//go:build linux

package tap

import (
	"fmt"
	"os"
	"syscall"
	"unsafe"
)

const (
	cIFF_TAP   = 0x0002
	cIFF_NO_PI = 0x1000
	cTUNSETIFF = 0x400454ca
	cSIOCSIFMTU = 0x8922
)

func openTap(name string, mtu int) (*Interface, error) {
	fd, err := syscall.Open("/dev/net/tun", syscall.O_RDWR, 0)
	if err != nil {
		return nil, fmt.Errorf("open /dev/net/tun: %w (is TUN/TAP supported?)", err)
	}

	ifr := make([]byte, 40)
	copy(ifr, []byte(name))
	*(*uint16)(unsafe.Pointer(&ifr[16])) = uint16(cIFF_TAP | cIFF_NO_PI)

	_, _, errno := syscall.Syscall(syscall.SYS_IOCTL, uintptr(fd), cTUNSETIFF, uintptr(unsafe.Pointer(&ifr[0])))
	if errno != 0 {
		syscall.Close(fd)
		return nil, fmt.Errorf("ioctl TUNSETIFF: %w", errno)
	}

	if mtu > 0 {
		mtuIfr := make([]byte, 40)
		copy(mtuIfr, ifr[:16])
		*(*int32)(unsafe.Pointer(&mtuIfr[16])) = int32(mtu)
		_, _, errno := syscall.Syscall(syscall.SYS_IOCTL, uintptr(fd), cSIOCSIFMTU, uintptr(unsafe.Pointer(&mtuIfr[0])))
		if errno != 0 {
			syscall.Close(fd)
			return nil, fmt.Errorf("ioctl SIOCSIFMTU: %w", errno)
		}
	}

	return &Interface{
		Name: name,
		MTU:  mtu,
		fd:   fd,
	}, nil
}

func readBuf(fd int, buf []byte) (int, error) {
	n, err := syscall.Read(fd, buf)
	if err != nil {
		return 0, os.NewSyscallError("read", err)
	}
	return n, nil
}

func writeBuf(fd int, buf []byte) (int, error) {
	n, err := syscall.Write(fd, buf)
	if err != nil {
		return 0, os.NewSyscallError("write", err)
	}
	return n, nil
}

func closeFD(fd int) error {
	return syscall.Close(fd)
}