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 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})

	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)
	}

	var pformat render.Pictformat
	for _, pd := range pformats.Screens[X.DefaultScreen].Depths {
		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{})

	// Brush shape.
	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(50), F64ToFixed(50)},
		render.Pointfix{F64ToFixed(50), F64ToFixed(50)},
		F64ToFixed(0),
		F64ToFixed(50),
		2, []render.Fixed{F64ToFixed(0), F64ToFixed(1)},
		[]render.Color{cFull, cTrans})

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

	// Unfortunately, XRender won't give us any /blending operators/, only
	// providing basic Porter-Duff ones with useless disjoint/conjoint variants,
	// but we need the "lighten" operator to draw brush strokes properly.
	//
	// https://www.cairographics.org/operators/
	// http://ssp.impulsetrain.com/porterduff.html
	printAt := func(x, y int16) {
		_ = render.Composite(X, render.PictOpOver,
			colorid, brushid, pid,
			0, 0, 0, 0, x-50, y-50, 100, 100)
	}

	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.ButtonPressEvent:
			if e.Detail == xproto.ButtonIndex1 {
				drawing = true
				printAt(e.EventX, e.EventY)
			}

		case xproto.MotionNotifyEvent:
			if drawing {
				printAt(e.EventX, e.EventY)
			}

		case xproto.ButtonReleaseEvent:
			if e.Detail == xproto.ButtonIndex1 {
				drawing = false
			}
		}
	}
}