package hostmode

import (
	"bufio"
	"errors"
	"fmt"
	"os"
	"os/exec"
	"strings"
)

// primaryMAC resolves the MAC of the iface that carries the default
// IPv4 route. Mirrors quick.sh.tmpl's primary_iface so the agent
// reports the same MAC that was registered (important on Proxmox
// where vmbr0 inherits its physical NIC's MAC).
func primaryMAC() (string, error) {
	iface, err := defaultRouteIface()
	if err != nil {
		return "", err
	}
	raw, err := os.ReadFile(fmt.Sprintf("/sys/class/net/%s/address", iface))
	if err != nil {
		return "", fmt.Errorf("read mac for %s: %w", iface, err)
	}
	return strings.ToLower(strings.TrimSpace(string(raw))), nil
}

// defaultRouteIface shells out to `ip` because reading /proc/net/route
// requires hex-swap logic and still misses the IPv4-only "dev"
// qualification. The service runs as root on a Linux box; `ip` is
// always present.
func defaultRouteIface() (string, error) {
	out, err := exec.Command("ip", "-o", "-4", "route", "show", "default").Output()
	if err != nil {
		return "", fmt.Errorf("ip route: %w", err)
	}
	scan := bufio.NewScanner(strings.NewReader(string(out)))
	for scan.Scan() {
		fields := strings.Fields(scan.Text())
		for i, f := range fields {
			if f == "dev" && i+1 < len(fields) {
				return fields[i+1], nil
			}
		}
	}
	return "", errors.New("no default IPv4 route")
}