package main

import (
	"context"
	"errors"
	"net"
	"net/netip"
	"strings"
	"time"
)

func resolveEndpoint(raw string) (netip.Addr, string, error) {
	host, port, err := net.SplitHostPort(raw)
	if err != nil || host == "" || port == "" {
		return netip.Addr{}, "", errors.New("VPN 公网端点格式无效")
	}
	if ip, err := netip.ParseAddr(host); err == nil {
		if !ip.Is4() || !ip.IsGlobalUnicast() {
			return netip.Addr{}, "", errors.New("VPN 公网端点需要有效 IPv4 地址")
		}
		return ip, port, nil
	}
	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
	defer cancel()
	addresses, err := net.DefaultResolver.LookupNetIP(ctx, "ip4", host)
	if err != nil {
		return netip.Addr{}, "", errors.New("无法在连接前解析 VPN 公网端点")
	}
	for _, address := range addresses {
		if address.Is4() && address.IsGlobalUnicast() {
			return address, port, nil
		}
	}
	return netip.Addr{}, "", errors.New("VPN 公网端点没有可用 IPv4 地址")
}

// networkPlan contains only public addressing information. The privileged
// network adapter still needs to discover the physical gateway and persist a
// rollback journal before applying any of these routes.
type networkPlan struct {
	Interface      string         `json:"interface"`
	ClientAddress  netip.Prefix   `json:"client_address"`
	Routes         []netip.Prefix `json:"routes"`
	Endpoint       netip.Addr     `json:"endpoint"`
	EndpointBypass bool           `json:"endpoint_bypass"`
	PhysicalExit   *physicalRoute `json:"physical_exit,omitempty"`
}

type physicalRoute struct {
	Interface string      `json:"interface"`
	Gateway   *netip.Addr `json:"gateway,omitempty"`
}

// macOS route -n get output is only an observation, never an executable
// command. Reject another VPN's utun as the physical endpoint exit.
func parsePhysicalRoute(output string) (physicalRoute, error) {
	var exit physicalRoute
	for _, line := range strings.Split(output, "\n") {
		key, value, ok := strings.Cut(strings.TrimSpace(line), ":")
		if !ok {
			continue
		}
		value = strings.TrimSpace(value)
		switch key {
		case "interface":
			exit.Interface = value
		case "gateway":
			gateway, err := netip.ParseAddr(value)
			if err != nil || !gateway.Is4() || gateway.IsUnspecified() || gateway.IsLoopback() {
				return physicalRoute{}, errors.New("物理网关地址无效")
			}
			exit.Gateway = &gateway
		}
	}
	if len(exit.Interface) < 2 || len(exit.Interface) > 16 || strings.HasPrefix(exit.Interface, "utun") || exit.Interface == "lo0" {
		return physicalRoute{}, errors.New("VPN 网关当前未经过物理网络出口")
	}
	for _, char := range exit.Interface {
		if !((char >= 'a' && char <= 'z') || (char >= 'A' && char <= 'Z') || (char >= '0' && char <= '9')) {
			return physicalRoute{}, errors.New("物理出口接口名称无效")
		}
	}
	return exit, nil
}

func planNativeRoutes(r *record, interfaceName string, endpoint netip.Addr) (networkPlan, error) {
	if !strings.HasPrefix(interfaceName, "utun") || len(interfaceName) < 5 {
		return networkPlan{}, errors.New("本机接口名称无效")
	}
	for _, digit := range interfaceName[4:] {
		if digit < '0' || digit > '9' {
			return networkPlan{}, errors.New("本机接口名称无效")
		}
	}
	address, err := netip.ParsePrefix(r.Config.ClientAddress)
	if err != nil || !address.Addr().Is4() || address.Addr().IsUnspecified() {
		return networkPlan{}, errors.New("本机 IPv4 地址无效")
	}
	if !endpoint.IsValid() || !endpoint.Is4() || !endpoint.IsGlobalUnicast() {
		return networkPlan{}, errors.New("VPN 公网端点必须解析为可达的 IPv4 地址")
	}
	plan := networkPlan{Interface: interfaceName, ClientAddress: address, Endpoint: endpoint, Routes: []netip.Prefix{}}
	seen := make(map[netip.Prefix]bool)
	for _, part := range strings.Split(r.Config.AllowedIPs, ",") {
		prefix, err := netip.ParsePrefix(strings.TrimSpace(part))
		if err != nil {
			return networkPlan{}, errors.New("WireGuard 授权路由无效")
		}
		if !prefix.Addr().Is4() {
			return networkPlan{}, errors.New("IPv6 系统路由尚未实现，请在专线中仅授权 IPv4 路由")
		}
		prefix = prefix.Masked()
		if prefix.Bits() == 0 {
			// These /1 routes override the physical default while retaining it
			// for explicit endpoint /32 bypass. The helper must install the bypass.
			for _, half := range []string{"0.0.0.0/1", "128.0.0.0/1"} {
				p := netip.MustParsePrefix(half)
				if !seen[p] {
					plan.Routes = append(plan.Routes, p)
					seen[p] = true
				}
			}
			plan.EndpointBypass = true
			continue
		}
		if !seen[prefix] {
			plan.Routes = append(plan.Routes, prefix)
			seen[prefix] = true
		}
		if prefix.Contains(endpoint) {
			plan.EndpointBypass = true
		}
	}
	if plan.EndpointBypass {
		// A physical /32 must be installed before any matching VPN route.
		// Otherwise a full-tunnel VPN can attempt to reach itself through TUN.
		if endpoint == address.Addr() {
			return networkPlan{}, errors.New("VPN 端点与本机地址冲突")
		}
	}
	return plan, nil
}