//go:build linux

package vpn

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

func fillPlatformDiag(r *DiagResult) {
	r.HasCapNetAdmin = ptrBool(checkCapNetAdmin())
	if r.HasCapNetAdmin == nil || !*r.HasCapNetAdmin {
		r.CapNetAdminNote = "CAP_NET_ADMIN 未授权，TUN 操作需 root 或显式授予该能力"
	}

	v := readIPForward()
	r.IPForward = v
	if v == nil || !*v {
		r.IPForwardNote = "未开启，执行: sysctl -w net.ipv4.ip_forward=1"
	}

	m, note := checkMasquerade()
	r.Masquerade = m
	r.MasqueradeNote = note

	v6 := readIP6Forward()
	r.IP6Forward = v6
	if v6 == nil || !*v6 {
		r.IP6ForwardNote = "未开启，执行: sysctl -w net.ipv6.conf.all.forwarding=1"
	}

	m6, note6 := checkMasquerade6()
	r.Masquerade6 = m6
	r.Masquerade6Note = note6

	ufwActive := checkUFWActive()
	r.UFWActive = &ufwActive
	if ufwActive {
		ufwOk, ufwNote := checkUFWForward()
		if !ufwOk {
			r.UFWForwardNote = ufwNote
		}
	}
}

func ptrBool(b bool) *bool { return &b }

func checkCapNetAdmin() bool {
	data, err := os.ReadFile("/proc/self/status")
	if err != nil {
		return false
	}
	for _, line := range strings.Split(string(data), "\n") {
		if !strings.HasPrefix(line, "CapEff:") {
			continue
		}
		fields := strings.Fields(line)
		if len(fields) < 2 {
			return false
		}
		val, err := strconv.ParseUint(fields[1], 16, 64)
		if err != nil {
			return false
		}
		return val&(1<<12) != 0
	}
	return false
}

func readIPForward() *bool {
	data, err := os.ReadFile("/proc/sys/net/ipv4/ip_forward")
	if err != nil {
		return nil
	}
	v := strings.TrimSpace(string(data)) == "1"
	return &v
}

// findExecutable 在常见路径中查找可执行文件，弥补 systemd 服务 PATH 不含 /usr/sbin、/sbin 的问题
func findExecutable(names ...string) string {
	for _, name := range names {
		if p, err := exec.LookPath(name); err == nil {
			return p
		}
		for _, dir := range []string{"/usr/sbin", "/sbin", "/usr/bin", "/bin"} {
			full := dir + "/" + name
			if fi, err := os.Stat(full); err == nil && !fi.IsDir() {
				return full
			}
		}
	}
	return ""
}

// checkMasquerade 检测 NAT masquerade 规则，优先 nft（原生、无兼容问题），回退 iptables
// 返回 (结果, 说明)；结果为 nil 表示无法判定
func checkMasquerade() (*bool, string) {
	// 优先 nft：Debian 12+ 的 iptables 是 nft 包装器，操作原生 nft nat 表会 "incompatible"
	nftPath := findExecutable("nft")
	if nftPath != "" {
		out, err := exec.Command(nftPath, "list", "ruleset").Output()
		if err == nil {
			has := strings.Contains(string(out), "masquerade")
			if has {
				return &has, ""
			}
			return &has, "未检测到 masquerade 规则，客户端无法出网"
		}
		// nft 存在但执行失败（权限不足等），仍回退 iptables 尝试
	}

	// 回退 iptables（老系统或 nft 不可用时）
	iptPath := findExecutable("iptables")
	if iptPath != "" {
		out, err := exec.Command(iptPath, "-t", "nat", "-L", "POSTROUTING", "-n").Output()
		if err != nil {
			// iptables-nft 与原生 nft 表不兼容时，若 nft 也读不到则判定为无法检测
			if nftPath != "" {
				return nil, "iptables 与原生 nft 表不兼容且 nft 不可执行，无法检测 MASQUERADE"
			}
			return nil, "iptables 不可执行（权限不足？），无法检测 MASQUERADE"
		}
		has := strings.Contains(string(out), "MASQUERADE")
		if has {
			return &has, ""
		}
		return &has, "未检测到 MASQUERADE 规则，客户端无法出网"
	}

	return nil, "iptables 与 nft 均未安装，无法检测 NAT 规则。Debian/Ubuntu 安装: apt install nftables"
}

func readIP6Forward() *bool {
	data, err := os.ReadFile("/proc/sys/net/ipv6/conf/all/forwarding")
	if err != nil {
		return nil
	}
	v := strings.TrimSpace(string(data)) == "1"
	return &v
}

func checkMasquerade6() (*bool, string) {
	nftPath := findExecutable("nft")
	if nftPath != "" {
		out, err := exec.Command(nftPath, "list", "ruleset").Output()
		if err == nil {
			s := string(out)
			if strings.Contains(s, "ip6 saddr") && strings.Contains(s, "masquerade") {
				return ptrBool(true), ""
			}
			return ptrBool(false), "未检测到 IPv6 masquerade 规则，IPv6 客户端无法出网"
		}
	}

	ip6tPath := findExecutable("ip6tables")
	if ip6tPath != "" {
		out, err := exec.Command(ip6tPath, "-t", "nat", "-L", "POSTROUTING", "-n").Output()
		if err != nil {
			if nftPath != "" {
				return nil, "ip6tables 与原生 nft 表不兼容且 nft 不可执行，无法检测 IPv6 MASQUERADE"
			}
			return nil, "ip6tables 不可执行（权限不足？），无法检测 IPv6 MASQUERADE"
		}
		has := strings.Contains(string(out), "MASQUERADE")
		if has {
			return &has, ""
		}
		return &has, "未检测到 IPv6 MASQUERADE 规则，IPv6 客户端无法出网"
	}

	return nil, "ip6tables 与 nft 均未安装，无法检测 IPv6 NAT 规则"
}

// checkUFWForward checks if VPN forward rules exist in UFW's user-forward chains.
func checkUFWForward() (bool, string) {
	nftPath := findExecutable("nft")
	if nftPath == "" {
		return true, ""
	}

	// Check IPv4
	out, err := exec.Command(nftPath, "list", "chain", "ip", "filter", "ufw-user-forward").Output()
	if err == nil {
		s := string(out)
		if !strings.Contains(s, "jump lmvpn-fwd") && !strings.Contains(s, "192.168.") {
			return false, "UFW 已启用但 IPv4 转发规则未配置，客户端可能无法上网"
		}
	}

	// Check IPv6
	out6, err := exec.Command(nftPath, "list", "chain", "ip6", "filter", "ufw6-user-forward").Output()
	if err == nil {
		s := string(out6)
		if !strings.Contains(s, "jump lmvpn6-fwd") && !strings.Contains(s, "fd00:") {
			return false, "UFW 已启用但 IPv6 转发规则未配置，IPv6 客户端可能无法上网"
		}
	}

	return true, ""
}