package openvpn

import (
	"context"
	"crypto/subtle"
	"net"
	"net/netip"
	"slices"
	"strings"

	"github.com/sagernet/sing-box/adapter"
	"github.com/sagernet/sing-box/adapter/endpoint"
	C "github.com/sagernet/sing-box/constant"
	"github.com/sagernet/sing-box/log"
	"github.com/sagernet/sing-box/option"
	ovpntransport "github.com/sagernet/sing-box/transport/openvpn"
	ovpn "github.com/sagernet/sing-openvpn"
	"github.com/sagernet/sing-tun"
	"github.com/sagernet/sing-tun/gtcpip/header"
	"github.com/sagernet/sing/common/auth"
	"github.com/sagernet/sing/common/bufio"
	E "github.com/sagernet/sing/common/exceptions"
	M "github.com/sagernet/sing/common/metadata"
	N "github.com/sagernet/sing/common/network"

	"go4.org/netipx"
)

func RegisterEndpoint(registry *endpoint.Registry) {
	endpoint.Register[option.OpenVPNClientEndpointOptions](registry, C.TypeOpenVPNClient, NewClientEndpoint)
	endpoint.Register[option.OpenVPNServerEndpointOptions](registry, C.TypeOpenVPNServer, NewServerEndpoint)
}

type endpointBase struct {
	endpoint.Adapter
	router adapter.Router
	logger log.ContextLogger
}

func (e *endpointBase) SupportsFlow(network string) bool {
	return slices.Contains(e.Network(), network)
}

func (e *endpointBase) newConnection(ctx context.Context, endpoint adapter.Endpoint, localAddresses []netip.Prefix, conn net.Conn, source M.Socksaddr, destination M.Socksaddr, onClose N.CloseHandlerFunc) {
	var metadata adapter.InboundContext
	metadata.Inbound = endpoint.Tag()
	metadata.InboundType = endpoint.Type()
	metadata.Source = source
	if isEndpointLocalAddress(localAddresses, destination.Addr) {
		metadata.OriginDestination = destination
		destination.Addr = loopbackAddressFor(destination.Addr)
	}
	metadata.Destination = destination
	e.logger.InfoContext(ctx, "inbound connection from ", source)
	e.logger.InfoContext(ctx, "inbound connection to ", metadata.Destination)
	e.router.RouteConnectionEx(ctx, conn, metadata, onClose)
}

func (e *endpointBase) newPacketConnection(ctx context.Context, endpoint adapter.Endpoint, localAddresses []netip.Prefix, conn N.PacketConn, source M.Socksaddr, destination M.Socksaddr, onClose N.CloseHandlerFunc) {
	var metadata adapter.InboundContext
	metadata.Inbound = endpoint.Tag()
	metadata.InboundType = endpoint.Type()
	metadata.Source = source
	if isEndpointLocalAddress(localAddresses, destination.Addr) {
		metadata.OriginDestination = destination
		destination.Addr = loopbackAddressFor(destination.Addr)
		conn = bufio.NewNATPacketConn(bufio.NewNetPacketConn(conn), metadata.OriginDestination, destination)
	}
	metadata.Destination = destination
	e.logger.InfoContext(ctx, "inbound packet connection from ", source)
	e.logger.InfoContext(ctx, "inbound packet connection to ", metadata.Destination)
	e.router.RoutePacketConnectionEx(ctx, conn, metadata, onClose)
}

func (e *endpointBase) newDNSPacket(ctx context.Context, endpoint adapter.Endpoint, payload []byte, source M.Socksaddr, destination M.Socksaddr, writer N.PacketWriter) {
	var metadata adapter.InboundContext
	metadata.Inbound = endpoint.Tag()
	metadata.InboundType = endpoint.Type()
	metadata.Network = N.NetworkUDP
	metadata.Source = source
	metadata.Destination = destination
	metadata.Protocol = C.ProtocolDNS
	e.logger.InfoContext(ctx, "inbound DNS packet from ", source)
	e.router.HijackDNSPacket(ctx, payload, writer, metadata)
}

func isEndpointLocalAddress(localAddresses []netip.Prefix, address netip.Addr) bool {
	for _, localPrefix := range localAddresses {
		if address == localPrefix.Addr() {
			return true
		}
	}
	return false
}

func loopbackAddressFor(address netip.Addr) netip.Addr {
	if address.Is4() {
		return netip.AddrFrom4([4]uint8{127, 0, 0, 1})
	}
	return netip.IPv6Loopback()
}

func judgeOpenVPNFlow(router adapter.Router, tag string, endpointType string, localAddresses []netip.Prefix, network uint8, source netip.AddrPort, destination netip.AddrPort, firstPacket []byte) tun.FlowVerdict {
	for _, localPrefix := range localAddresses {
		if destination.Addr() == localPrefix.Addr() {
			return tun.FlowVerdict{Action: tun.ActionAccept}
		}
	}
	return adapter.JudgeFlow(router, tag, endpointType, network, source, destination, firstPacket)
}

func keyDirectionValue(direction string) (int, error) {
	switch direction {
	case "":
		return -1, nil
	case "server":
		return 0, nil
	case "client":
		return 1, nil
	default:
		return 0, E.New("unsupported OpenVPN key direction: ", direction, " (expected \"server\" or \"client\")")
	}
}

func openVPNClientRemoteIsDomain(options option.OpenVPNClientEndpointOptions) bool {
	if options.Server != "" && options.ServerIsDomain() {
		return true
	}
	for _, remoteOptions := range options.Servers {
		if remoteOptions.Build().IsDomain() {
			return true
		}
	}
	return false
}

func materialSource(name string, inlineValues []string, path string) (ovpn.Material, error) {
	material := ovpn.Material{Path: path}
	if len(inlineValues) > 0 {
		material.Content = []byte(strings.Join(inlineValues, "\n"))
	}
	return material, material.Validate(name)
}

func requiredMaterialSource(name string, inlineValues []string, path string) (ovpn.Material, error) {
	material, err := materialSource(name, inlineValues, path)
	if err != nil {
		return ovpn.Material{}, err
	}
	if !material.IsSet() {
		return ovpn.Material{}, E.New("missing `", name, "` or `", name, "_path`")
	}
	return material, nil
}

func configurationFromClientEvent(event ovpn.TunnelConfigurationEvent, logger log.ContextLogger) ovpntransport.Configuration {
	configuration := event.Configuration
	var addresses []netip.Prefix
	addresses = append(addresses, configuration.LocalIPv4...)
	addresses = append(addresses, configuration.LocalIPv6...)
	mtu := configuration.TunMTU
	if mtu == 0 {
		mtu = ovpntransport.DefaultMTU
	}
	var routes []ovpntransport.Route
	inet4DefaultRoute := netip.PrefixFrom(netip.IPv4Unspecified(), 0)
	inet6DefaultRoute := netip.PrefixFrom(netip.IPv6Unspecified(), 0)
	var hasInet4DefaultRoute bool
	var hasInet6DefaultRoute bool
	for _, route := range configuration.IPv4Routes {
		routes = append(routes, ovpntransport.Route{
			Prefix:  route.Prefix,
			Gateway: route.Gateway,
			Metric:  route.Metric,
		})
		if route.Prefix == inet4DefaultRoute {
			hasInet4DefaultRoute = true
		}
	}
	for _, route := range configuration.IPv6Routes {
		routes = append(routes, ovpntransport.Route{
			Prefix:  route.Prefix,
			Gateway: route.Gateway,
			Metric:  route.Metric,
		})
		if route.Prefix == inet6DefaultRoute {
			hasInet6DefaultRoute = true
		}
	}
	if configuration.RedirectGateway {
		if !hasOpenVPNFlag(configuration.RedirectGatewayFlags, "!ipv4") && !hasInet4DefaultRoute {
			routes = append(routes, ovpntransport.Route{
				Prefix:  inet4DefaultRoute,
				Gateway: configuration.VPNGateway,
				Metric:  configuration.RouteMetric,
			})
		}
		if hasOpenVPNFlag(configuration.RedirectGatewayFlags, "ipv6") && !hasInet6DefaultRoute {
			routes = append(routes, ovpntransport.Route{
				Prefix:  inet6DefaultRoute,
				Gateway: configuration.VPNGatewayIPv6,
				Metric:  configuration.RouteMetric,
			})
			hasInet6DefaultRoute = true
		}
	}
	if configuration.BlockIPv6 && !hasInet6DefaultRoute {
		routes = append(routes, ovpntransport.Route{
			Prefix:  inet6DefaultRoute,
			Gateway: configuration.VPNGatewayIPv6,
			Metric:  configuration.RouteMetric,
		})
	}
	var ignoredOptions []string
	for _, flag := range configuration.RedirectGatewayFlags {
		switch strings.ToLower(flag) {
		case "!ipv4", "ipv6":
		default:
			if flag != "" {
				ignoredOptions = append(ignoredOptions, "redirect-gateway "+flag)
			}
		}
	}
	if configuration.RedirectPrivate {
		ignoredOptions = append(ignoredOptions, "redirect-private")
	}
	if configuration.BlockOutsideDNS {
		ignoredOptions = append(ignoredOptions, "block-outside-dns")
	}
	for _, dhcpOption := range configuration.DHCPOptions {
		fields := strings.Fields(dhcpOption)
		if len(fields) == 0 || strings.EqualFold(fields[0], "DNS") || strings.EqualFold(fields[0], "DNS6") {
			continue
		}
		ignoredOptions = append(ignoredOptions, "dhcp-option "+strings.TrimSpace(dhcpOption))
	}
	if len(ignoredOptions) > 0 && logger != nil {
		logger.Debug("ignored pushed OpenVPN options: ", strings.Join(ignoredOptions, ", "))
	}
	return ovpntransport.Configuration{
		MTU:       mtu,
		Address:   addresses,
		Routes:    routes,
		DNS:       configuration.DNS,
		Topology:  configuration.Topology,
		BlockIPv6: configuration.BlockIPv6,
	}
}

func buildIPSet(routes []ovpntransport.Route) (*netipx.IPSet, error) {
	var builder netipx.IPSetBuilder
	for _, route := range routes {
		builder.AddPrefix(route.Prefix)
	}
	return builder.IPSet()
}

func hasOpenVPNFlag(flags []string, flag string) bool {
	for _, value := range flags {
		if strings.EqualFold(value, flag) {
			return true
		}
	}
	return false
}

func packetSourceAddress(packet []byte, inet4Address netip.Addr, inet6Address netip.Addr) netip.Addr {
	if header.IPVersion(packet) == header.IPv6Version {
		return inet6Address
	}
	return inet4Address
}

func authenticatorFromUsers(users []auth.User) ovpn.UserPassAuthenticator {
	if len(users) == 0 {
		return nil
	}
	passwordByUsername := make(map[string]string, len(users))
	for _, user := range users {
		passwordByUsername[user.Username] = user.Password
	}
	return func(ctx context.Context, username string, password string) error {
		expectedPassword, found := passwordByUsername[username]
		if !found || subtle.ConstantTimeCompare([]byte(expectedPassword), []byte(password)) != 1 {
			return E.New("invalid username or password")
		}
		return nil
	}
}