package main

import (
	"encoding/binary"
	"fmt"
	"io"
	"net/http"
	"os"
	"path/filepath"
	"strconv"
	"strings"
	"unsafe"

	"golang.org/x/sys/unix"
)

const (
	evSyn    = 0x00
	evKey    = 0x01
	evAbs    = 0x03
	btnTouch = 0x14a
	absX     = 0x00
	absY     = 0x01

	inputPropDirect  = 0x01
	sizeofInputEvent = 24
)

func evIoc(nr, size uintptr) uintptr {
	const iocRead = 2
	return iocRead<<30 | size<<16 | 'E'<<8 | nr
}

type touchDev struct {
	f                      *os.File
	minX, maxX, minY, maxY int32
}

var touchscreen *touchDev

// Picks the direct-input device that reports absolute X and Y — the same
// probe cabletest runs, so both ends name the same hardware. Events written
// to the node are fed into the input core as if the hardware produced them,
// so cabletest sees a finger without carrying any injection code.
func openTouchscreen() (*touchDev, error) {
	paths, err := filepath.Glob("/dev/input/event*")
	if err != nil {
		return nil, err
	}
	for _, p := range paths {
		f, err := os.OpenFile(p, os.O_RDWR, 0)
		if err != nil {
			continue
		}
		var props, absBits uint64
		if _, _, errno := unix.Syscall(unix.SYS_IOCTL, f.Fd(),
			evIoc(0x09, 8), uintptr(unsafe.Pointer(&props))); errno != 0 {
			f.Close()
			continue
		}
		if _, _, errno := unix.Syscall(unix.SYS_IOCTL, f.Fd(),
			evIoc(0x20+evAbs, 8), uintptr(unsafe.Pointer(&absBits))); errno != 0 {
			f.Close()
			continue
		}
		if props&(1<<inputPropDirect) == 0 ||
			absBits&(1<<absX) == 0 || absBits&(1<<absY) == 0 {
			f.Close()
			continue
		}
		t := &touchDev{f: f}
		if t.minX, t.maxX, err = axisRange(f, absX); err != nil {
			f.Close()
			return nil, fmt.Errorf("%s x range: %w", p, err)
		}
		if t.minY, t.maxY, err = axisRange(f, absY); err != nil {
			f.Close()
			return nil, fmt.Errorf("%s y range: %w", p, err)
		}
		if t.maxX <= t.minX || t.maxY <= t.minY {
			f.Close()
			return nil, fmt.Errorf("%s: degenerate axes x %d..%d y %d..%d",
				p, t.minX, t.maxX, t.minY, t.maxY)
		}
		return t, nil
	}
	return nil, fmt.Errorf("no touchscreen found among /dev/input/event*")
}

func axisRange(f *os.File, axis uintptr) (min, max int32, err error) {
	var info [6]int32
	if _, _, errno := unix.Syscall(unix.SYS_IOCTL, f.Fd(),
		evIoc(0x40+axis, 24), uintptr(unsafe.Pointer(&info[0]))); errno != 0 {
		return 0, 0, errno
	}
	return info[1], info[2], nil
}

type inputEvent struct {
	typ, code uint16
	val       int32
}

// Timestamps are left zero: evdev stamps injected events at delivery.
func (t *touchDev) send(evs []inputEvent) error {
	buf := make([]byte, len(evs)*sizeofInputEvent)
	for i, e := range evs {
		o := i * sizeofInputEvent
		binary.NativeEndian.PutUint16(buf[o+16:], e.typ)
		binary.NativeEndian.PutUint16(buf[o+18:], e.code)
		binary.NativeEndian.PutUint32(buf[o+20:], uint32(e.val))
	}
	_, err := t.f.Write(buf)
	return err
}

// The page speaks portrait canvas pixels; the device speaks the panel's
// landscape axes, so the quarter turn cabletest applies on the way in is
// inverted on the way out.
func touch(w http.ResponseWriter, r *http.Request) {
	body, err := io.ReadAll(r.Body)
	if err != nil {
		return
	}
	f := strings.Fields(string(body))
	if len(f) == 1 && f[0] == "up" {
		if err := touchscreen.send([]inputEvent{{evKey, btnTouch, 0}, {evSyn, 0, 0}}); err != nil {
			http.Error(w, err.Error(), http.StatusInternalServerError)
		}
		return
	}
	if len(f) != 3 || (f[0] != "down" && f[0] != "move") {
		http.Error(w, "want down x y, move x y, or up", http.StatusBadRequest)
		return
	}
	cx, errX := strconv.Atoi(f[1])
	cy, errY := strconv.Atoi(f[2])
	if errX != nil || errY != nil {
		http.Error(w, "bad coordinates", http.StatusBadRequest)
		return
	}
	latestMu.RLock()
	k := latest
	latestMu.RUnlock()
	if k.buf == nil {
		http.Error(w, "no frame to locate the touch on", http.StatusServiceUnavailable)
		return
	}
	cx = min(max(cx, 0), k.ph-1)
	cy = min(max(cy, 0), k.pw-1)
	xl, yl := cy, k.ph-1-cx
	t := touchscreen
	rawX := t.minX + int32(int64(xl)*int64(t.maxX-t.minX)/int64(k.pw-1))
	rawY := t.minY + int32(int64(yl)*int64(t.maxY-t.minY)/int64(k.ph-1))
	evs := []inputEvent{{evAbs, absX, rawX}, {evAbs, absY, rawY}}
	if f[0] == "down" {
		evs = append(evs, inputEvent{evKey, btnTouch, 1})
	}
	evs = append(evs, inputEvent{evSyn, 0, 0})
	if err := t.send(evs); err != nil {
		http.Error(w, err.Error(), http.StatusInternalServerError)
	}
}