package worker

import (
	"context"
	"crypto/tls"
	"crypto/x509"
	"errors"
	"fmt"
	"net"
	"os"
	"reflect"
	"strings"
	"time"

	controlplanev1 "proxy-pool/gen/controlplane/v1"
	"proxy-pool/internal/config"
	"proxy-pool/internal/controlplane/tlsreload"

	"google.golang.org/grpc"
	"google.golang.org/grpc/credentials"
	"google.golang.org/grpc/keepalive"
)

var ErrInvalidServer = errors.New("invalid worker control server configuration")

type ServerOptions struct {
	ShutdownTimeout time.Duration
	Snapshots       SnapshotSource
	Checker         controlplanev1.CheckerControlPlaneServer
}

func DefaultServerOptions() ServerOptions {
	return ServerOptions{ShutdownTimeout: 15 * time.Second}
}

type Server struct {
	listen          string
	grpcServer      *grpc.Server
	shutdownTimeout time.Duration
}

func NewServer(controlPlane config.ControlPlane, service Service, options ServerOptions) (*Server, error) {
	if service == nil || !controlPlane.Enabled || !validServerConfig(controlPlane) {
		return nil, ErrInvalidServer
	}
	if options.ShutdownTimeout < 0 {
		return nil, fmt.Errorf("%w: shutdown timeout must not be negative", ErrInvalidServer)
	}
	if options.ShutdownTimeout == 0 {
		options = DefaultServerOptions()
	}

	identity, serverOptions, err := serverTransportOptions(controlPlane)
	if err != nil {
		return nil, err
	}
	snapshots := options.Snapshots
	if snapshots == nil {
		if provider, ok := service.(interface{ SnapshotSource() SnapshotSource }); ok {
			snapshots = provider.SnapshotSource()
		}
	}
	if snapshots == nil {
		initial, initialErr := NewInitialSnapshotSource(service, controlPlane.MaxStaleAge.Value(), time.Now)
		if initialErr != nil {
			return nil, fmt.Errorf("%w: build initial snapshot source: %v", ErrInvalidServer, initialErr)
		}
		snapshots, err = NewRefreshingSnapshotSource(initial, snapshotRefreshEvery(controlPlane.MaxStaleAge.Value()))
		if err != nil {
			return nil, fmt.Errorf("%w: build refreshing snapshot source: %v", ErrInvalidServer, err)
		}
	}
	serverOptions = append(serverOptions,
		grpc.MaxRecvMsgSize(controlPlane.MaxMessageBytes),
		grpc.MaxSendMsgSize(controlPlane.MaxMessageBytes),
		grpc.MaxConcurrentStreams(controlPlane.MaxConcurrentStreams),
		grpc.KeepaliveEnforcementPolicy(keepalive.EnforcementPolicy{
			MinTime:             10 * time.Second,
			PermitWithoutStream: false,
		}),
	)
	grpcServer := grpc.NewServer(serverOptions...)
	controlplanev1.RegisterWorkerControlPlaneServer(grpcServer, NewGRPCHandler(service, identity, snapshots))
	if !nilService(options.Checker) {
		controlplanev1.RegisterCheckerControlPlaneServer(grpcServer, options.Checker)
	}
	return &Server{listen: controlPlane.Listen, grpcServer: grpcServer, shutdownTimeout: options.ShutdownTimeout}, nil
}

func nilService(value any) bool {
	if value == nil {
		return true
	}
	reflected := reflect.ValueOf(value)
	switch reflected.Kind() {
	case reflect.Chan, reflect.Func, reflect.Interface, reflect.Map, reflect.Pointer, reflect.Slice:
		return reflected.IsNil()
	default:
		return false
	}
}

func (server *Server) Run(ctx context.Context) error {
	if server == nil || server.grpcServer == nil || server.listen == "" {
		return ErrInvalidServer
	}
	listener, err := net.Listen("tcp", server.listen)
	if err != nil {
		return fmt.Errorf("listen worker control plane: %w", err)
	}
	return server.Serve(ctx, listener)
}

func (server *Server) Serve(ctx context.Context, listener net.Listener) error {
	if server == nil || server.grpcServer == nil || listener == nil || ctx == nil {
		return ErrInvalidServer
	}
	completed := make(chan struct{})
	go func() {
		select {
		case <-ctx.Done():
			server.gracefulStop()
		case <-completed:
		}
	}()

	err := server.grpcServer.Serve(listener)
	close(completed)
	if ctx.Err() != nil || errors.Is(err, grpc.ErrServerStopped) {
		return nil
	}
	return err
}

func (server *Server) gracefulStop() {
	stopped := make(chan struct{})
	go func() {
		server.grpcServer.GracefulStop()
		close(stopped)
	}()
	timer := time.NewTimer(server.shutdownTimeout)
	defer timer.Stop()
	select {
	case <-stopped:
	case <-timer.C:
		server.grpcServer.Stop()
		<-stopped
	}
}

func serverTransportOptions(controlPlane config.ControlPlane) (IdentityAuthorizer, []grpc.ServerOption, error) {
	switch controlPlane.TLS.Mode {
	case "disabled":
		if !loopbackListen(controlPlane.Listen) {
			return nil, nil, fmt.Errorf("%w: plaintext listener must be loopback", ErrInvalidServer)
		}
		return AllowLoopbackIdentity{}, nil, nil
	case "mtls":
		identity, err := NewSPIFFEIdentityAuthorizer(controlPlane.TLS.TrustDomain, controlPlane.TLS.Environment)
		if err != nil {
			return nil, nil, fmt.Errorf("%w: %v", ErrInvalidServer, err)
		}
		certificate, err := tlsreload.NewCertificateProvider(controlPlane.TLS.CertFile, controlPlane.TLS.KeyFile)
		if err != nil {
			return nil, nil, fmt.Errorf("%w: load server certificate: %v", ErrInvalidServer, err)
		}
		caPEM, err := os.ReadFile(controlPlane.TLS.ClientCAFile)
		if err != nil {
			return nil, nil, fmt.Errorf("%w: read client ca: %v", ErrInvalidServer, err)
		}
		clientCAs := x509.NewCertPool()
		if !clientCAs.AppendCertsFromPEM(caPEM) {
			return nil, nil, fmt.Errorf("%w: parse client ca", ErrInvalidServer)
		}
		transport := credentials.NewTLS(&tls.Config{
			MinVersion:     tls.VersionTLS13,
			GetCertificate: certificate.ServerCertificate,
			ClientAuth:     tls.RequireAndVerifyClientCert,
			ClientCAs:      clientCAs,
		})
		return identity, []grpc.ServerOption{grpc.Creds(transport)}, nil
	default:
		return nil, nil, fmt.Errorf("%w: unsupported tls mode", ErrInvalidServer)
	}
}

func validServerConfig(controlPlane config.ControlPlane) bool {
	return controlPlane.Listen != "" && controlPlane.ProtocolVersion == 1 &&
		controlPlane.HeartbeatInterval.Value() > 0 && controlPlane.SessionTTL.Value() > 0 &&
		controlPlane.MaxStaleAge.Value() > 0 && controlPlane.MaxMessageBytes > 0 &&
		controlPlane.MaxRuntimeCounters > 0 && controlPlane.MaxConcurrentStreams > 0
}

func loopbackListen(listen string) bool {
	host, _, err := net.SplitHostPort(listen)
	if err != nil {
		return false
	}
	host = strings.Trim(host, "[]")
	if strings.EqualFold(host, "localhost") {
		return true
	}
	ip := net.ParseIP(host)
	return ip != nil && ip.IsLoopback()
}