Add up/down keynav

Paragraph jumping could be better, but that can be refined later.
Progress on #10

internal/system/textbox.go

@@ -1,6 +1,7 @@
 package system
 
 import "image"
+import "unicode"
 import "image/color"
 import "golang.org/x/image/font"
 import "git.tebibyte.media/tomo/tomo"
@@ -31,9 +32,11 @@ type textBox struct {
 	selecting   bool
 	selectStart int
 	dot         text.Dot
+	desiredX    fixed.Int26_6
 
-	drawer typeset.Drawer
+	drawer     typeset.Drawer
-	face   util.Cycler[font.Face]
+	face       util.Cycler[font.Face]
+	lineHeight util.Memo[fixed.Int26_6]
 
 	on struct {
 		contentBoundsChange event.FuncBroadcaster
@@ -46,6 +49,12 @@ func (this *System) NewTextBox () tomo.TextBox {
 	box.box = this.newBox(box)
 	box.attrTextColor.SetFallback(tomo.ATextColor(color.Black))
 	box.attrDotColor.SetFallback(tomo.ADotColor(color.RGBA { G: 255, B: 255, A: 255}))
+	box.lineHeight = util.NewMemo(func () fixed.Int26_6 {
+		face := box.face.Value()
+		if face == nil { return 0 }
+		metrics := face.Metrics()
+		return metrics.Height
+	})
 	return box
 }
 
@@ -89,13 +98,20 @@ func (this *textBox) SetSelectable (selectable bool) {
 }
 
 func (this *textBox) Select (dot text.Dot) {
-	if !this.selectable { return }
+	if this.selectWithoutResettingDesiredX(dot) {
-	if this.dot == dot  { return }
+		this.desiredX = fixed.I(0)
+	}
+}
+
+func (this *textBox) selectWithoutResettingDesiredX (dot text.Dot) bool {
+	if !this.selectable { return false }
+	if this.dot == dot  { return false }
 	this.SetFocused(true)
 	this.dot = dot
 	this.scrollToDot()
 	this.on.dotChange.Broadcast()
 	this.invalidateDraw()
+	return true
 }
 
 func (this *textBox) Dot () text.Dot {
@@ -351,6 +367,42 @@ func (this *textBox) handleKeyDown (key input.Key, numberPad bool) bool {
 	dot       := this.Dot()
 	sel       := modifiers.Shift
 	word      := modifiers.Control
+
+	moveVertically := func (delta fixed.Int26_6) {
+		currentDot := 0
+		if sel {
+			currentDot = dot.End
+		} else {
+			currentDot = dot.Canon().Start
+			if delta > fixed.I(0) { currentDot = dot.Canon().End }
+		}
+
+		nextDot := 0
+		if word {
+			if delta > fixed.I(0) {
+				nextDot = nextParagraph(this.runes, currentDot)
+			} else {
+				nextDot = previousParagraph(this.runes, currentDot)
+			}
+		} else {
+			currentPosition := this.drawer.PositionAt(currentDot)
+			if this.desiredX != fixed.I(0) {
+				currentPosition.X = this.desiredX
+			}
+			nextPosition   := currentPosition
+			nextPosition.Y += this.lineHeight.Value().Mul(delta)
+			this.desiredX   = nextPosition.X
+			nextDot         = this.drawer.AtPosition(nextPosition)
+		}
+
+		if sel {
+			dot.End = nextDot
+			this.selectWithoutResettingDesiredX(dot)
+		} else {
+			this.selectWithoutResettingDesiredX(text.EmptyDot(nextDot))
+		}
+	}
+	
 	switch {
 	case key == input.KeyHome || (modifiers.Alt && key == input.KeyLeft):
 		dot.End = 0
@@ -376,6 +428,12 @@ func (this *textBox) handleKeyDown (key input.Key, numberPad bool) bool {
 			this.Select(text.MoveRight(this.runes, dot, word))
 		}
 		return true
+	case key == input.KeyUp:
+		moveVertically(fixed.I(-1))
+		return true
+	case key == input.KeyDown:
+		moveVertically(fixed.I(1))
+		return true
 	case key == input.Key('a') && modifiers.Control:
 		dot.Start = 0
 		dot.End   = len(this.text)
@@ -396,6 +454,10 @@ func (this *textBox) handleKeyUp (key input.Key, numberPad bool) bool {
 		return true
 	case key == input.KeyEnd || (modifiers.Alt && key == input.KeyRight):
 		return true
+	case key == input.KeyUp:
+		return true
+	case key == input.KeyDown:
+		return true
 	case key == input.KeyLeft:
 		return true
 	case key == input.KeyRight:
@@ -502,6 +564,7 @@ func (this *textBox) handleFaceChange () {
 	this.drawer.SetFace(face)
 	this.invalidateMinimum()
 	this.invalidateLayout()
+	this.lineHeight.Invalidate()
 }
 
 func (this *textBox) recursiveReApply () {
@@ -523,3 +586,32 @@ func (this *textBox) recursiveReApply () {
 		}
 	}
 }
+
+func previousParagraph (text []rune, index int) int {
+	consecLF := 0
+	if index >= len(text) { index = len(text) - 1 }
+	for ; index > 0; index -- {
+		char := text[index]
+		if char == '\n' {
+			consecLF ++
+		} else if !unicode.IsSpace(char) {
+			if consecLF >= 2 { return index + 1 }
+			consecLF  = 0
+		}
+	}
+	return index
+}
+
+func nextParagraph (text []rune, index int) int {
+	consecLF := 0
+	for ; index < len(text); index ++ {
+		char := text[index]
+		if char == '\n' {
+			consecLF ++
+		} else if !unicode.IsSpace(char) {
+			if consecLF >= 2 { return index }
+			consecLF = 0
+		}
+	}
+	return index
+}