//go:build linux

package sdk

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

type tapInterface struct {
	Name string
	MTU  int
	fd   int
}

func openTap(name string, mtu int) (*tapInterface, error) {
	fd, err := syscall.Open("/dev/net/tun", syscall.O_RDWR, 0)
	if err != nil {
		return nil, fmt.Errorf("open /dev/net/tun: %w", err)
	}
	var ifr struct {
		name  [16]byte
		flags uint16
	}
	copy(ifr.name[:], []byte(name))
	ifr.flags = 0x0002 | 0x1000 // IFF_TAP | IFF_NO_PI
	if _, _, errno := syscall.Syscall(syscall.SYS_IOCTL, uintptr(fd), 0x400454ca, uintptr(unsafe.Pointer(&ifr))); errno != 0 {
		syscall.Close(fd)
		return nil, fmt.Errorf("TUNSETIFF: %w", errno)
	}
	devName := string(ifr.name[:])
	if idx := indexOfZero(devName); idx >= 0 {
		devName = devName[:idx]
	}
	ti := &tapInterface{Name: devName, MTU: mtu, fd: fd}

	s, err := syscall.Socket(syscall.AF_INET, syscall.SOCK_DGRAM, 0)
	if err == nil {
		defer syscall.Close(s)
		var mtuReq struct {
			name [16]byte
			mtu  int32
		}
		copy(mtuReq.name[:], []byte(devName))
		mtuReq.mtu = int32(mtu)
		syscall.Syscall(syscall.SYS_IOCTL, uintptr(s), 0x400c4a52, uintptr(unsafe.Pointer(&mtuReq)))
	}
	return ti, nil
}

func (ti *tapInterface) Read(buf []byte) (int, error) {
	return syscall.Read(ti.fd, buf)
}

func (ti *tapInterface) Write(buf []byte) error {
	_, err := syscall.Write(ti.fd, buf)
	return err
}

func (ti *tapInterface) Close() error {
	return syscall.Close(ti.fd)
}

func (ti *tapInterface) SetMAC(mac [6]byte) error {
	macStr := fmt.Sprintf("%02x:%02x:%02x:%02x:%02x:%02x", mac[0], mac[1], mac[2], mac[3], mac[4], mac[5])
	return exec.Command("ip", "link", "set", ti.Name, "address", macStr).Run()
}

func (ti *tapInterface) SetIP(cidr string) error {
	if err := exec.Command("ip", "addr", "add", cidr, "dev", ti.Name).Run(); err != nil {
		return fmt.Errorf("add ip: %w", err)
	}
	if err := exec.Command("ip", "link", "set", ti.Name, "up").Run(); err != nil {
		return fmt.Errorf("link up: %w", err)
	}
	return nil
}

func indexOfZero(s string) int {
	for i, c := range []byte(s) {
		if c == 0 {
			return i
		}
	}
	return -1
}