// Package server accepts PostgreSQL TLS sessions and routes them by SNI.
package server

import (
	"context"
	"crypto/tls"
	"errors"
	"fmt"
	"log/slog"
	"net"
	"strings"
	"sync"
	"time"

	"github.com/barkstack/pawsql/internal/pgwire"
	"github.com/barkstack/pawsql/internal/proxy"
	"github.com/barkstack/pawsql/internal/router"
)

const (
	defaultHandshakeTimeout = 15 * time.Second
	defaultResolveTimeout   = 60 * time.Second
	defaultDialTimeout      = 10 * time.Second
)

// Server terminates client TLS and transparently proxies PostgreSQL bytes.
type Server struct {
	TLSConfig        *tls.Config
	Resolver         router.BackendResolver
	Logger           *slog.Logger
	HandshakeTimeout time.Duration
	ResolveTimeout   time.Duration
	DialTimeout      time.Duration
	mu               sync.Mutex
	listener         net.Listener
	handlers         sync.WaitGroup
}

// New validates server dependencies and applies safe protocol defaults. A nil
// TLS configuration selects plaintext mode: PostgreSQL SSL negotiation is not
// offered and routes are selected by database name only.
func New(tlsConfig *tls.Config, resolver router.BackendResolver, logger *slog.Logger) (*Server, error) {
	if resolver == nil {
		return nil, errors.New("backend resolver is required")
	}
	if logger == nil {
		logger = slog.Default()
	}
	if tlsConfig != nil {
		copy := tlsConfig.Clone()
		if copy.MinVersion == 0 {
			copy.MinVersion = tls.VersionTLS12
		}
		tlsConfig = copy
	}
	return &Server{TLSConfig: tlsConfig, Resolver: resolver, Logger: logger, HandshakeTimeout: defaultHandshakeTimeout, ResolveTimeout: defaultResolveTimeout, DialTimeout: defaultDialTimeout}, nil
}

// Serve accepts connections until Shutdown closes its listener.
func (s *Server) Serve(listener net.Listener) error {
	s.mu.Lock()
	if s.listener != nil {
		s.mu.Unlock()
		return errors.New("server is already serving")
	}
	s.listener = listener
	s.mu.Unlock()

	for {
		connection, err := listener.Accept()
		if err != nil {
			if errors.Is(err, net.ErrClosed) {
				return nil
			}
			if temporary, ok := err.(interface{ Temporary() bool }); ok && temporary.Temporary() {
				s.Logger.Warn("temporary accept failure", "error", err)
				continue
			}
			return fmt.Errorf("accept connection: %w", err)
		}
		s.handlers.Add(1)
		go func() {
			defer s.handlers.Done()
			s.handle(connection)
		}()
	}
}

// Shutdown stops accepting new connections. Existing sessions continue until they close.
func (s *Server) Shutdown() error {
	s.mu.Lock()
	listener := s.listener
	s.mu.Unlock()
	if listener == nil {
		return nil
	}
	return listener.Close()
}

// Wait waits for existing routed connections to finish.
func (s *Server) Wait() { s.handlers.Wait() }

func (s *Server) handle(connection net.Conn) {
	started := time.Now()
	remote := connection.RemoteAddr().String()
	var sni string
	var backend router.Backend
	result := "rejected"
	defer func() {
		_ = connection.Close()
		s.Logger.Info("connection finished", "remote_address", remote, "sni", sni, "database", backend.DatabaseName, "upstream", backend.Address, "duration", time.Since(started), "result", result)
	}()

	handshakeTimeout := s.HandshakeTimeout
	if handshakeTimeout <= 0 {
		handshakeTimeout = defaultHandshakeTimeout
	}
	_ = connection.SetDeadline(time.Now().Add(handshakeTimeout))
	var (
		proxyClient net.Conn = connection
		err         error
	)
	if s.TLSConfig != nil {
		if err := pgwire.ReadSSLRequest(connection); err != nil {
			s.Logger.Warn("connection rejected", "remote_address", remote, "result", "invalid_ssl_request", "error", err)
			return
		}
		if _, err := connection.Write([]byte{'S'}); err != nil {
			s.Logger.Warn("connection rejected", "remote_address", remote, "result", "ssl_response_failed", "error", err)
			return
		}
		tlsConnection := tls.Server(connection, s.TLSConfig)
		if err := tlsConnection.Handshake(); err != nil {
			s.Logger.Warn("connection rejected", "remote_address", remote, "result", "tls_handshake_failed", "error", err)
			return
		}
		_ = tlsConnection.SetDeadline(time.Time{})
		sni = normalizeHostname(tlsConnection.ConnectionState().ServerName)
		proxyClient = tlsConnection
	}
	resolveTimeout := s.ResolveTimeout
	if resolveTimeout <= 0 {
		resolveTimeout = defaultResolveTimeout
	}
	resolveContext, cancelResolve := context.WithTimeout(context.Background(), resolveTimeout)
	defer cancelResolve()

	if s.TLSConfig == nil {
		negotiated, negotiateErr := pgwire.NegotiatePlainPostgreSQL(connection)
		if negotiateErr != nil {
			s.Logger.Warn("connection rejected", "remote_address", remote, "result", "invalid_negotiation", "error", negotiateErr)
			return
		}
		proxyClient = negotiated
	}
	if sni != "" {
		backend, err = s.Resolver.Resolve(resolveContext, sni)
		if err != nil {
			s.Logger.Warn("connection rejected", "remote_address", remote, "sni", sni, "result", "unknown_hostname", "error", err)
			return
		}
	} else {
		startup, startupErr := pgwire.ReadStartupMessage(proxyClient)
		if startupErr != nil {
			s.Logger.Warn("connection rejected", "remote_address", remote, "result", "invalid_startup_message", "error", startupErr)
			return
		}
		backend, err = s.Resolver.ResolveDatabase(resolveContext, startup.Database)
		if err != nil {
			s.Logger.Warn("connection rejected", "remote_address", remote, "result", "unknown_database", "error", err)
			return
		}
		proxyClient = pgwire.Replay(proxyClient, startup.Bytes())
	}

	if leases, ok := s.Resolver.(router.ConnectionLeaseManager); ok {
		defer leases.ReleaseConnection(backend.DatabaseName)
	}

	dialTimeout := s.DialTimeout
	if dialTimeout <= 0 {
		dialTimeout = defaultDialTimeout
	}
	ctx, cancel := context.WithTimeout(context.Background(), dialTimeout)
	upstream, err := (&net.Dialer{}).DialContext(ctx, "tcp", backend.Address)
	cancel()
	if err != nil {
		s.Logger.Warn("connection rejected", "remote_address", remote, "sni", sni, "database", backend.DatabaseName, "upstream", backend.Address, "result", "upstream_connect_failed", "error", err)
		return
	}
	defer upstream.Close()
	s.Logger.Info("connection routed", "remote_address", remote, "sni", sni, "database", backend.DatabaseName, "upstream", backend.Address)
	var proxyErr error
	if meter, ok := s.Resolver.(router.TrafficMeter); ok {
		proxyErr = proxy.BidirectionalWithTraffic(proxyClient, upstream, func(clientToBackend bool, bytes int64) {
			meter.RecordTraffic(backend.DatabaseName, clientToBackend, bytes)
		})
	} else {
		proxyErr = proxy.Bidirectional(proxyClient, upstream)
	}
	if proxyErr != nil {
		result = "proxy_error"
		s.Logger.Warn("connection proxy failed", "remote_address", remote, "sni", sni, "database", backend.DatabaseName, "upstream", backend.Address, "error", proxyErr)
		return
	}
	result = "closed"
}

func normalizeHostname(hostname string) string {
	return strings.ToLower(strings.TrimSuffix(hostname, "."))
}