//go:build linux

package dnstt

import (
	"fmt"
	"syscall"
)

// OpenRawSocket creates an AF_PACKET raw socket for sniffing IPv4 packets.
func OpenRawSocket() (int, error) {
	fd, err := syscall.Socket(syscall.AF_PACKET, syscall.SOCK_DGRAM, int(htons(ethPAll)))
	if err != nil {
		return -1, fmt.Errorf("open raw socket: %w", err)
	}

	tv := syscall.Timeval{Sec: 0, Usec: 500000}
	_ = syscall.SetsockoptTimeval(fd, syscall.SOL_SOCKET, syscall.SO_RCVTIMEO, &tv)
	return fd, nil
}

// CaptureLoop records matching packets until stop is closed.
func CaptureLoop(fd int, port int, collector *Collector, stop <-chan struct{}) {
	buf := make([]byte, 65535)
	for {
		select {
		case <-stop:
			return
		default:
		}

		n, _, err := syscall.Recvfrom(fd, buf, 0)
		if err != nil {
			if err == syscall.EAGAIN || err == syscall.EWOULDBLOCK || err == syscall.EINTR {
				continue
			}
			return
		}
		ProcessPacket(buf[:n], port, collector)
	}
}

// CloseRawSocket closes a socket opened by OpenRawSocket.
func CloseRawSocket(fd int) error {
	return syscall.Close(fd)
}

func htons(v uint16) uint16 {
	return (v << 8) | (v >> 8)
}

const ethPAll = 0x0003