// Package evdev is the kernel input-event ABI.
package evdev

import (
	"fmt"
	"os"
	"path/filepath"
	"unsafe"

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

const (
	Syn       = 0x00
	Key       = 0x01
	Abs       = 0x03
	SynReport = 0x00
	BtnTouch  = 0x14a
	AbsX      = 0x00
	AbsY      = 0x01

	propDirect = 0x01

	EventSize = 24
)

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

type Touchscreen struct {
	F                      *os.File
	MinX, MaxX, MinY, MaxY int32
}

// Picks the direct-input device that reports absolute X and Y, which is the
// touchscreen; a mouse is relative and a keyboard has no absolute axes.
func FindTouchscreen(flag int) (*Touchscreen, error) {
	paths, err := filepath.Glob("/dev/input/event*")
	if err != nil {
		return nil, err
	}
	for _, p := range paths {
		f, err := os.OpenFile(p, flag, 0)
		if err != nil {
			continue
		}
		var props, absBits uint64
		if _, _, errno := unix.Syscall(unix.SYS_IOCTL, f.Fd(),
			ioc(0x09, 8), uintptr(unsafe.Pointer(&props))); errno != 0 {
			f.Close()
			continue
		}
		if _, _, errno := unix.Syscall(unix.SYS_IOCTL, f.Fd(),
			ioc(0x20+Abs, 8), uintptr(unsafe.Pointer(&absBits))); errno != 0 {
			f.Close()
			continue
		}
		if props&(1<<propDirect) == 0 ||
			absBits&(1<<AbsX) == 0 || absBits&(1<<AbsY) == 0 {
			f.Close()
			continue
		}
		t := &Touchscreen{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(),
		ioc(0x40+axis, 24), uintptr(unsafe.Pointer(&info[0]))); errno != 0 {
		return 0, 0, errno
	}
	return info[1], info[2], nil
}