package main

import (
	"bytes"
	"encoding/binary"
	"fmt"
	"image"
	"image/color"
	"image/png"
	"net"
	"net/http"
	"os"

	"g.fc.run/theater/cabletest/internal/drm"
)

const streamAddr = ":8080"

const page = `<!doctype html>
<meta name="viewport" content="width=device-width">
<title>cabletest</title>
<body style="margin:0;background:#000;min-height:100vh;display:grid;place-items:center">
<canvas style="max-width:100vw;max-height:100vh"></canvas>
<script>
(async () => {
  const cv = document.querySelector('canvas'), g = cv.getContext('2d');
  const rd = (await fetch('frames')).body.getReader();
  let buf = new Uint8Array(0);
  for (;;) {
    const {value, done} = await rd.read();
    if (done) return;
    const b = new Uint8Array(buf.length + value.length);
    b.set(buf); b.set(value, buf.length); buf = b;
    for (;;) {
      if (buf.length < 12) break;
      const dv = new DataView(buf.buffer, buf.byteOffset);
      const x = dv.getUint32(0, true), y = dv.getUint32(4, true), n = dv.getUint32(8, true);
      if (buf.length < 12 + n) break;
      const bmp = await createImageBitmap(new Blob([buf.slice(12, 12 + n)], {type: 'image/png'}));
      if (x === 0 && y === 0 && cv.width !== bmp.width) { cv.width = bmp.width; cv.height = bmp.height; }
      g.drawImage(bmp, x, y);
      buf = buf.slice(12 + n);
    }
  }
})();
</script>
`

func serveStream() error {
	ln, err := net.Listen("tcp", streamAddr)
	if err != nil {
		return err
	}
	host, err := os.Hostname()
	if err != nil {
		host = "localhost"
	}
	fmt.Printf("panel stream at http://%s%s/\n", host, streamAddr)
	mux := http.NewServeMux()
	mux.HandleFunc("/{$}", func(w http.ResponseWriter, r *http.Request) {
		fmt.Fprint(w, page)
	})
	mux.HandleFunc("/frames", frames)
	go http.Serve(ln, mux)
	return nil
}

func frames(w http.ResponseWriter, r *http.Request) {
	g, err := openGrabber()
	if err != nil {
		http.Error(w, err.Error(), http.StatusServiceUnavailable)
		return
	}
	defer g.close()

	cur, pitch, pw, ph, err := g.rawFrame(nil)
	if err != nil {
		http.Error(w, err.Error(), http.StatusServiceUnavailable)
		return
	}

	w.Header().Set("Content-Type", "application/octet-stream")
	if err := send(w, image.Pt(0, 0), decode(cur, pitch, pw, ph)); err != nil {
		return
	}

	ctx := r.Context()
	prev := make([]byte, len(cur))
	for {
		prev, cur = cur, prev
		for {
			if ctx.Err() != nil {
				return
			}
			if cur, pitch, pw, ph, err = g.rawFrame(cur); err != nil {
				fmt.Fprintf(os.Stderr, "stream %s: %v\n", r.RemoteAddr, err)
				return
			}
			rects := diffRects(prev, cur, pitch, pw, ph)
			if len(rects) == 0 {
				continue
			}
			for _, rect := range rects {
				if err := send(w, rect.Min, maskRect(prev, cur, pitch, ph, rect)); err != nil {
					return
				}
			}
			break
		}
	}
}

// Encoding happens inside the vblank budget, and on rect-sized parts the
// dense level trades 3-6x the cpu for a few percent of size — enough to spill
// past a vblank on the once-per-second bursts and stutter the stream.
var encoder = png.Encoder{CompressionLevel: png.DefaultCompression}

func send(w http.ResponseWriter, at image.Point, img *image.NRGBA) error {
	var buf bytes.Buffer
	if err := encoder.Encode(&buf, img); err != nil {
		return err
	}
	var hdr [12]byte
	binary.LittleEndian.PutUint32(hdr[0:], uint32(at.X))
	binary.LittleEndian.PutUint32(hdr[4:], uint32(at.Y))
	binary.LittleEndian.PutUint32(hdr[8:], uint32(buf.Len()))
	if _, err := w.Write(hdr[:]); err != nil {
		return err
	}
	if _, err := w.Write(buf.Bytes()); err != nil {
		return err
	}
	w.(http.Flusher).Flush()
	return nil
}

// Distant regions changing together (a spinner up top, counters at the foot)
// would union into one near-full-height box, so changed portrait rows are
// merged into bands only across small gaps and each band gets its own box.
const bandGap = 32

// Compares in panel space, where a portrait column is one contiguous row, and
// reports the changed bounding boxes in portrait coordinates.
func diffRects(prev, cur []byte, pitch, pw, ph int) []image.Rectangle {
	var rows []int
	for row := 0; row < ph; row++ {
		if !bytes.Equal(prev[row*pitch:row*pitch+pw*4], cur[row*pitch:row*pitch+pw*4]) {
			rows = append(rows, row)
		}
	}
	if len(rows) == 0 {
		return nil
	}

	changed := make([]bool, pw)
	for _, row := range rows {
		a := prev[row*pitch : row*pitch+pw*4]
		b := cur[row*pitch : row*pitch+pw*4]
		for y := 0; y < pw; y++ {
			if !changed[y] && !bytes.Equal(a[y*4:y*4+4], b[y*4:y*4+4]) {
				changed[y] = true
			}
		}
	}

	var rects []image.Rectangle
	for y := 0; y < pw; {
		if !changed[y] {
			y++
			continue
		}
		y0, y1 := y, y+1
		for next := y1; next < pw && next < y1+bandGap; next++ {
			if changed[next] {
				y1 = next + 1
			}
		}
		rMin, rMax := -1, -1
		for _, row := range rows {
			if !bytes.Equal(prev[row*pitch+y0*4:row*pitch+y1*4], cur[row*pitch+y0*4:row*pitch+y1*4]) {
				if rMin < 0 {
					rMin = row
				}
				rMax = row
			}
		}
		rects = append(rects, image.Rect(ph-1-rMax, y0, ph-rMin, y1))
		y = y1
	}
	return rects
}

// The box holds only the pixels that changed, transparent everywhere else, so
// the client composites it straight over the last frame.
func maskRect(prev, cur []byte, pitch, ph int, rect image.Rectangle) *image.NRGBA {
	img := image.NewNRGBA(image.Rect(0, 0, rect.Dx(), rect.Dy()))
	for x := rect.Min.X; x < rect.Max.X; x++ {
		row := (ph - 1 - x) * pitch
		for y := rect.Min.Y; y < rect.Max.Y; y++ {
			o := row + y*4
			if bytes.Equal(prev[o:o+4], cur[o:o+4]) {
				continue
			}
			v := uint32(cur[o]) | uint32(cur[o+1])<<8 |
				uint32(cur[o+2])<<16 | uint32(cur[o+3])<<24
			cr, cg, cb := drm.Unpack(v)
			img.SetNRGBA(x-rect.Min.X, y-rect.Min.Y, color.NRGBA{R: cr, G: cg, B: cb, A: 255})
		}
	}
	return img
}