line segment all by itself. If it is, keep a
				// record that it's an individual segment because we need to
				// adjust later in the / and \ cases.
				if !currentLine.goesSomewhere() && lastSeenRune == segment {
					if !c.partOfRoundedCorner(currentLine.start) {
						currentLine.setStop(idx)
						currentLine.lonely = true
					}
				}
				currentLine = snip(currentLine)
			} else if isPassThrough {
				// Snip the existing line but include the current pass-through
				// character because we may be continuing the line.
				currentLine.setStop(idx)
				currentLine = snip(currentLine)
				currentLine.setStart(idx)
			} else if shouldKeep {
				// Keep the line going and extend it by this character.
				currentLine.setStop(idx)
			}
		}

		lastSeenRune = r
	}

	return lines
}

// Triangles returns a slice of all detectable Triangles.
func (c *Canvas) Triangles() []Drawable {
	var triangles []Drawable

	o := NONE

	for idx := range upDown(c.Width, c.Height) {
		needsNudging := false
		start := idx

		r := c.runeAt(idx)

		if !isTriangle(r) {
			continue
		}

		// Identify our orientation and nudge the triangle to touch any
		// adjacent walls.
		switch r {
		case '^':
			o = N
			//  ^  and ^
			// /        \
			if c.runeAt(start.sWest()) == '/' {
				o = NE
			} else if c.runeAt(start.sEast()) == '\\' {
				o = NW
			}
		case 'v':
			o = S
			//  /  and \
			// v        v
			if c.runeAt(start.nEast()) == '/' {
				o = SW
			} else if c.runeAt(start.nWest()) == '\\' {
				o = SE
			}
		case '<':
			o = W
		case '>':
			o = E
		}

		// Determine if we need to snap the triangle to something and, if so,
		// draw a tail if we need to.
		switch o {
		case N:
			r := c.runeAt(start.north())
			if r == '-' || isJoint(r) && !isDot(r) {
				needsNudging = true
				triangles = append(triangles, newHalfStep(start, N))
			}
		case NW:
			r := c.runeAt(start.nWest())
			// Need to draw a tail.
			if r == '-' || isJoint(r) && !isDot(r) {
				needsNudging = true
				triangles = append(
					triangles,
					Line{
						start:       start.nWest(),
						stop:        start,
						orientation: SE,
					},
				)
			}
		case NE:
			r := c.runeAt(start.nEast())
			if r == '-' || isJoint(r) && !isDot(r) {
				needsNudging = true
				triangles = append(
					triangles,
					Line{
						start:       start,
						stop:        start.nEast(),
						orientation: NE,
					},
				)
			}
		case S:
			r := c.runeAt(start.south())
			if r == '-' || isJoint(r) && !isDot(r) {
				needsNudging = true
				triangles = append(triangles, newHalfStep(start, S))
			}
		case SE:
			r := c.runeAt(start.sEast())
			if r == '-' || isJoint(r) && !isDot(r) {
				needsNudging = true
				triangles = append(
					triangles,
					Line{
						start:       start,
						stop:        start.sEast(),
						orientation: SE,
					},
				)
			}
		case SW:
			r := c.runeAt(start.sWest())
			if r == '-' || isJoint(r) && !isDot(r) {
				needsNudging = true
				triangles = append(
					triangles,
					Line{
						start:       start.sWest(),
						stop:        start,
						orientation: NE,
					},
				)
			}
		case W:
			r := c.runeAt(start.west())
			if isDot(r) {
				needsNudging = true
			}
		case E:
			r := c.runeAt(start.east())
			if isDot(r) {
				needsNudging = true
			}
		}

		triangles = append(
			triangles,
			Triangle{
				start:        start,
				orientation:  o,
				needsNudging: needsNudging,
			},
		)
	}

	return triangles
}

// Circles returns a slice of all 'o' and '*' characters not considered text.
func (c *Canvas) Circles() []Circle {
	var circles []Circle

	for idx := range upDown(c.Width, c.Height) {
		// TODO INCOMING
		if c.runeAt(idx) == 'o' {
			circles = append(circles, Circle{start: idx})
		} else if c.runeAt(idx) == '*' {
			circles = append(circles, Circle{start: idx, bold: true})
		}
	}

	return circles
}

// RoundedCorners returns a slice of all curvy corners in the diagram.
func (c *Canvas) RoundedCorners() []RoundedCorner {
	var corners []RoundedCorner

	for idx := range leftRight(c.Width, c.Height) {
		if o := c.isRoundedCorner(idx); o != NONE {
			corners = append(
				corners,
				RoundedCorner{start: idx, orientation: o},
			)
		}
	}

	return corners
}

// For . and ' characters this will return a non-NONE orientation if the
// character falls on a rounded corner.
func (c *Canvas) isRoundedCorner(i Index) Orientation {
	r := c.runeAt(i)

	if !isJoint(r) {
		return NONE
	}

	left := i.west()
	right := i.east()
	lowerLeft := i.sWest()
	lowerRight := i.sEast()
	upperLeft := i.nWest()
	upperRight := i.nEast()

	opensUp := r == '\'' || r == '+'
	opensDown := r == '.' || r == '+'

	dashRight := c.runeAt(right) == '-' || c.runeAt(right) == '+' || c.runeAt(right) == '_' || c.runeAt(upperRight) == '_'
	dashLeft := c.runeAt(left) == '-' || c.runeAt(left) == '+' || c.runeAt(left) == '_' || c.runeAt(upperLeft) == '_'

	isVerticalSegment := func(i Index) bool {
		r := c.runeAt(i)
		return r == '|' || r == '+' || r == ')' || r == '(' || isDot(r)
	}

	//  .- or  .-
	// |      +
	if opensDown && dashRight && isVerticalSegment(lowerLeft) {
		return NW
	}

	// -. or -.  or -.  or _.  or -.
	//   |     +      )      )      o
	if opensDown && dashLeft && isVerticalSegment(lowerRight) {
		return NE
	}

	//   | or   + or   | or   + or   + or_ )
	// -'     -'     +'     +'     ++     '
	if opensUp && dashLeft && isVerticalSegment(upperRight) {
		return SE
	}

	// |  or +
	//  '-    '-
	if opensUp && dashRight && isVerticalSegment(upperLeft) {
		return SW
	}

	return NONE
}

// A wrapper to enable sorting.
type indexRuneDrawable struct {
	i Index
	r rune
	Drawable
}

// Text returns a slice of all text characters not belonging to part of the diagram.
// How these characters are identified is rather complicated.
func (c *Canvas) Text() []Drawable {
	newLine := func(i Index, r rune, o Orientation) Drawable {
		stop := i

		switch o {
		case NE:
			stop = i.nEast()
		case SE:
			stop = i.sEast()
		}

		l := Line{
			start:       i,
			stop:        stop,
			lonely:      true,
			orientation: o,
		}

		return indexRuneDrawable{
			Drawable: l,
			i:        i,
			r:        r,
		}
	}

	text := make([]Drawable, len(c.text))
	var j int

	for i, r := range c.text {
		switch r {
		// Weird unicode edge cases that markdeep handles. These get
		// substituted with lines.
		case '╱':
			text[j] = newLine(i, r, NE)
		case '╲':
			text[j] = newLine(i, r, SE)
		case '╳':
			text[j] = newLine(i, r, NE)
		default:
			text[j] = indexRuneDrawable{Drawable: Text{start: i, contents: string(r)}, i: i, r: r}
		}
		j++
	}

	sort.Slice(text, func(i, j int) bool {
		ti, tj := text[i].(indexRuneDrawable), text[j].(indexRuneDrawable)

		if ti.i.x == tj.i.x {
			return ti.i.y < tj.i.y || (ti.i.y == tj.i.y && ti.r < tj.r)
		}

		return ti.i.x < tj.i.x
	})

	return text
}

// Bridges returns a slice of all bridges, "-)-" or "-(-".
func (c *Canvas) Bridges() []Drawable {
	var bridges []Drawable

	for idx := range leftRight(c.Width, c.Height) {
		if o := c.isBridge(idx); o != NONE {
			bridges = append(
				bridges,
				newHalfStep(idx.north(), S),
				newHalfStep(idx.south(), N),
				Bridge{
					start:       idx,
					orientation: o,
				},
			)
		}
	}

	return bridges
}

// -)- or -(- or
func (c *Canvas) isBridge(i Index) Orientation {
	r := c.runeAt(i)

	left := c.runeAt(i.west())
	right := c.runeAt(i.east())

	if left != '-' || right != '-' {
		return NONE
	}

	if r == '(' {
		return W
	}

	if r == ')' {
		return E
	}

	return NONE
}

func (c *Canvas) isText(i Index) bool {
	// Short circuit, we already saw this index and called it text.
	if _, isText := c.text[i]; isText {
		return true
	}

	if c.runeAt(i) == ' ' {
		return false
	}

	if !c.isReserved(i) {
		return true
	}

	// This is a reserved character with an incoming line (e.g., "|") above it,
	// so call it non-text.
	if c.hasLineAboveOrBelow(i) {
		return false
	}

	// Reserved characters like "o" or "*" with letters sitting next to them
	// are probably text.
	// TODO: Fix this to count contiguous blocks of text. If we had a bunch of
	// reserved characters previously that were counted as text then this
	// should be as well, e.g., "A----B".

	// We're reserved but surrounded by text and probably part of an existing
	// word. Use a hash lookup on the left to preserve chains of
	// reserved-but-text characters like "foo----bar".
	if _, textLeft := c.text[i.west()]; textLeft || !c.isReserved(i.east()) {
		return true
	}

	w := i.west()
	e := i.east()

	if !(c.runeAt(w) == ' ' && c.runeAt(e) == ' ') {
		return false
	}

	// Circles surrounded by whitespace shouldn't be shown as text.
	if c.runeAt(i) == 'o' || c.runeAt(i) == '*' {
		return false
	}

	// We're surrounded by whitespace + text on either side.
	if !c.isReserved(w.west()) || !c.isReserved(e.east()) {
		return true
	}

	return false
}

// Returns true if the character at this index is not reserved for diagrams.
// Characters like "o" need more context (e.g., are other text characters
// nearby) to determine whether they're part of a diagram.
func (c *Canvas) isReserved(i Index) bool {
	r := c.runeAt(i)
	_, isReserved := reservedRunes[r]
	return isReserved
}

// Returns true if it looks like this character belongs to anything besides a
// horizontal line. This is the context we use to determine if a reserved
// character is text or not.
func (c *Canvas) hasLineAboveOrBelow(i Index) bool {
	r := c.runeAt(i)

	switch r {
	case '*', 'o', '+', 'v', '^':
		return c.partOfDiagonalLine(i) || c.partOfVerticalLine(i)
	case '|':
		return c.partOfVerticalLine(i) || c.partOfRoundedCorner(i)
	case '/', '\\':
		return c.partOfDiagonalLine(i)
	case '-':
		return c.partOfRoundedCorner(i)
	case '(', ')':
		return c.partOfVerticalLine(i)
	}

	return false
}

// Returns true if a "|" segment passes through this index.
func (c *Canvas) partOfVerticalLine(i Index) bool {
	this := c.runeAt(i)
	north := c.runeAt(i.north())
	south := c.runeAt(i.south())

	jointAboveMe := this == '|' && isJoint(north)

	if north == '|' || jointAboveMe {
		return true
	}

	jointBelowMe := this == '|' && isJoint(south)

	if south == '|' || jointBelowMe {
		return true
	}

	return false
}

// Return true if a "--" segment passes through this index.
func (c *Canvas) partOfHorizontalLine(i Index) bool {
	return c.runeAt(i.east()) == '-' || c.runeAt(i.west()) == '-'
}

func (c *Canvas) partOfDiagonalLine(i Index) bool {
	r := c.runeAt(i)

	n := c.runeAt(i.north())
	s := c.runeAt(i.south())
	nw := c.runeAt(i.nWest())
	se := c.runeAt(i.sEast())
	ne := c.runeAt(i.nEast())
	sw := c.runeAt(i.sWest())

	switch r {
	// Diagonal segments can be connected to joint or other segments.
	case '/':
		return ne == r || sw == r || isJoint(ne) || isJoint(sw) || n == '\\' || s == '\\'
	case '\\':
		return nw == r || se == r || isJoint(nw) || isJoint(se) || n == '/' || s == '/'

	// For everything else just check if we have segments next to us.
	default:
		return nw == '\\' || ne == '/' || sw == '/' || se == '\\'
	}
}

// For "-" and "|" characters returns true if they could be part of a rounded
// corner.
func (c *Canvas) partOfRoundedCorner(i Index) bool {
	r := c.runeAt(i)

	switch r {
	case '-':
		dotNext := c.runeAt(i.west()) == '.' || c.runeAt(i.east()) == '.'
		hyphenNext := c.runeAt(i.west()) == '\'' || c.runeAt(i.east()) == '\''
		return dotNext || hyphenNext

	case '|':
		dotAbove := c.runeAt(i.nWest()) == '.' || c.runeAt(i.nEast()) == '.'
		hyphenBelow := c.runeAt(i.sWest()) == '\'' || c.runeAt(i.sEast()) == '\''
		return dotAbove || hyphenBelow
	}

	return false
}

// TODO: Have this take care of all the vertical line nudging.
func (c *Canvas) partOfHalfStep(i Index) Orientation {
	r := c.runeAt(i)
	if r != '\'' && r != '.' && r != '|' {
		return NONE
	}

	if c.isRoundedCorner(i) != NONE {
		return NONE
	}

	w := c.runeAt(i.west())
	e := c.runeAt(i.east())
	n := c.runeAt(i.north())
	s := c.runeAt(i.south())
	nw := c.runeAt(i.nWest())
	ne := c.runeAt(i.nEast())

	switch r {
	case '\'':
		//  _      _
		//   '-  -'
		if (nw == '_' && e == '-') || (w == '-' && ne == '_') {
			return N
		}
	case '.':
		// _.-  -._
		if (w == '-' && e == '_') || (w == '_' && e == '-') {
			return S
		}
	case '|':
		//// _   _
		////  | |
		if n != '|' && (ne == '_' || nw == '_') {
			return N
		}

		if n == '-' {
			return N
		}

		//// _| |_
		if s != '|' && (w == '_' || e == '_') {
			return S
		}

		if s == '-' {
			return S
		}
	}
	return NONE
}