// SPDX-License-Identifier: Unlicense OR MIT

package widget

import (
	"image"
	"math"
	"strings"
	"time"
	"unicode/utf8"

	"gioui.org/f32"
	"gioui.org/gesture"
	"gioui.org/io/key"
	"gioui.org/io/pointer"
	"gioui.org/layout"
	"gioui.org/op"
	"gioui.org/op/paint"
	"gioui.org/text"
	"gioui.org/unit"

	"golang.org/x/image/math/fixed"
)

// Editor implements an editable and scrollable text area.
type Editor struct {
	Alignment text.Alignment
	// SingleLine force the text to stay on a single line.
	// SingleLine also sets the scrolling direction to
	// horizontal.
	SingleLine bool
	// Submit enabled translation of carriage return keys to SubmitEvents.
	// If not enabled, carriage returns are inserted as newlines in the text.
	Submit bool

	eventKey     int
	font         text.Font
	shaper       text.Shaper
	textSize     fixed.Int26_6
	blinkStart   time.Time
	focused      bool
	rr           editBuffer
	maxWidth     int
	viewSize     image.Point
	valid        bool
	lines        []text.Line
	shapes       []line
	dims         layout.Dimensions
	requestFocus bool

	caret struct {
		on     bool
		scroll bool

		// xoff is the offset to the current caret
		// position when moving between lines.
		xoff fixed.Int26_6
	}

	scroller  gesture.Scroll
	scrollOff image.Point

	clicker gesture.Click

	// events is the list of events not yet processed.
	events []EditorEvent
	// prevEvents is the number of events from the previous frame.
	prevEvents int
}

type EditorEvent interface {
	isEditorEvent()
}

// A ChangeEvent is generated for every user change to the text.
type ChangeEvent struct{}

// A SubmitEvent is generated when Submit is set
// and a carriage return key is pressed.
type SubmitEvent struct {
	Text string
}

type line struct {
	offset f32.Point
	clip   op.CallOp
}

const (
	blinksPerSecond  = 1
	maxBlinkDuration = 10 * time.Second
)

// Events returns available editor events.
func (e *Editor) Events() []EditorEvent {
	events := e.events
	e.events = nil
	e.prevEvents = 0
	return events
}

func (e *Editor) processEvents(gtx layout.Context) {
	// Flush events from before the previous Layout.
	n := copy(e.events, e.events[e.prevEvents:])
	e.events = e.events[:n]
	e.prevEvents = n

	if e.shaper == nil {
		// Can't process events without a shaper.
		return
	}
	e.makeValid()
	e.processPointer(gtx)
	e.processKey(gtx)
}

func (e *Editor) makeValid() {
	if !e.valid {
		e.lines, e.dims = e.layoutText(e.shaper)
		e.valid = true
	}
}

func (e *Editor) processPointer(gtx layout.Context) {
	sbounds := e.scrollBounds()
	var smin, smax int
	var axis gesture.Axis
	if e.SingleLine {
		axis = gesture.Horizontal
		smin, smax = sbounds.Min.X, sbounds.Max.X
	} else {
		axis = gesture.Vertical
		smin, smax = sbounds.Min.Y, sbounds.Max.Y
	}
	sdist := e.scroller.Scroll(gtx.Metric, gtx, gtx.Now, axis)
	var soff int
	if e.SingleLine {
		e.scrollRel(sdist, 0)
		soff = e.scrollOff.X
	} else {
		e.scrollRel(0, sdist)
		soff = e.scrollOff.Y
	}
	for _, evt := range e.clicker.Events(gtx) {
		switch {
		case evt.Type == gesture.TypePress && evt.Source == pointer.Mouse,
			evt.Type == gesture.TypeClick && evt.Source == pointer.Touch:
			e.blinkStart = gtx.Now
			e.moveCoord(image.Point{
				X: int(math.Round(float64(evt.Position.X))),
				Y: int(math.Round(float64(evt.Position.Y))),
			})
			e.requestFocus = true
			if e.scroller.State() != gesture.StateFlinging {
				e.caret.scroll = true
			}
		}
	}
	if (sdist > 0 && soff >= smax) || (sdist < 0 && soff <= smin) {
		e.scroller.Stop()
	}
}

func (e *Editor) processKey(gtx layout.Context) {
	if e.rr.Changed() {
		e.events = append(e.events, ChangeEvent{})
	}
	for _, ke := range gtx.Events(&e.eventKey) {
		e.blinkStart = gtx.Now
		switch ke := ke.(type) {
		case key.FocusEvent:
			e.focused = ke.Focus
		case key.Event:
			if !e.focused {
				break
			}
			if e.Submit && (ke.Name == key.NameReturn || ke.Name == key.NameEnter) {
				if !ke.Modifiers.Contain(key.ModShift) {
					e.events = append(e.events, SubmitEvent{
						Text: e.Text(),
					})
					return
				}
			}
			if e.command(ke) {
				e.caret.scroll = true
				e.scroller.Stop()
			}
		case key.EditEvent:
			e.caret.scroll = true
			e.scroller.Stop()
			e.append(ke.Text)
		}
		if e.rr.Changed() {
			e.events = append(e.events, ChangeEvent{})
		}
	}
}

func (e *Editor) command(k key.Event) bool {
	switch k.Name {
	case key.NameReturn, key.NameEnter:
		e.append("\n")
	case key.NameDeleteBackward:
		e.Delete(-1)
	case key.NameDeleteForward:
		e.Delete(1)
	case key.NameUpArrow:
		line, _, carX, _ := e.layoutCaret()
		e.caret.xoff = e.moveToLine(carX+e.caret.xoff, line-1)
	case key.NameDownArrow:
		line, _, carX, _ := e.layoutCaret()
		e.caret.xoff = e.moveToLine(carX+e.caret.xoff, line+1)
	case key.NameLeftArrow:
		e.Move(-1)
	case key.NameRightArrow:
		e.Move(1)
	case key.NamePageUp:
		e.movePages(-1)
	case key.NamePageDown:
		e.movePages(+1)
	case key.NameHome:
		e.moveStart()
	case key.NameEnd:
		e.moveEnd()
	default:
		return false
	}
	return true
}

// Focus requests the input focus for the Editor.
func (e *Editor) Focus() {
	e.requestFocus = true
}

// Focused returns whether the editor is focused or not.
func (e *Editor) Focused() bool {
	return e.focused
}

// Layout lays out the editor.
func (e *Editor) Layout(gtx layout.Context, sh text.Shaper, font text.Font, size unit.Value) layout.Dimensions {
	textSize := fixed.I(gtx.Px(size))
	if e.font != font || e.textSize != textSize {
		e.invalidate()
		e.font = font
		e.textSize = textSize
	}
	maxWidth := gtx.Constraints.Max.X
	if e.SingleLine {
		maxWidth = inf
	}
	if maxWidth != e.maxWidth {
		e.maxWidth = maxWidth
		e.invalidate()
	}
	if sh != e.shaper {
		e.shaper = sh
		e.invalidate()
	}

	e.processEvents(gtx)
	return e.layout(gtx)
}

func (e *Editor) layout(gtx layout.Context) layout.Dimensions {
	e.makeValid()

	e.viewSize = gtx.Constraints.Constrain(e.dims.Size)
	// Adjust scrolling for new viewport and layout.
	e.scrollRel(0, 0)

	if e.caret.scroll {
		e.caret.scroll = false
		e.scrollToCaret()
	}

	off := image.Point{
		X: -e.scrollOff.X,
		Y: -e.scrollOff.Y,
	}
	clip := textPadding(e.lines)
	clip.Max = clip.Max.Add(e.viewSize)
	it := lineIterator{
		Lines:     e.lines,
		Clip:      clip,
		Alignment: e.Alignment,
		Width:     e.viewSize.X,
		Offset:    off,
	}
	e.shapes = e.shapes[:0]
	for {
		_, _, layout, off, ok := it.Next()
		if !ok {
			break
		}
		path := e.shaper.Shape(e.font, e.textSize, layout)
		e.shapes = append(e.shapes, line{off, path})
	}

	key.InputOp{Tag: &e.eventKey, Focus: e.requestFocus}.Add(gtx.Ops)
	e.requestFocus = false
	pointerPadding := gtx.Px(unit.Dp(4))
	r := image.Rectangle{Max: e.viewSize}
	r.Min.X -= pointerPadding
	r.Min.Y -= pointerPadding
	r.Max.X += pointerPadding
	r.Max.X += pointerPadding
	pointer.Rect(r).Add(gtx.Ops)
	e.scroller.Add(gtx.Ops)
	e.clicker.Add(gtx.Ops)
	e.caret.on = false
	if e.focused {
		now := gtx.Now
		dt := now.Sub(e.blinkStart)
		blinking := dt < maxBlinkDuration
		const timePerBlink = time.Second / blinksPerSecond
		nextBlink := now.Add(timePerBlink/2 - dt%(timePerBlink/2))
		if blinking {
			redraw := op.InvalidateOp{At: nextBlink}
			redraw.Add(gtx.Ops)
		}
		e.caret.on = e.focused && (!blinking || dt%timePerBlink < timePerBlink/2)
	}

	return layout.Dimensions{Size: e.viewSize, Baseline: e.dims.Baseline}
}

func (e *Editor) PaintText(gtx layout.Context) {
	clip := textPadding(e.lines)
	clip.Max = clip.Max.Add(e.viewSize)
	for _, shape := range e.shapes {
		stack := op.Push(gtx.Ops)
		op.TransformOp{}.Offset(shape.offset).Add(gtx.Ops)
		shape.clip.Add(gtx.Ops)
		paint.PaintOp{Rect: layout.FRect(clip).Sub(shape.offset)}.Add(gtx.Ops)
		stack.Pop()
	}
}

func (e *Editor) PaintCaret(gtx layout.Context) {
	if !e.caret.on {
		return
	}
	carWidth := fixed.I(gtx.Px(unit.Dp(1)))
	carLine, _, carX, carY := e.layoutCaret()

	stack := op.Push(gtx.Ops)
	carX -= carWidth / 2
	carAsc, carDesc := -e.lines[carLine].Bounds.Min.Y, e.lines[carLine].Bounds.Max.Y
	carRect := image.Rectangle{
		Min: image.Point{X: carX.Ceil(), Y: carY - carAsc.Ceil()},
		Max: image.Point{X: carX.Ceil() + carWidth.Ceil(), Y: carY + carDesc.Ceil()},
	}
	carRect = carRect.Add(image.Point{
		X: -e.scrollOff.X,
		Y: -e.scrollOff.Y,
	})
	clip := textPadding(e.lines)
	// Account for caret width to each side.
	whalf := (carWidth / 2).Ceil()
	if clip.Max.X < whalf {
		clip.Max.X = whalf
	}
	if clip.Min.X > -whalf {
		clip.Min.X = -whalf
	}
	clip.Max = clip.Max.Add(e.viewSize)
	carRect = clip.Intersect(carRect)
	if !carRect.Empty() {
		paint.PaintOp{Rect: layout.FRect(carRect)}.Add(gtx.Ops)
	}
	stack.Pop()
}

// Len is the length of the editor contents.
func (e *Editor) Len() int {
	return e.rr.len()
}

// Text returns the contents of the editor.
func (e *Editor) Text() string {
	return e.rr.String()
}

// SetText replaces the contents of the editor.
func (e *Editor) SetText(s string) {
	e.rr = editBuffer{}
	e.caret.xoff = 0
	e.prepend(s)
}

func (e *Editor) scrollBounds() image.Rectangle {
	var b image.Rectangle
	if e.SingleLine {
		if len(e.lines) > 0 {
			b.Min.X = align(e.Alignment, e.lines[0].Width, e.viewSize.X).Floor()
			if b.Min.X > 0 {
				b.Min.X = 0
			}
		}
		b.Max.X = e.dims.Size.X + b.Min.X - e.viewSize.X
	} else {
		b.Max.Y = e.dims.Size.Y - e.viewSize.Y
	}
	return b
}

func (e *Editor) scrollRel(dx, dy int) {
	e.scrollAbs(e.scrollOff.X+dx, e.scrollOff.Y+dy)
}

func (e *Editor) scrollAbs(x, y int) {
	e.scrollOff.X = x
	e.scrollOff.Y = y
	b := e.scrollBounds()
	if e.scrollOff.X > b.Max.X {
		e.scrollOff.X = b.Max.X
	}
	if e.scrollOff.X < b.Min.X {
		e.scrollOff.X = b.Min.X
	}
	if e.scrollOff.Y > b.Max.Y {
		e.scrollOff.Y = b.Max.Y
	}
	if e.scrollOff.Y < b.Min.Y {
		e.scrollOff.Y = b.Min.Y
	}
}

func (e *Editor) moveCoord(pos image.Point) {
	var (
		prevDesc fixed.Int26_6
		carLine  int
		y        int
	)
	for _, l := range e.lines {
		y += (prevDesc + l.Ascent).Ceil()
		prevDesc = l.Descent
		if y+prevDesc.Ceil() >= pos.Y+e.scrollOff.Y {
			break
		}
		carLine++
	}
	x := fixed.I(pos.X + e.scrollOff.X)
	e.carXOff = 0
	e.moveToLine(x, carLine)
}

func (e *Editor) layoutText(s text.Shaper) ([]text.Line, layout.Dimensions) {
	e.rr.Reset()
	lines, _ := s.Layout(e.font, e.textSize, e.maxWidth, &e.rr)
	dims := linesDimens(lines)
	for i := 0; i < len(lines)-1; i++ {
		// To avoid layout flickering while editing, assume a soft newline takes
		// up all available space.
		if layout := lines[i].Layout; len(layout) > 0 {
			r := layout[len(layout)-1].Rune
			if r != '\n' {
				dims.Size.X = e.maxWidth
				break
			}
		}
	}
	return lines, dims
}

// CaretPos returns the line & column numbers of the caret.
func (e *Editor) CaretPos() (line, col int) {
	line, col, _, _ = e.layoutCaret()
	return
}

// CaretCoords returns the coordinates of the caret, relative to the
// editor itself.
func (e *Editor) CaretCoords() f32.Point {
	_, _, x, y := e.layoutCaret()
	return f32.Pt(float32(x)/64, float32(y))
}

func (e *Editor) layoutCaret() (carLine, carCol int, x fixed.Int26_6, y int) {
	var idx int
	var prevDesc fixed.Int26_6
loop:
	for carLine = 0; carLine < len(e.lines); carLine++ {
		l := e.lines[carLine]
		y += (prevDesc + l.Ascent).Ceil()
		prevDesc = l.Descent
		if carLine == len(e.lines)-1 || idx+l.Len > e.rr.caret {
			for _, g := range l.Layout {
				if idx == e.rr.caret {
					break loop
				}
				x += g.Advance
				idx += utf8.RuneLen(g.Rune)
				carCol++
			}
			break
		}
		idx += l.Len
	}
	x += align(e.Alignment, e.lines[carLine].Width, e.viewSize.X)
	return
}

func (e *Editor) invalidate() {
	e.valid = false
}

// Delete runes from the caret position. The sign of runes specifies the
// direction to delete: positive is forward, negative is backward.
func (e *Editor) Delete(runes int) {
	e.rr.deleteRunes(runes)
	e.caret.xoff = 0
	e.invalidate()
}

// Insert inserts text at the caret, moving the caret forward.
func (e *Editor) Insert(s string) {
	e.append(s)
	e.caret.scroll = true
	e.invalidate()
}

func (e *Editor) append(s string) {
	if e.SingleLine {
		s = strings.ReplaceAll(s, "\n", "")
	}
	e.prepend(s)
	e.rr.caret += len(s)
}

func (e *Editor) prepend(s string) {
	e.rr.prepend(s)
	e.caret.xoff = 0
	e.invalidate()
}

func (e *Editor) movePages(pages int) {
	_, _, carX, carY := e.layoutCaret()
	y := carY + pages*e.viewSize.Y
	var (
		prevDesc fixed.Int26_6
		carLine2 int
	)
	y2 := e.lines[0].Ascent.Ceil()
	for i := 1; i < len(e.lines); i++ {
		if y2 >= y {
			break
		}
		l := e.lines[i]
		h := (prevDesc + l.Ascent).Ceil()
		prevDesc = l.Descent
		if y2+h-y >= y-y2 {
			break
		}
		y2 += h
		carLine2++
	}
	e.caret.xoff = e.moveToLine(carX+e.caret.xoff, carLine2)
}

func (e *Editor) moveToLine(carX fixed.Int26_6, carLine2 int) fixed.Int26_6 {
	carLine, carCol, _, _ := e.layoutCaret()
	if carLine2 < 0 {
		carLine2 = 0
	}
	if carLine2 >= len(e.lines) {
		carLine2 = len(e.lines) - 1
	}
	// Move to start of line.
	for i := carCol - 1; i >= 0; i-- {
		_, s := e.rr.runeBefore(e.rr.caret)
		e.rr.caret -= s
	}
	if carLine2 != carLine {
		// Move to start of line2.
		if carLine2 > carLine {
			for i := carLine; i < carLine2; i++ {
				e.rr.caret += e.lines[i].Len
			}
		} else {
			for i := carLine - 1; i >= carLine2; i-- {
				e.rr.caret -= e.lines[i].Len
			}
		}
	}
	l2 := e.lines[carLine2]
	carX2 := align(e.Alignment, l2.Width, e.viewSize.X)
	// Only move past the end of the last line
	end := 0
	if carLine2 < len(e.lines)-1 {
		end = 1
	}
	// Move to rune closest to previous horizontal position.
	for i := 0; i < len(l2.Layout)-end; i++ {
		g := l2.Layout[i]
		if carX2 >= carX {
			break
		}
		if carX2+g.Advance-carX >= carX-carX2 {
			break
		}
		carX2 += g.Advance
		_, s := e.rr.runeAt(e.rr.caret)
		e.rr.caret += s
	}
	return carX - carX2
}

// Move the caret: positive distance moves forward, negative distance moves
// backward.
func (e *Editor) Move(distance int) {
	e.rr.move(distance)
	e.caret.xoff = 0
}

func (e *Editor) moveStart() {
	carLine, carCol, x, _ := e.layoutCaret()
	layout := e.lines[carLine].Layout
	for i := carCol - 1; i >= 0; i-- {
		_, s := e.rr.runeBefore(e.rr.caret)
		e.rr.caret -= s
		x -= layout[i].Advance
	}
	e.caret.xoff = -x
}

func (e *Editor) moveEnd() {
	carLine, carCol, x, _ := e.layoutCaret()
	l := e.lines[carLine]
	// Only move past the end of the last line
	end := 0
	if carLine < len(e.lines)-1 {
		end = 1
	}
	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
	}
	a := align(e.Alignment, l.Width, e.viewSize.X)
	e.caret.xoff = l.Width + a - x
}

func (e *Editor) scrollToCaret() {
	carLine, _, x, y := e.layoutCaret()
	l := e.lines[carLine]
	if e.SingleLine {
		var dist int
		if d := x.Floor() - e.scrollOff.X; d < 0 {
			dist = d
		} else if d := x.Ceil() - (e.scrollOff.X + e.viewSize.X); d > 0 {
			dist = d
		}
		e.scrollRel(dist, 0)
	} else {
		miny := y - l.Ascent.Ceil()
		maxy := y + l.Descent.Ceil()
		var dist int
		if d := miny - e.scrollOff.Y; d < 0 {
			dist = d
		} else if d := maxy - (e.scrollOff.Y + e.viewSize.Y); d > 0 {
			dist = d
		}
		e.scrollRel(0, dist)
	}
}

// NumLines returns the number of lines in the editor.
func (e *Editor) NumLines() int {
	e.makeValid()
	return len(e.lines)
}

func (s ChangeEvent) isEditorEvent() {}
func (s SubmitEvent) isEditorEvent() {}