//go:build linux && !android

package dns

import (
	"context"
	"fmt"
	"net"
	"net/netip"
	"time"

	dbus "github.com/godbus/dbus/v5"
	"golang.org/x/sys/unix"
)

const (
	systemdResolvedDest      = "org.freedesktop.resolve1"
	systemdDbusObjectNode    = "/org/freedesktop/resolve1"
	systemdDbusManagerIface  = "org.freedesktop.resolve1.Manager"
	systemdDbusGetLinkMethod = systemdDbusManagerIface + ".GetLink"
	systemdDbusLinkInterface = "org.freedesktop.resolve1.Link"
	systemdDbusSetDNSMethod  = systemdDbusLinkInterface + ".SetDNS"
	systemdDbusRevertMethod  = systemdDbusLinkInterface + ".Revert"
)

// systemdDbusDNSInput maps to (iay) dbus input for SetDNS method
type systemdDbusDNSInput struct {
	Family  int32
	Address []byte
}

// SystemdResolvedDNSConfigurator manages DNS settings using systemd-resolved D-Bus API
type SystemdResolvedDNSConfigurator struct {
	ifaceName      string
	dbusLinkObject dbus.ObjectPath
	originalState  *DNSState
}

// NewSystemdResolvedDNSConfigurator creates a new systemd-resolved DNS configurator
func NewSystemdResolvedDNSConfigurator(ifaceName string) (*SystemdResolvedDNSConfigurator, error) {
	// Get network interface
	iface, err := net.InterfaceByName(ifaceName)
	if err != nil {
		return nil, fmt.Errorf("get interface: %w", err)
	}

	// Connect to D-Bus
	conn, err := dbus.SystemBus()
	if err != nil {
		return nil, fmt.Errorf("connect to system bus: %w", err)
	}
	defer conn.Close()

	obj := conn.Object(systemdResolvedDest, systemdDbusObjectNode)

	// Get the link object for this interface
	var linkPath string
	if err := obj.Call(systemdDbusGetLinkMethod, 0, iface.Index).Store(&linkPath); err != nil {
		return nil, fmt.Errorf("get link: %w", err)
	}

	return &SystemdResolvedDNSConfigurator{
		ifaceName:      ifaceName,
		dbusLinkObject: dbus.ObjectPath(linkPath),
	}, nil
}

// Name returns the configurator name
func (s *SystemdResolvedDNSConfigurator) Name() string {
	return "systemd-resolved"
}

// SetDNS sets the DNS servers and returns the original servers
func (s *SystemdResolvedDNSConfigurator) SetDNS(servers []netip.Addr) ([]netip.Addr, error) {
	// Get current DNS settings before overriding
	originalServers, err := s.GetCurrentDNS()
	if err != nil {
		// If we can't get current DNS, proceed anyway
		originalServers = []netip.Addr{}
	}

	// Store original state
	s.originalState = &DNSState{
		OriginalServers:  originalServers,
		ConfiguratorName: s.Name(),
	}

	// Apply new DNS servers
	if err := s.applyDNSServers(servers); err != nil {
		return nil, fmt.Errorf("apply DNS servers: %w", err)
	}

	return originalServers, nil
}

// RestoreDNS restores the original DNS configuration
func (s *SystemdResolvedDNSConfigurator) RestoreDNS() error {
	// Call Revert method to restore systemd-resolved defaults
	conn, err := dbus.SystemBus()
	if err != nil {
		return fmt.Errorf("connect to system bus: %w", err)
	}
	defer conn.Close()

	obj := conn.Object(systemdResolvedDest, s.dbusLinkObject)

	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
	defer cancel()

	if err := obj.CallWithContext(ctx, systemdDbusRevertMethod, 0).Store(); err != nil {
		return fmt.Errorf("revert DNS settings: %w", err)
	}

	return nil
}

// GetCurrentDNS returns the currently configured DNS servers
// Note: systemd-resolved doesn't easily expose current per-link DNS servers via D-Bus
// This is a placeholder that returns an empty list
func (s *SystemdResolvedDNSConfigurator) GetCurrentDNS() ([]netip.Addr, error) {
	// systemd-resolved's D-Bus API doesn't have a simple way to query current DNS servers
	// We would need to parse resolvectl status output or read from /run/systemd/resolve/
	// For now, return empty list
	return []netip.Addr{}, nil
}

// applyDNSServers applies DNS server configuration via systemd-resolved
func (s *SystemdResolvedDNSConfigurator) applyDNSServers(servers []netip.Addr) error {
	if len(servers) == 0 {
		return fmt.Errorf("no DNS servers provided")
	}

	// Convert servers to systemd-resolved format
	var dnsInputs []systemdDbusDNSInput
	for _, server := range servers {
		family := unix.AF_INET
		if server.Is6() {
			family = unix.AF_INET6
		}

		dnsInputs = append(dnsInputs, systemdDbusDNSInput{
			Family:  int32(family),
			Address: server.AsSlice(),
		})
	}

	// Connect to D-Bus
	conn, err := dbus.SystemBus()
	if err != nil {
		return fmt.Errorf("connect to system bus: %w", err)
	}
	defer conn.Close()

	obj := conn.Object(systemdResolvedDest, s.dbusLinkObject)

	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
	defer cancel()

	// Call SetDNS method
	if err := obj.CallWithContext(ctx, systemdDbusSetDNSMethod, 0, dnsInputs).Store(); err != nil {
		return fmt.Errorf("set DNS servers: %w", err)
	}

	return nil
}

// IsSystemdResolvedAvailable checks if systemd-resolved is available and responsive
func IsSystemdResolvedAvailable() bool {
	conn, err := dbus.SystemBus()
	if err != nil {
		return false
	}
	defer conn.Close()

	obj := conn.Object(systemdResolvedDest, systemdDbusObjectNode)

	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
	defer cancel()

	// Try to ping systemd-resolved
	if err := obj.CallWithContext(ctx, "org.freedesktop.DBus.Peer.Ping", 0).Store(); err != nil {
		return false
	}

	return true
}