// Package draw speaks the Plan 9 /dev/draw protocol directly.
//
// There is no C, no cgo and no libdraw here: /dev/draw is a file, its
// protocol is a handful of little-endian messages, and this file is the
// whole of what libdraw's init.c, alloc.c and draw.c actually do.
// See draw(3) and /sys/src/libdraw for the authority.
package draw

import (
	"encoding/binary"
	"fmt"
	"os"
	"strconv"
	"sync"
)

// Channel descriptors, from /sys/include/draw.h.
const (
	chGrey1  = 0x31
	chRGB24  = 0x081828
	chRGBA32 = 0x08182848
)

type Point struct{ X, Y int32 }

type Rectangle struct{ Min, Max Point }

func Rect(x0, y0, x1, y1 int32) Rectangle {
	return Rectangle{Point{x0, y0}, Point{x1, y1}}
}

func (r Rectangle) Dx() int32 { return r.Max.X - r.Min.X }
func (r Rectangle) Dy() int32 { return r.Max.Y - r.Min.Y }

// Add offsets a rectangle by p.
func (r Rectangle) Add(p Point) Rectangle {
	return Rectangle{
		Point{r.Min.X + p.X, r.Min.Y + p.Y},
		Point{r.Max.X + p.X, r.Max.Y + p.Y},
	}
}

var ZP Point

// Colors are RGBA, as draw(2) writes them.
const (
	White = 0xFFFFFFFF
	Black = 0x000000FF
	Red   = 0xFF0000FF
	Green = 0x00FF00FF
	Blue  = 0x0000FFFF
)

type Image struct {
	d     *Display
	id    uint32
	R     Rectangle
	Clipr Rectangle
	chn   uint32
	repl  bool
}

func (i *Image) Rect() Rectangle { return i.R }

type Display struct {
	ctl, data *os.File
	dirno     int

	// Screen is the display's own image, id 0.  Without a window system
	// that is the physical screen; under rio it is the whole screen and
	// the window comes from /dev/winname instead.
	Screen *Image

	// opaque is libdraw's display->opaque: a replicated all-ones GREY1
	// pixel.  Draw with a nil mask means "mask with this", not "mask with
	// image 0" -- image 0 is the screen.
	opaque *Image

	mu     sync.Mutex
	nextid uint32
}

// Init attaches to the draw device under dev (normally "/dev").
func Init(dev string) (*Display, error) {
	if dev == "" {
		dev = "/dev"
	}
	ctl, err := os.OpenFile(dev+"/draw/new", os.O_RDWR, 0)
	if err != nil {
		return nil, fmt.Errorf("open draw/new: %w", err)
	}
	// 12 fields of 12 bytes: id, imageid, chan, repl, r[4], clipr[4].
	var info [12 * 12]byte
	n, err := ctl.Read(info[:])
	if err != nil || n < 12 {
		ctl.Close()
		return nil, fmt.Errorf("read draw/new: short read %d: %w", n, err)
	}
	fld := func(i int) int32 {
		s := string(info[i*12 : i*12+12])
		v, _ := strconv.Atoi(trim(s))
		return int32(v)
	}
	d := &Display{ctl: ctl, dirno: int(fld(0))}
	data, err := os.OpenFile(fmt.Sprintf("%s/draw/%d/data", dev, d.dirno), os.O_RDWR, 0)
	if err != nil {
		ctl.Close()
		return nil, fmt.Errorf("open draw/%d/data: %w", d.dirno, err)
	}
	d.data = data
	if n >= len(info) {
		d.Screen = &Image{
			d:     d,
			id:    0,
			chn:   strToChan(trim(string(info[2*12 : 3*12]))),
			repl:  fld(3) != 0,
			R:     Rect(fld(4), fld(5), fld(6), fld(7)),
			Clipr: Rect(fld(8), fld(9), fld(10), fld(11)),
		}
	}
	if d.opaque, err = d.Alloc(Rect(0, 0, 1, 1), chGrey1, true, White); err != nil {
		d.Close()
		return nil, err
	}
	return d, nil
}

func (d *Display) Close() error {
	d.data.Close()
	return d.ctl.Close()
}

func trim(s string) string {
	i, j := 0, len(s)
	for i < j && s[i] == ' ' {
		i++
	}
	for j > i && (s[j-1] == ' ' || s[j-1] == '\n') {
		j--
	}
	return s[i:j]
}

// strToChan parses "r8g8b8" and friends.  Only what the screen actually
// reports is needed, so unknown names fall back to RGB24.
func strToChan(s string) uint32 {
	typ := map[byte]uint32{'r': 0, 'g': 1, 'b': 2, 'k': 3, 'a': 4, 'm': 5, 'x': 6}
	var c uint32
	for i := 0; i+1 < len(s); i += 2 {
		t, ok := typ[s[i]]
		if !ok {
			return chRGB24
		}
		nb := uint32(s[i+1] - '0')
		c = c<<8 | (t&15)<<4 | nb&15
	}
	if c == 0 {
		return chRGB24
	}
	return c
}

func put32(b []byte, off int, v uint32) {
	binary.LittleEndian.PutUint32(b[off:], v)
}

func (d *Display) write(msg []byte) error {
	d.mu.Lock()
	defer d.mu.Unlock()
	_, err := d.data.Write(msg)
	return err
}

// Alloc creates a new image on the server.  A 1x1 replicated image is how
// you make a solid colour: it tiles to fill whatever you draw it into.
func (d *Display) Alloc(r Rectangle, chn uint32, repl bool, col uint32) (*Image, error) {
	d.mu.Lock()
	d.nextid++
	id := d.nextid
	d.mu.Unlock()

	clipr := r
	if repl {
		// Huge but not infinite, so offsets stay huge instead of overflowing.
		clipr = Rect(-0x3FFFFFFF, -0x3FFFFFFF, 0x3FFFFFFF, 0x3FFFFFFF)
	}
	b := make([]byte, 51)
	b[0] = 'b'
	put32(b, 1, id)
	put32(b, 5, 0) // screenid: not a window
	b[9] = 0       // refresh: Refbackup
	put32(b, 10, chn)
	if repl {
		b[14] = 1
	}
	put32(b, 15, uint32(r.Min.X))
	put32(b, 19, uint32(r.Min.Y))
	put32(b, 23, uint32(r.Max.X))
	put32(b, 27, uint32(r.Max.Y))
	put32(b, 31, uint32(clipr.Min.X))
	put32(b, 35, uint32(clipr.Min.Y))
	put32(b, 39, uint32(clipr.Max.X))
	put32(b, 43, uint32(clipr.Max.Y))
	put32(b, 47, col)
	if err := d.write(b); err != nil {
		return nil, fmt.Errorf("alloc image: %w", err)
	}
	return &Image{d: d, id: id, R: r, Clipr: clipr, chn: chn, repl: repl}, nil
}

// Color is the common case of Alloc: one replicated pixel of a solid colour.
func (d *Display) Color(col uint32) (*Image, error) {
	return d.Alloc(Rect(0, 0, 1, 1), chRGBA32, true, col)
}

// Free releases the image.  All of a client's images go away by themselves
// when its data fd closes, so this is only for long-running programs.
func (i *Image) Free() error {
	b := make([]byte, 5)
	b[0] = 'f'
	put32(b, 1, i.id)
	return i.d.write(b)
}

// Draw copies src (through mask, or opaquely if mask is nil) into r on dst.
func Draw(dst *Image, r Rectangle, src *Image, mask *Image, p Point) error {
	b := make([]byte, 45)
	b[0] = 'd'
	put32(b, 1, dst.id)
	put32(b, 5, src.id)
	if mask == nil {
		mask = dst.d.opaque
	}
	put32(b, 9, mask.id)
	put32(b, 13, uint32(r.Min.X))
	put32(b, 17, uint32(r.Min.Y))
	put32(b, 21, uint32(r.Max.X))
	put32(b, 25, uint32(r.Max.Y))
	put32(b, 29, uint32(p.X))
	put32(b, 33, uint32(p.Y))
	put32(b, 37, uint32(p.X))
	put32(b, 41, uint32(p.Y))
	return dst.d.write(b)
}

// Flush makes queued drawing visible.  devdraw executes each write as it
// arrives, so this only matters for the screen refresh.
func (d *Display) Flush() error {
	return d.write([]byte{'v'})
}