// Network-friendly drawing application based on XRender.
//
// TODO
//  - use double buffering to remove flicker
//    (more pronounced over X11 forwarding)
//  - maybe keep the pixmap as large as the window
package main

import (
	"github.com/BurntSushi/xgb"
	"github.com/BurntSushi/xgb/render"
	"github.com/BurntSushi/xgb/xproto"
	"log"
)

func F64ToFixed(f float64) render.Fixed { return render.Fixed(f * 65536) }
func FixedToF64(f render.Fixed) float64 { return float64(f) / 65536 }

func findPictureFormat(formats []render.Pictforminfo,
	depth byte, direct render.Directformat) render.Pictformat {
	for _, pf := range formats {
		if pf.Depth == depth && pf.Direct == direct {
			return pf.Id
		}
	}
	return 0
}

func main() {
	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
	if depth < 24 {
		log.Fatalln("need more colors")
	}

	wid, err := xproto.NewWindowId(X)
	if err != nil {
		log.Fatalln(err)
	}

	_ = xproto.CreateWindow(X, depth, wid, screen.Root,
		0, 0, 500, 500, 0, xproto.WindowClassInputOutput,
		visual, xproto.CwBackPixel|xproto.CwEventMask,
		[]uint32{0xffffffff, xproto.EventMaskButtonPress |
			xproto.EventMaskButtonMotion | xproto.EventMaskButtonRelease |
			xproto.EventMaskStructureNotify | xproto.EventMaskExposure})

	title := []byte("Draw")
	_ = 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)
	}

	// Find appropriate picture formats.
	var pformat, pformatAlpha, pformatRGB render.Pictformat
	for _, pd := range pformats.Screens[X.DefaultScreen].Depths {
		for _, pv := range pd.Visuals {
			if pv.Visual == visual {
				pformat = pv.Format
			}
		}
	}
	if pformatAlpha = findPictureFormat(pformats.Formats, 8,
		render.Directformat{
			AlphaShift: 0,
			AlphaMask:  0xff,
		}); pformat == 0 {
		log.Fatalln("required picture format not found")
	}
	if pformatRGB = findPictureFormat(pformats.Formats, 24,
		render.Directformat{
			RedShift:   16,
			RedMask:    0xff,
			GreenShift: 8,
			GreenMask:  0xff,
			BlueShift:  0,
			BlueMask:   0xff,
		}); pformatRGB == 0 {
		log.Fatalln("required picture format not found")
	}

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

	// Brush shape.
	const brushRadius = 5

	brushid, err := render.NewPictureId(X)
	if err != nil {
		log.Fatalln(err)
	}

	cFull := render.Color{0xffff, 0xffff, 0xffff, 0xffff}
	cTrans := render.Color{0xffff, 0xffff, 0xffff, 0}
	_ = render.CreateRadialGradient(X, brushid,
		render.Pointfix{F64ToFixed(brushRadius), F64ToFixed(brushRadius)},
		render.Pointfix{F64ToFixed(brushRadius), F64ToFixed(brushRadius)},
		F64ToFixed(0),
		F64ToFixed(brushRadius),
		3, []render.Fixed{F64ToFixed(0), F64ToFixed(0.1), F64ToFixed(1)},
		[]render.Color{cFull, cFull, cTrans})

	// Brush color.
	colorid, err := render.NewPictureId(X)
	if err != nil {
		log.Fatalln(err)
	}
	_ = render.CreateSolidFill(X, colorid, render.Color{
		Red:   0xeeee,
		Green: 0x8888,
		Blue:  0x4444,
		Alpha: 0xffff,
	})

	// Pixmaps.
	const (
		pixWidth  = 1000
		pixHeight = 1000
	)

	pixid, err := xproto.NewPixmapId(X)
	if err != nil {
		log.Fatalln(err)
	}
	_ = xproto.CreatePixmap(X, 24, pixid, xproto.Drawable(screen.Root),
		pixWidth, pixHeight)

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

	pixmaskid, err := xproto.NewPixmapId(X)
	if err != nil {
		log.Fatalln(err)
	}
	_ = xproto.CreatePixmap(X, 8, pixmaskid, xproto.Drawable(screen.Root),
		pixWidth, pixHeight)

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

	// Pixmaps come uninitialized.
	render.FillRectangles(X,
		render.PictOpSrc, pixpictid, render.Color{
			Red: 0xffff, Green: 0xffff, Blue: 0xffff, Alpha: 0xffff,
		}, []xproto.Rectangle{{Width: pixWidth, Height: pixHeight}})

	// This is the only method we can use to render proper brush strokes.
	//
	// ConjointOver is defined as: A = Aa * 1 + Ab * max(1-Aa/Ab,0)
	// which basically resolves to: A = max(Aa, Ab)
	// which equals "lighten" with one channel only.
	//
	// Resources:
	//  - https://www.cairographics.org/operators/
	//  - http://ssp.impulsetrain.com/porterduff.html
	//  - https://keithp.com/~keithp/talks/renderproblems/renderproblems/render-title.html
	//  - https://keithp.com/~keithp/talks/cairo2003.pdf
	drawPointAt := func(x, y int16) {
		_ = render.Composite(X, render.PictOpConjointOver,
			brushid, render.PictureNone, pixmaskpictid,
			0, 0, 0, 0, x-brushRadius, y-brushRadius,
			brushRadius*2, brushRadius*2)

		_ = render.SetPictureClipRectangles(X, pid,
			x-brushRadius, y-brushRadius, []xproto.Rectangle{
				{Width: brushRadius * 2, Height: brushRadius * 2}})

		_ = render.Composite(X, render.PictOpSrc,
			pixpictid, render.PictureNone, pid,
			0, 0, 0, 0, 0 /* dst-x */, 0, /* dst-y */
			pixWidth, pixHeight)
		_ = render.Composite(X, render.PictOpOver,
			colorid, pixmaskpictid, pid,
			0, 0, 0, 0, 0 /* dst-x */, 0, /* dst-y */
			pixWidth, pixHeight)
	}

	// Integer version of Bresenham's line drawing algorithm
	drawLine := func(x0, y0, x1, y1 int16) {
		dx, dy := x1-x0, y1-y0
		if dx < 0 {
			dx = -dx
		}
		if dy < 0 {
			dy = -dy
		}

		steep := dx < dy
		if steep {
			// Flip the coordinate system on input
			x0, y0 = y0, x0
			x1, y1 = y1, x1
			dx, dy = dy, dx
		}

		var stepX, stepY int16 = 1, 1
		if x0 > x1 {
			stepX = -1
		}
		if y0 > y1 {
			stepY = -1
		}

		dpr := dy * 2
		delta := dpr - dx
		dpru := delta - dx

		for ; dx > 0; dx-- {
			// Unflip the coordinate system on output
			if steep {
				drawPointAt(y0, x0)
			} else {
				drawPointAt(x0, y0)
			}

			x0 += stepX
			if delta > 0 {
				y0 += stepY
				delta += dpru
			} else {
				delta += dpr
			}
		}
	}

	var startX, startY int16 = 0, 0
	drawing := false
	for {
		ev, xerr := X.WaitForEvent()
		if xerr != nil {
			log.Printf("Error: %s\n", xerr)
			return
		}
		if ev == nil {
			return
		}

		log.Printf("Event: %s\n", ev)
		switch e := ev.(type) {
		case xproto.UnmapNotifyEvent:
			return

		case xproto.ExposeEvent:
			_ = render.SetPictureClipRectangles(X, pid, int16(e.X), int16(e.Y),
				[]xproto.Rectangle{{Width: e.Width, Height: e.Height}})

			_ = render.Composite(X, render.PictOpSrc,
				pixpictid, render.PictureNone, pid,
				0, 0, 0, 0, 0 /* dst-x */, 0, /* dst-y */
				pixWidth, pixHeight)

			if drawing {
				_ = render.Composite(X, render.PictOpOver,
					colorid, pixmaskpictid, pid,
					0, 0, 0, 0, 0 /* dst-x */, 0, /* dst-y */
					pixWidth, pixHeight)
			}

		case xproto.ButtonPressEvent:
			if e.Detail == xproto.ButtonIndex1 {
				render.FillRectangles(X,
					render.PictOpSrc, pixmaskpictid, render.Color{},
					[]xproto.Rectangle{{Width: pixWidth, Height: pixHeight}})

				drawing = true
				drawPointAt(e.EventX, e.EventY)
				startX, startY = e.EventX, e.EventY
			}

		case xproto.MotionNotifyEvent:
			if drawing {
				drawLine(startX, startY, e.EventX, e.EventY)
				startX, startY = e.EventX, e.EventY
			}

		case xproto.ButtonReleaseEvent:
			if e.Detail == xproto.ButtonIndex1 {
				_ = render.Composite(X, render.PictOpOver,
					colorid, pixmaskpictid, pixpictid,
					0, 0, 0, 0, 0 /* dst-x */, 0, /* dst-y */
					pixWidth, pixHeight)

				drawing = false
			}
		}
	}
}