package session

import (
	"context"
	"crypto/rand"
	"encoding/base64"
	"errors"
	"fmt"
	"net/http"
	"sync"
	"time"

	"github.com/coder/websocket"
	rvboxv1 "github.com/rvbox/rvbox/gen/go/rvbox/v1"
	"github.com/rvbox/rvbox/internal/agentproto"
	"github.com/rvbox/rvbox/internal/domain"
	"github.com/rvbox/rvbox/internal/server/store"
	"google.golang.org/protobuf/proto"
	"google.golang.org/protobuf/types/known/timestamppb"
)

const (
	defaultAgentPath = "/v1/agent"
	defaultWriteWait = 10 * time.Second
)

var (
	ErrUnexpectedOrigin  = errors.New("agent connections must not send an Origin header")
	ErrUnexpectedMessage = errors.New("agent message is not a binary protobuf envelope")
	ErrStaleSession      = errors.New("agent message is for an unknown or fenced session")
)

// AgentServer is the transport edge for agent WebSocket sessions. Command
// processing is intentionally injected above this admission/fencing boundary.
type AgentServer struct {
	Store             *store.Store
	Registry          *Registry
	Path              string
	Limits            agentproto.Limits
	SupportedProtocol *rvboxv1.ProtocolRange
	WriteDeadline     time.Duration
	HeartbeatIdle     time.Duration
	LivenessTimeout   time.Duration
	Now               func() time.Time
}

func (server *AgentServer) ServeHTTP(response http.ResponseWriter, request *http.Request) {
	if request.URL.Path != server.agentPath() {
		http.NotFound(response, request)
		return
	}
	if request.Header.Get("Origin") != "" {
		http.Error(response, ErrUnexpectedOrigin.Error(), http.StatusForbidden)
		return
	}
	if server.Store == nil {
		http.Error(response, "agent server storage is unavailable", http.StatusServiceUnavailable)
		return
	}
	if server.Registry == nil {
		http.Error(response, "agent session registry is unavailable", http.StatusServiceUnavailable)
		return
	}
	connection, err := websocket.Accept(response, request, &websocket.AcceptOptions{
		CompressionMode: websocket.CompressionDisabled,
	})
	if err != nil {
		return
	}
	defer connection.CloseNow()
	server.serveConnection(request.Context(), connection)
}

func (server *AgentServer) serveConnection(parent context.Context, connection *websocket.Conn) {
	started := time.Now()
	messageType, payload, err := connection.Read(parent)
	if err != nil {
		return
	}
	if messageType != websocket.MessageBinary {
		server.close(connection, websocket.StatusUnsupportedData, ErrUnexpectedMessage.Error())
		return
	}
	helloEnvelope, err := agentproto.DecodeEnvelope(payload, server.limits(), rvboxv1.Platform_PLATFORM_UNSPECIFIED)
	if err != nil || helloEnvelope.GetClientHello() == nil {
		server.close(connection, websocket.StatusPolicyViolation, "invalid ClientHello")
		return
	}
	hello := helloEnvelope.GetClientHello()
	instanceID, err := domain.ParseUUIDv7(hello.GetClientInstanceId())
	if err != nil {
		server.close(connection, websocket.StatusPolicyViolation, "invalid client instance ID")
		return
	}
	selected, err := domain.SelectProtocol(server.protocolRange(), hello.GetSupportedProtocol())
	if err != nil {
		server.close(connection, websocket.StatusPolicyViolation, "unsupported protocol")
		return
	}
	sessionID, err := randomSessionID()
	if err != nil {
		server.close(connection, websocket.StatusInternalError, "could not allocate session")
		return
	}
	shells, err := proto.Marshal(&rvboxv1.ClientHello{SupportedShells: hello.GetSupportedShells()})
	if err != nil {
		server.close(connection, websocket.StatusInternalError, "could not persist client capabilities")
		return
	}
	registration, err := server.Store.RegisterClientSession(parent, store.ClientRegistration{
		ClientID: hello.GetClientId(), Platform: uint32(hello.GetPlatform()), Architecture: hello.GetArchitecture(),
		DaemonVersion: hello.GetDaemonVersion(), DaemonCWD: hello.GetDaemonCwd(), SupportedShells: shells,
		ClientInstanceID: [16]byte(instanceID), SessionID: sessionID, ConnectedAt: server.now(),
	})
	if err != nil {
		server.close(connection, websocket.StatusPolicyViolation, sessionCloseReason(err))
		return
	}
	registry := server.Registry
	handle, err := registry.Install(hello.GetClientId(), sessionID, registration.Generation)
	if err != nil {
		_ = server.Store.CloseLiveSession(context.Background(), sessionID, registration.Generation, "registry installation failed", server.now())
		server.close(connection, websocket.StatusInternalError, "could not install session")
		return
	}
	defer func() {
		registry.Remove(handle)
		_ = server.Store.CloseLiveSession(context.Background(), sessionID, registration.Generation, "connection closed", server.now())
	}()

	sessionContext, cancel := context.WithCancel(parent)
	stopFence := context.AfterFunc(handle.Context, cancel)
	defer func() {
		stopFence()
		cancel()
	}()

	queue := NewWriterQueue(16, 64, 8)
	defer queue.Close()
	encodedSessionID := encodeSessionID(sessionID)
	welcome, err := proto.Marshal(&rvboxv1.AgentEnvelope{
		SessionId: encodedSessionID, SessionGeneration: registration.Generation,
		Payload: &rvboxv1.AgentEnvelope_ServerWelcome{ServerWelcome: &rvboxv1.ServerWelcome{
			SelectedProtocol: selected, ServerTime: timestamppb.New(server.now()),
		}},
	})
	if err != nil {
		server.close(connection, websocket.StatusInternalError, "could not encode welcome")
		return
	}
	if err := queue.EnqueueControl(Frame{Kind: FrameControl, Payload: welcome}); err != nil {
		server.close(connection, websocket.StatusInternalError, "could not queue welcome")
		return
	}

	heartbeat := newSynchronizedHeartbeat(server.heartbeatIdle(), server.livenessTimeout(), 0)
	writerDone := make(chan struct{})
	go func() {
		defer close(writerDone)
		server.writeLoop(sessionContext, connection, queue, heartbeat, started)
		cancel()
	}()
	defer func() { <-writerDone }()

	for {
		messageType, payload, err = connection.Read(sessionContext)
		if err != nil {
			return
		}
		// Elapsed time is monotonic; peer-provided wall timestamps are never used
		// to determine liveness.
		heartbeat.Observe(time.Since(started))
		if messageType != websocket.MessageBinary {
			server.close(connection, websocket.StatusUnsupportedData, ErrUnexpectedMessage.Error())
			return
		}
		envelope, err := agentproto.DecodeEnvelope(payload, server.limits(), hello.GetPlatform())
		if err != nil || envelope.GetClientHello() != nil || envelope.GetSessionId() != encodedSessionID || envelope.GetSessionGeneration() != registration.Generation {
			server.close(connection, websocket.StatusPolicyViolation, ErrStaleSession.Error())
			return
		}
		clientID, err := server.Store.ValidateLiveSession(sessionContext, sessionID, registration.Generation)
		if err != nil || clientID != hello.GetClientId() {
			server.close(connection, websocket.StatusPolicyViolation, ErrStaleSession.Error())
			return
		}
		if snapshot := envelope.GetReconcileSnapshot(); snapshot != nil {
			result, reconcileErr := server.Store.ReconcileClientSnapshot(sessionContext, hello.GetClientId(), snapshot)
			if reconcileErr != nil {
				server.close(connection, websocket.StatusPolicyViolation, "reconciliation failed")
				return
			}
			encoded, err := proto.Marshal(&rvboxv1.AgentEnvelope{
				SessionId: encodedSessionID, SessionGeneration: registration.Generation,
				Payload: &rvboxv1.AgentEnvelope_ReconcileResult{ReconcileResult: result},
			})
			if err != nil || queue.EnqueueControl(Frame{Kind: FrameControl, Payload: encoded}) != nil {
				server.close(connection, websocket.StatusInternalError, "could not queue reconciliation result")
				return
			}
		}
	}
}

func (server *AgentServer) writeLoop(ctx context.Context, connection *websocket.Conn, queue *WriterQueue, heartbeat *synchronizedHeartbeat, started time.Time) {
	for {
		frameContext, cancel := context.WithTimeout(ctx, server.heartbeatPollInterval())
		frame, err := queue.Next(frameContext)
		cancel()
		if err == nil {
			writeContext, writeCancel := context.WithTimeout(ctx, server.writeDeadline())
			err = connection.Write(writeContext, websocket.MessageBinary, frame.Payload)
			writeCancel()
			if err != nil {
				return
			}
			continue
		}
		if !errors.Is(err, context.DeadlineExceeded) {
			return
		}
		switch heartbeat.Check(time.Since(started)) {
		case HeartbeatPing:
			pingContext, pingCancel := context.WithTimeout(ctx, server.writeDeadline())
			err = connection.Ping(pingContext)
			pingCancel()
			if err != nil {
				return
			}
			heartbeat.Observe(time.Since(started))
		case HeartbeatClose:
			return
		}
	}
}

func (server *AgentServer) close(connection *websocket.Conn, status websocket.StatusCode, reason string) {
	_ = connection.Close(status, reason)
}

func (server *AgentServer) agentPath() string {
	if server.Path == "" {
		return defaultAgentPath
	}
	return server.Path
}

func (server *AgentServer) limits() agentproto.Limits {
	if server.Limits.MaxEnvelopeBytes == 0 {
		return agentproto.DefaultLimits()
	}
	return server.Limits
}

func (server *AgentServer) protocolRange() *rvboxv1.ProtocolRange {
	if server.SupportedProtocol == nil {
		return &rvboxv1.ProtocolRange{Major: 1, MinMinor: 0, MaxMinor: 0}
	}
	return server.SupportedProtocol
}

func (server *AgentServer) writeDeadline() time.Duration {
	if server.WriteDeadline <= 0 {
		return defaultWriteWait
	}
	return server.WriteDeadline
}

func (server *AgentServer) heartbeatIdle() time.Duration {
	if server.HeartbeatIdle <= 0 {
		return 10 * time.Second
	}
	return server.HeartbeatIdle
}

func (server *AgentServer) livenessTimeout() time.Duration {
	if server.LivenessTimeout <= server.heartbeatIdle() {
		return 30 * time.Second
	}
	return server.LivenessTimeout
}

func (server *AgentServer) heartbeatPollInterval() time.Duration {
	interval := server.heartbeatIdle() / 2
	if interval <= 0 || interval > server.writeDeadline() {
		return server.writeDeadline()
	}
	return interval
}

func (server *AgentServer) now() time.Time {
	if server.Now != nil {
		return server.Now().UTC()
	}
	return time.Now().UTC()
}

func randomSessionID() ([16]byte, error) {
	var result [16]byte
	if _, err := rand.Read(result[:]); err != nil {
		return result, fmt.Errorf("read session entropy: %w", err)
	}
	return result, nil
}

func encodeSessionID(value [16]byte) string { return base64.RawURLEncoding.EncodeToString(value[:]) }

func sessionCloseReason(err error) string {
	if errors.Is(err, store.ErrTakeoverRequired) {
		return "client takeover authorization required"
	}
	return "client registration rejected"
}

type synchronizedHeartbeat struct {
	mu        sync.Mutex
	heartbeat *Heartbeat
}

func newSynchronizedHeartbeat(idle, timeout, now time.Duration) *synchronizedHeartbeat {
	return &synchronizedHeartbeat{heartbeat: NewHeartbeat(idle, timeout, now)}
}

func (heartbeat *synchronizedHeartbeat) Observe(now time.Duration) {
	heartbeat.mu.Lock()
	defer heartbeat.mu.Unlock()
	heartbeat.heartbeat.ObserveInbound(now)
}

func (heartbeat *synchronizedHeartbeat) Check(now time.Duration) HeartbeatAction {
	heartbeat.mu.Lock()
	defer heartbeat.mu.Unlock()
	return heartbeat.heartbeat.Check(now)
}