package discovery

import (
	"os"
	"os/exec"
	"runtime"
	"strings"
)

// lookupMAC 从 ARP 表获取 IP 对应的 MAC 地址（跨平台）
// 返回空字符串表示无法获取
func lookupMAC(ip string) string {
	switch runtime.GOOS {
	case "linux":
		return macFromProcNetArp(ip)
	case "windows":
		return macFromWindowsArp(ip)
	default:
		return macFromUnixArp(ip)
	}
}

// macFromProcNetArp 解析 Linux /proc/net/arp
func macFromProcNetArp(ip string) string {
	data, err := os.ReadFile("/proc/net/arp")
	if err != nil {
		return ""
	}
	for _, line := range strings.Split(string(data), "\n")[1:] {
		fields := strings.Fields(line)
		if len(fields) >= 4 && fields[0] == ip {
			mac := fields[3]
			if mac != "" && mac != "00:00:00:00:00:00" {
				return mac
			}
		}
	}
	return ""
}

// macFromWindowsArp 解析 Windows `arp -a` 输出
func macFromWindowsArp(ip string) string {
	out, err := exec.Command("arp", "-a").Output()
	if err != nil {
		return ""
	}
	for _, line := range strings.Split(string(out), "\n") {
		fields := strings.Fields(line)
		if len(fields) >= 2 && fields[0] == ip {
			mac := strings.ReplaceAll(fields[1], "-", ":")
			if mac != "" && mac != "00:00:00:00:00:00" {
				return strings.ToLower(mac)
			}
		}
	}
	return ""
}

// macFromUnixArp 解析 macOS/Linux `arp -n` 输出
func macFromUnixArp(ip string) string {
	out, err := exec.Command("arp", "-n", ip).Output()
	if err != nil {
		return ""
	}
	for _, line := range strings.Split(string(out), "\n") {
		fields := strings.Fields(line)
		for i, f := range fields {
			if f == "("+ip+")" && i+2 < len(fields) {
				mac := fields[i+2]
				if mac != "" && mac != "ff:ff:ff:ff:ff:ff" {
					return strings.ToLower(mac)
				}
			}
		}
	}
	return ""
}