// This is an amalgamation of xgb-xrender.go and xgb-keys.go and more of a demo,
// some comments have been stripped.
package main

import (
	"github.com/BurntSushi/xgb"
	"github.com/BurntSushi/xgb/render"
	"github.com/BurntSushi/xgb/xproto"
	"github.com/golang/freetype"
	"github.com/golang/freetype/truetype"
	"golang.org/x/image/font"
	"golang.org/x/image/font/gofont/goregular"
	"golang.org/x/image/math/fixed"
	"image"
	"image/draw"
	"io/ioutil"
	"log"
	"os"
	"strings"
)

func glyphListBytes(buf []byte, runes []rune, size int) int {
	b := 0
	for _, r := range runes {
		switch size {
		default:
			buf[b] = byte(r)
			b += 1
		case 2:
			xgb.Put16(buf[b:], uint16(r))
			b += 2
		case 4:
			xgb.Put32(buf[b:], uint32(r))
			b += 4
		}
	}
	return xgb.Pad(b)
}

// When the len is 255, a GLYPHABLE follows, otherwise a list of CARD8/16/32.
func glyphEltHeaderBytes(buf []byte, len byte, deltaX, deltaY int16) int {
	b := 0
	buf[b] = len
	b += 4
	xgb.Put16(buf[b:], uint16(deltaX))
	b += 2
	xgb.Put16(buf[b:], uint16(deltaY))
	b += 2
	return xgb.Pad(b)
}

type xgbCookie interface{ Check() error }

// compositeString makes an appropriate render.CompositeGlyphs request,
// assuming that glyphs equal Unicode codepoints.
func compositeString(c *xgb.Conn, op byte, src, dst render.Picture,
	maskFormat render.Pictformat, glyphset render.Glyphset, srcX, srcY int16,
	destX, destY int16, text string) xgbCookie {
	runes := []rune(text)

	var highest rune
	for _, r := range runes {
		if r > highest {
			highest = r
		}
	}

	size := 1
	switch {
	case highest > 1<<16:
		size = 4
	case highest > 1<<8:
		size = 2
	}

	// They gave up on the XCB protocol API and we need to serialize explicitly.

	// To spare us from caring about the padding, use the largest number lesser
	// than 255 that is divisible by 4 (for size 2 and 4 the requirements are
	// less strict but this works in the general case).
	const maxPerChunk = 252

	buf := make([]byte, (len(runes)+maxPerChunk-1)/maxPerChunk*8+len(runes)*size)
	b := 0

	for len(runes) > maxPerChunk {
		b += glyphEltHeaderBytes(buf[b:], maxPerChunk, 0, 0)
		b += glyphListBytes(buf[b:], runes[:maxPerChunk], size)
		runes = runes[maxPerChunk:]
	}
	if len(runes) > 0 {
		b += glyphEltHeaderBytes(buf[b:], byte(len(runes)), destX, destY)
		b += glyphListBytes(buf[b:], runes, size)
	}

	switch size {
	default:
		return render.CompositeGlyphs8(c, op, src, dst, maskFormat, glyphset,
			srcX, srcY, buf)
	case 2:
		return render.CompositeGlyphs16(c, op, src, dst, maskFormat, glyphset,
			srcX, srcY, buf)
	case 4:
		return render.CompositeGlyphs32(c, op, src, dst, maskFormat, glyphset,
			srcX, srcY, buf)
	}
}

type textRenderer struct {
	f    *truetype.Font
	opts *truetype.Options
	face font.Face

	bounds fixed.Rectangle26_6 // outer bounds for all the font's glyph
	buf    *image.RGBA         // rendering buffer

	X      *xgb.Conn
	gsid   render.Glyphset
	loaded map[rune]bool
}

func newTextRenderer(X *xgb.Conn, ttf []byte, opts *truetype.Options) (
	*textRenderer, error) {
	pformats, err := render.QueryPictFormats(X).Reply()
	if err != nil {
		return nil, err
	}

	// We use RGBA here just so that lines are padded to 32 bits.
	// Since there's no subpixel antialiasing and alpha is premultiplied,
	// it doesn't even mater that RGBA is interpreted as ARGB or BGRA.
	var rgbFormat render.Pictformat
	for _, pf := range pformats.Formats {
		if pf.Depth == 32 && pf.Direct.AlphaMask != 0 {
			rgbFormat = pf.Id
			break
		}
	}

	tr := &textRenderer{opts: opts, X: X, loaded: make(map[rune]bool)}
	if tr.f, err = freetype.ParseFont(goregular.TTF); err != nil {
		return nil, err
	}

	tr.face = truetype.NewFace(tr.f, opts)
	tr.bounds = tr.f.Bounds(fixed.Int26_6(opts.Size * float64(opts.DPI) *
		(64.0 / 72.0)))

	if tr.gsid, err = render.NewGlyphsetId(X); err != nil {
		return nil, err
	}
	if err := render.CreateGlyphSetChecked(X, tr.gsid, rgbFormat).
		Check(); err != nil {
		return nil, err
	}

	tr.buf = image.NewRGBA(image.Rect(
		+tr.bounds.Min.X.Floor(),
		-tr.bounds.Min.Y.Floor(),
		+tr.bounds.Max.X.Ceil(),
		-tr.bounds.Max.Y.Ceil(),
	))
	return tr, nil
}

func (tr *textRenderer) addRune(r rune) bool {
	dr, mask, maskp, advance, ok := tr.face.Glyph(
		fixed.P(0, 0) /* subpixel destination location */, r)
	if !ok {
		return false
	}

	for i := 0; i < len(tr.buf.Pix); i++ {
		tr.buf.Pix[i] = 0
	}

	// Copying, since there are absolutely no guarantees.
	draw.Draw(tr.buf, dr, mask, maskp, draw.Src)

	_ = render.AddGlyphs(tr.X, tr.gsid, 1, []uint32{uint32(r)},
		[]render.Glyphinfo{{
			Width:  uint16(tr.buf.Rect.Size().X),
			Height: uint16(tr.buf.Rect.Size().Y),
			X:      int16(-tr.bounds.Min.X.Floor()),
			Y:      int16(+tr.bounds.Max.Y.Ceil()),
			XOff:   int16(advance.Ceil()),
			YOff:   int16(0),
		}}, []byte(tr.buf.Pix))
	return true
}

func (tr *textRenderer) render(src, dst render.Picture,
	srcX, srcY, destX, destY int16, text string) xgbCookie {
	// XXX: You're really supposed to handle tabs differently from this.
	text = strings.Replace(text, "\t", "    ", -1)

	for _, r := range text {
		if !tr.loaded[r] {
			tr.addRune(r)
			tr.loaded[r] = true
		}
	}

	return compositeString(tr.X, render.PictOpOver, src, dst,
		0 /* TODO: mask Pictureformat? */, tr.gsid,
		srcX, srcY, destX, destY, text)
}

const (
	ksEscape     = 0xff1b
	ksUp         = 0xff52
	ksDown       = 0xff54
	ksPageUp     = 0xff55
	ksPageDown   = 0xff56
	ksModeSwitch = 0xff7e
)

type keyMapper struct {
	X       *xgb.Conn
	setup   *xproto.SetupInfo
	mapping *xproto.GetKeyboardMappingReply

	modeSwitchMask uint16
}

func newKeyMapper(X *xgb.Conn) (*keyMapper, error) {
	m := &keyMapper{X: X, setup: xproto.Setup(X)}
	if err := m.update(); err != nil {
		return nil, err
	}
	return m, nil
}

func (km *keyMapper) update() error {
	var err error
	km.mapping, err = xproto.GetKeyboardMapping(km.X, km.setup.MinKeycode,
		byte(km.setup.MaxKeycode-km.setup.MinKeycode+1)).Reply()
	if err != nil {
		return err
	}

	km.modeSwitchMask = 0

	// The order is "Shift, Lock, Control, Mod1, Mod2, Mod3, Mod4, and Mod5."
	mm, err := xproto.GetModifierMapping(km.X).Reply()
	if err != nil {
		return err
	}

	perMod := int(mm.KeycodesPerModifier)
	perKc := int(km.mapping.KeysymsPerKeycode)
	for mod := 0; mod < 8; mod++ {
		for _, kc := range mm.Keycodes[mod*perMod : (mod+1)*perMod] {
			if kc == 0 {
				continue
			}

			k := int(kc - km.setup.MinKeycode)
			for _, ks := range km.mapping.Keysyms[k*perKc : (k+1)*perKc] {
				if ks == ksModeSwitch {
					km.modeSwitchMask |= 1 << uint(mod)
				}
			}
		}
	}
	return nil
}

func (km *keyMapper) decode(e xproto.KeyPressEvent) (result xproto.Keysym) {
	step := int(km.mapping.KeysymsPerKeycode)
	from := int(e.Detail-km.setup.MinKeycode) * step
	ks := km.mapping.Keysyms[from : from+step]

	// Strip trailing NoSymbol entries.
	for len(ks) > 0 && ks[len(ks)-1] == 0 {
		ks = ks[:len(ks)-1]
	}

	// Expand back to at least 4.
	switch {
	case len(ks) == 1:
		ks = append(ks, 0, ks[0], 0)
	case len(ks) == 2:
		ks = append(ks, ks[0], ks[1])
	case len(ks) == 3:
		ks = append(ks, 0)
	}

	// Other silly expansion rules, only applied to basic ASCII since we
	// don't have translation tables to Unicode here for brevity.
	if ks[1] == 0 {
		ks[1] = ks[0]
		if ks[0] >= 'A' && ks[0] <= 'Z' ||
			ks[0] >= 'a' && ks[0] <= 'z' {
			ks[0] = ks[0] | 32
			ks[1] = ks[0] &^ 32
		}
	}

	if ks[3] == 0 {
		ks[3] = ks[2]
		if ks[2] >= 'A' && ks[2] <= 'Z' ||
			ks[2] >= 'a' && ks[2] <= 'z' {
			ks[2] = ks[2] | 32
			ks[3] = ks[2] &^ 32
		}
	}

	offset := 0
	if e.State&km.modeSwitchMask != 0 {
		offset += 2
	}

	shift := e.State&xproto.ModMaskShift != 0
	lock := e.State&xproto.ModMaskLock != 0
	switch {
	case !shift && !lock:
		result = ks[offset+0]
	case !shift && lock:
		if ks[offset+0] >= 'a' && ks[offset+0] <= 'z' {
			result = ks[offset+1]
		} else {
			result = ks[offset+0]
		}
	case shift && lock:
		if ks[offset+1] >= 'a' && ks[offset+1] <= 'z' {
			result = ks[offset+1] &^ 32
		} else {
			result = ks[offset+1]
		}
	case shift:
		result = ks[offset+1]
	}
	return
}

func main() {
	if len(os.Args) < 2 {
		log.Fatalln("no filename given")
	}

	text, err := ioutil.ReadFile(os.Args[1])
	if err != nil {
		log.Fatalln(err)
	}
	lines := strings.Split(string(text), "\n")

	X, err := xgb.NewConn()
	if err != nil {
		log.Fatalln(err)
	}
	if err := render.Init(X); err != nil {
		log.Fatalln(err)
	}

	setup := xproto.Setup(X)
	screen := setup.DefaultScreen(X)

	visual, depth := screen.RootVisual, screen.RootDepth
	// TODO: We should check that we find it, though we don't /need/ alpha here,
	// it's just a minor improvement--affects the backpixel value.
	for _, i := range screen.AllowedDepths {
		// TODO: Could/should check other parameters.
		for _, v := range i.Visuals {
			if i.Depth == 32 && v.Class == xproto.VisualClassTrueColor {
				visual, depth = v.VisualId, i.Depth
				break
			}
		}
	}

	mid, err := xproto.NewColormapId(X)
	if err != nil {
		log.Fatalln(err)
	}
	_ = xproto.CreateColormap(
		X, xproto.ColormapAllocNone, mid, screen.Root, visual)

	wid, err := xproto.NewWindowId(X)
	if err != nil {
		log.Fatalln(err)
	}
	// Border pixel and colormap are required when depth differs from parent.
	_ = xproto.CreateWindow(X, depth, wid, screen.Root,
		0, 0, 500, 500, 0, xproto.WindowClassInputOutput,
		visual, xproto.CwBackPixel|xproto.CwBorderPixel|xproto.CwEventMask|
			xproto.CwColormap, []uint32{0xf0f0f0f0, 0,
			xproto.EventMaskStructureNotify | xproto.EventMaskKeyPress |
				/* KeymapNotify */ xproto.EventMaskKeymapState |
				xproto.EventMaskExposure | xproto.EventMaskButtonPress,
			uint32(mid)})

	title := []byte("Viewer")
	_ = xproto.ChangeProperty(X, xproto.PropModeReplace, wid, xproto.AtomWmName,
		xproto.AtomString, 8, uint32(len(title)), title)

	_ = xproto.MapWindow(X, wid)

	pformats, err := render.QueryPictFormats(X).Reply()
	if err != nil {
		log.Fatalln(err)
	}

	// Similar to XRenderFindVisualFormat.
	// The DefaultScreen is almost certain to be zero.
	var pformat render.Pictformat
	for _, pd := range pformats.Screens[X.DefaultScreen].Depths {
		// This check seems to be slightly extraneous.
		if pd.Depth != depth {
			continue
		}
		for _, pv := range pd.Visuals {
			if pv.Visual == visual {
				pformat = pv.Format
			}
		}
	}

	pid, err := render.NewPictureId(X)
	if err != nil {
		log.Fatalln(err)
	}
	render.CreatePicture(X, pid, xproto.Drawable(wid), pformat, 0, []uint32{})

	blackid, err := render.NewPictureId(X)
	if err != nil {
		log.Fatalln(err)
	}
	_ = render.CreateSolidFill(X, blackid, render.Color{Alpha: 0xffff})

	tr, err := newTextRenderer(X, goregular.TTF, &truetype.Options{
		Size: 10,
		DPI: float64(screen.WidthInPixels) /
			float64(screen.WidthInMillimeters) * 25.4,
		Hinting: font.HintingFull,
	})
	if err != nil {
		log.Fatalln(err)
	}

	scroll := 0 // index of the top line

	var w, h uint16
	redraw := func() {
		y, ascent, step := 5, tr.bounds.Max.Y.Ceil(),
			tr.bounds.Max.Y.Ceil()-tr.bounds.Min.Y.Floor()
		for _, line := range lines[scroll:] {
			if uint16(y) >= h {
				break
			}
			_ = tr.render(blackid, pid, 0, 0, 5, int16(y+ascent), line)
			y += step
		}

		vis := float64(h-10) / float64(step)
		if vis < float64(len(lines)) {
			length := float64(step) * (vis + 1) * vis / float64(len(lines))
			start := float64(step) * float64(scroll) * vis / float64(len(lines))

			_ = render.FillRectangles(X, render.PictOpSrc, pid,
				render.Color{Alpha: 0xffff}, []xproto.Rectangle{{
					X: int16(w - 15), Y: int16(start),
					Width: 15, Height: uint16(length + 10)}})
		}
	}

	km, err := newKeyMapper(X)
	if err != nil {
		log.Fatalln(err)
	}

	for {
		ev, xerr := X.WaitForEvent()
		if xerr != nil {
			log.Printf("Error: %s\n", xerr)
			return
		}
		if ev == nil {
			return
		}

		switch e := ev.(type) {
		case xproto.UnmapNotifyEvent:
			return

		case xproto.ConfigureNotifyEvent:
			w, h = e.Width, e.Height

		case xproto.MappingNotifyEvent:
			_ = km.update()

		case xproto.KeyPressEvent:
			_ = xproto.ClearArea(X, true /* ExposeEvent */, wid, 0, 0, w, h)

			const pageJump = 40
			switch km.decode(e) {
			case ksEscape:
				return
			case ksUp:
				if scroll >= 1 {
					scroll--
				}
			case ksDown:
				if scroll+1 < len(lines) {
					scroll++
				}
			case ksPageUp:
				if scroll >= pageJump {
					scroll -= pageJump
				}
			case ksPageDown:
				if scroll+pageJump < len(lines) {
					scroll += pageJump
				}
			}

		case xproto.ButtonPressEvent:
			_ = xproto.ClearArea(X, true /* ExposeEvent */, wid, 0, 0, w, h)

			switch e.Detail {
			case xproto.ButtonIndex4:
				if scroll > 0 {
					scroll--
				}
			case xproto.ButtonIndex5:
				if scroll+1 < len(lines) {
					scroll++
				}
			}

		case xproto.ExposeEvent:
			// FIXME: The window's context haven't necessarily been destroyed.
			if e.Count == 0 {
				redraw()
			}
		}
	}
}