package proto_socket

import (
	"errors"
	"fmt"
	"reflect"
	"sync"
	"sync/atomic"
	"time"

	"google.golang.org/protobuf/proto"

	"git.toki-labs.com/toki/proto-socket/go/packets"
)

var ErrNotConnected = errors.New("not connected")

const MaxNonce int32 = 1<<31 - 1

type ParserMap map[string]func([]byte) (proto.Message, error)

type Transport interface {
	WritePacket(base *packets.PacketBase) error
	Close() error
}

type pendingRequest struct {
	expectedTypeName string
	ch               chan proto.Message
	errCh            chan error
}

type queuedPacket struct {
	base *packets.PacketBase
	done chan error
}

type inboundItem struct {
	typeName      string
	data          []byte
	incomingNonce int32
	responseNonce int32
}

type Communicator struct {
	mu              sync.RWMutex
	nonce           atomic.Int32
	frameSeq        atomic.Int64
	isAlive         atomic.Bool
	drainOnClose    atomic.Bool
	gateway         InboundGateway
	parserMap       ParserMap
	handlers        map[string][]func(proto.Message)
	reqHandlers     map[string]func(proto.Message, int32)
	pendingRequests map[int32]*pendingRequest
	writeQueue      chan queuedPacket
	inboundQueue    chan inboundItem
	closed          chan struct{}
	receiveDone     chan struct{}
	closeOnce       sync.Once
	transport       Transport
	writeErrHandler func(error)
	frameErrHandler func(error)
}

func TypeNameOf(m proto.Message) string {
	return string(proto.MessageName(m))
}

func NewCommunicator(transport Transport, parserMap ParserMap) *Communicator {
	c := &Communicator{}
	c.Initialize(transport, parserMap)
	return c
}

func (c *Communicator) Initialize(transport Transport, parserMap ParserMap) {
	c.transport = transport
	c.parserMap = make(ParserMap, len(parserMap)+1)
	for k, v := range parserMap {
		c.parserMap[k] = v
	}
	c.parserMap[TypeNameOf(&packets.HeartBeat{})] = func(b []byte) (proto.Message, error) {
		m := &packets.HeartBeat{}
		return m, proto.Unmarshal(b, m)
	}
	c.handlers = make(map[string][]func(proto.Message))
	c.reqHandlers = make(map[string]func(proto.Message, int32))
	c.pendingRequests = make(map[int32]*pendingRequest)
	c.writeQueue = make(chan queuedPacket, 64)
	c.inboundQueue = make(chan inboundItem, 64)
	c.closed = make(chan struct{})
	c.receiveDone = make(chan struct{})
	c.isAlive.Store(true)
	go c.writeLoop()
	go c.receiveLoop()
}

func (c *Communicator) IsAlive() bool {
	return c.isAlive.Load()
}

func (c *Communicator) SetWriteErrorHandler(fn func(error)) {
	c.mu.Lock()
	defer c.mu.Unlock()
	c.writeErrHandler = fn
}

// SetFrameErrorHandler registers a handler invoked when an inbound gateway fails
// to decode a raw frame. Transports use it to apply their parse-error disconnect
// semantics for the gateway path, mirroring SetWriteErrorHandler for the write
// path. Without a gateway, OnReceivedFrame surfaces decode errors via its return
// value instead and this handler is unused.
func (c *Communicator) SetFrameErrorHandler(fn func(error)) {
	c.mu.Lock()
	defer c.mu.Unlock()
	c.frameErrHandler = fn
}

func (c *Communicator) onFrameError(err error) {
	c.mu.RLock()
	handler := c.frameErrHandler
	c.mu.RUnlock()
	if handler != nil {
		handler(err)
	}
}

func (c *Communicator) nextNonce() int32 {
	for {
		current := c.nonce.Load()
		var next int32
		if current >= MaxNonce {
			next = 1
		} else {
			next = current + 1
		}
		if c.nonce.CompareAndSwap(current, next) {
			return next
		}
	}
}

func (c *Communicator) shutdown() {
	c.isAlive.Store(false)
	c.closeOnce.Do(func() {
		close(c.closed)
	})
}

func (c *Communicator) ForceShutdown() {
	c.drainOnClose.Store(false)
	c.shutdown()
	c.closeGateway()
	if c.receiveDone != nil {
		<-c.receiveDone
	}
	c.cancelPendingRequests()
}

func (c *Communicator) Close() error {
	c.drainOnClose.Store(true)
	c.shutdown()
	c.closeGateway()
	if c.receiveDone != nil {
		<-c.receiveDone
	}
	c.cancelPendingRequests()
	return nil
}

func (c *Communicator) cancelPendingRequests() {
	c.mu.Lock()
	defer c.mu.Unlock()
	for k, pending := range c.pendingRequests {
		pending.errCh <- ErrNotConnected
		delete(c.pendingRequests, k)
	}
}

func (c *Communicator) writeLoop() {
	for {
		select {
		case item := <-c.writeQueue:
			err := c.transport.WritePacket(item.base)
			item.done <- err
			if err != nil {
				c.mu.RLock()
				handler := c.writeErrHandler
				c.mu.RUnlock()
				if handler != nil {
					handler(err)
				}
			}
		case <-c.closed:
			for {
				select {
				case item := <-c.writeQueue:
					err := c.transport.WritePacket(item.base)
					item.done <- err
				case <-c.receiveDone:
					for {
						select {
						case item := <-c.writeQueue:
							err := c.transport.WritePacket(item.base)
							item.done <- err
						default:
							return
						}
					}
				}
			}
		}
	}
}

func (c *Communicator) QueuePacket(base *packets.PacketBase) error {
	select {
	case <-c.receiveDone:
		return ErrNotConnected
	default:
	}
	done := make(chan error, 1)
	select {
	case c.writeQueue <- queuedPacket{base: base, done: done}:
	case <-c.receiveDone:
		return ErrNotConnected
	}
	select {
	case err := <-done:
		return err
	case <-c.receiveDone:
		return ErrNotConnected
	}
}

func (c *Communicator) Send(m proto.Message) error {
	if !c.IsAlive() {
		return ErrNotConnected
	}
	data, err := proto.Marshal(m)
	if err != nil {
		return err
	}
	return c.QueuePacket(&packets.PacketBase{
		TypeName: TypeNameOf(m),
		Nonce:    c.nextNonce(),
		Data:     data,
	})
}

func (c *Communicator) SendRequest(req proto.Message, resType proto.Message, timeout time.Duration) (proto.Message, error) {
	if !c.IsAlive() {
		return nil, ErrNotConnected
	}
	if timeout <= 0 {
		timeout = 30 * time.Second
	}

	requestNonce := c.nextNonce()
	pending := &pendingRequest{
		expectedTypeName: TypeNameOf(resType),
		ch:               make(chan proto.Message, 1),
		errCh:            make(chan error, 1),
	}

	c.mu.Lock()
	c.pendingRequests[requestNonce] = pending
	c.mu.Unlock()

	data, err := proto.Marshal(req)
	if err != nil {
		c.removePending(requestNonce)
		return nil, err
	}
	err = c.QueuePacket(&packets.PacketBase{
		TypeName: TypeNameOf(req),
		Nonce:    requestNonce,
		Data:     data,
	})
	if err != nil {
		c.removePending(requestNonce)
		return nil, err
	}

	timer := time.NewTimer(timeout)
	defer timer.Stop()

	select {
	case res := <-pending.ch:
		return res, nil
	case err := <-pending.errCh:
		return nil, err
	case <-timer.C:
		c.removePending(requestNonce)
		return nil, fmt.Errorf("request timeout for nonce %d", requestNonce)
	case <-c.closed:
		c.removePending(requestNonce)
		return nil, ErrNotConnected
	}
}

func (c *Communicator) AddListener(typeName string, fn func(proto.Message)) {
	c.mu.Lock()
	defer c.mu.Unlock()

	if _, ok := c.reqHandlers[typeName]; ok {
		panic(fmt.Sprintf("type %s is already registered with AddRequestListener", typeName))
	}
	c.handlers[typeName] = append(c.handlers[typeName], fn)
}

func (c *Communicator) RemoveListeners(typeName string) {
	c.mu.Lock()
	defer c.mu.Unlock()
	delete(c.handlers, typeName)
}

func (c *Communicator) AddRequestListener(typeName string, fn func(proto.Message, int32)) {
	c.mu.Lock()
	defer c.mu.Unlock()

	if len(c.handlers[typeName]) > 0 {
		panic(fmt.Sprintf("type %s is already registered with AddListener", typeName))
	}
	if _, ok := c.reqHandlers[typeName]; ok {
		panic(fmt.Sprintf("type %s is already registered with AddRequestListener", typeName))
	}
	c.reqHandlers[typeName] = fn
}

func (c *Communicator) OnReceivedData(typeName string, data []byte, incomingNonce, responseNonce int32) {
	if responseNonce > 0 {
		c.handleResponse(typeName, data, responseNonce)
		return
	}
	c.EnqueueInbound(typeName, data, incomingNonce, responseNonce)
}

// EnableInboundGateway attaches a goroutine worker-pool gateway in front of the
// receive coordinator. Raw frames submitted via OnReceivedFrame are decoded by
// the pool and reordered by an internal seq before reaching OnReceivedData, so
// the coordinator keeps FIFO dispatch and sole ownership of stateful handling.
//
// The gateway performs pure PacketBase envelope decode only; it never touches
// the pending-request map, listeners, or the write queue. Opt-in: without it
// OnReceivedFrame decodes inline on the calling goroutine.
func (c *Communicator) EnableInboundGateway(workers, queueSize int) {
	gateway := NewWorkerGateway(workers, queueSize,
		func(f DecodedFrame) {
			c.OnReceivedData(f.TypeName, f.Data, f.IncomingNonce, f.ResponseNonce)
		},
		func(err error) {
			// Surface gateway decode errors to the transport's frame error
			// handler so the parse-error disconnect semantics match the inline
			// path. The gateway never tears down the transport itself.
			c.onFrameError(err)
		},
	)
	c.AttachInboundGateway(gateway)
}

// AttachInboundGateway installs a custom InboundGateway in front of the receive
// coordinator. The gateway's sink must forward decoded frames to OnReceivedData
// to preserve coordinator ownership. Use EnableInboundGateway for the default
// worker-pool gateway.
func (c *Communicator) AttachInboundGateway(gateway InboundGateway) {
	c.mu.Lock()
	c.gateway = gateway
	c.mu.Unlock()
}

// OnReceivedFrame ingests a raw PacketBase frame from a transport read loop.
//
// When a gateway is attached the frame is submitted with an internal seq for
// off-coordinator decode + reorder and nil is returned; a frame that fails to
// decode is reported asynchronously, in seq order, to the frame error handler
// registered via SetFrameErrorHandler so the transport can apply the same
// parse-error disconnect semantics as the inline path. The gateway never tears
// down the transport itself.
//
// Without a gateway the frame is decoded inline on the calling goroutine and
// forwarded to OnReceivedData; a decode error is returned so the transport can
// apply its existing parse-error disconnect semantics.
func (c *Communicator) OnReceivedFrame(raw []byte) error {
	if !c.IsAlive() {
		return nil
	}
	c.mu.RLock()
	gateway := c.gateway
	c.mu.RUnlock()
	if gateway != nil {
		gateway.Submit(InboundFrame{Seq: c.frameSeq.Add(1), Bytes: raw})
		return nil
	}
	base, err := decodePacketBase(raw)
	if err != nil {
		return err
	}
	c.OnReceivedData(base.GetTypeName(), base.GetData(), base.GetNonce(), base.GetResponseNonce())
	return nil
}

func (c *Communicator) closeGateway() {
	c.mu.Lock()
	gateway := c.gateway
	c.gateway = nil
	c.mu.Unlock()
	if gateway != nil {
		gateway.Close()
	}
}

func (c *Communicator) EnqueueInbound(typeName string, data []byte, incomingNonce, responseNonce int32) {
	if !c.IsAlive() {
		return
	}
	item := inboundItem{
		typeName:      typeName,
		data:          data,
		incomingNonce: incomingNonce,
		responseNonce: responseNonce,
	}
	select {
	case c.inboundQueue <- item:
	case <-c.closed:
	}
}

func (c *Communicator) receiveLoop() {
	defer close(c.receiveDone)
	for {
		select {
		case item := <-c.inboundQueue:
			c.dispatchInbound(item)
		case <-c.closed:
			if c.drainOnClose.Load() {
				// drain remaining in queue
				for {
					select {
					case item := <-c.inboundQueue:
						c.dispatchInbound(item)
					default:
						return
					}
				}
			} else {
				return
			}
		}
	}
}

func (c *Communicator) dispatchInbound(item inboundItem) {
	c.mu.RLock()
	reqHandler := c.reqHandlers[item.typeName]
	listeners := append([]func(proto.Message){}, c.handlers[item.typeName]...)
	c.mu.RUnlock()

	if reqHandler != nil {
		msg, err := c.parse(item.typeName, item.data)
		if err != nil {
			return
		}
		reqHandler(msg, item.incomingNonce)
		return
	}

	if len(listeners) == 0 {
		return
	}
	msg, err := c.parse(item.typeName, item.data)
	if err != nil {
		return
	}
	for _, listener := range listeners {
		listener(msg)
	}
}

func (c *Communicator) handleResponse(typeName string, data []byte, responseNonce int32) {
	pending := c.removePending(responseNonce)
	if pending == nil {
		return
	}
	if typeName != pending.expectedTypeName {
		pending.errCh <- fmt.Errorf("response type mismatch for nonce %d: expected %s, got %s", responseNonce, pending.expectedTypeName, typeName)
		return
	}
	msg, err := c.parse(typeName, data)
	if err != nil {
		pending.errCh <- err
		return
	}
	pending.ch <- msg
}

func (c *Communicator) parse(typeName string, data []byte) (proto.Message, error) {
	c.mu.RLock()
	parser := c.parserMap[typeName]
	c.mu.RUnlock()
	if parser == nil {
		return nil, fmt.Errorf("protobuf parser is not registered for type %s", typeName)
	}
	return parser(data)
}

func (c *Communicator) removePending(nonce int32) *pendingRequest {
	c.mu.Lock()
	defer c.mu.Unlock()
	pending := c.pendingRequests[nonce]
	delete(c.pendingRequests, nonce)
	return pending
}

func AddListenerTyped[T proto.Message](c *Communicator, fn func(T)) {
	typeName := TypeNameOf(newMessageOf[T]())
	c.AddListener(typeName, func(m proto.Message) {
		typed, ok := m.(T)
		if !ok {
			panic(fmt.Sprintf("received %T for listener %s", m, typeName))
		}
		fn(typed)
	})
}

func AddRequestListenerTyped[Req proto.Message, Res proto.Message](c *Communicator, fn func(Req) (Res, error)) {
	reqTypeName := TypeNameOf(newMessageOf[Req]())
	c.AddRequestListener(reqTypeName, func(m proto.Message, requestNonce int32) {
		req, ok := m.(Req)
		if !ok {
			return
		}
		res, err := fn(req)
		if err != nil {
			return
		}
		select {
		case <-c.receiveDone:
			return
		default:
		}
		data, err := proto.Marshal(res)
		if err != nil {
			return
		}
		_ = c.QueuePacket(&packets.PacketBase{
			TypeName:      TypeNameOf(res),
			Nonce:         c.nextNonce(),
			ResponseNonce: requestNonce,
			Data:          data,
		})
	})
}

func SendRequestTyped[Req proto.Message, Res proto.Message](c *Communicator, req Req, timeout time.Duration) (Res, error) {
	resType := newMessageOf[Res]()
	msg, err := c.SendRequest(req, resType, timeout)
	if err != nil {
		var zero Res
		return zero, err
	}
	res, ok := msg.(Res)
	if !ok {
		var zero Res
		return zero, fmt.Errorf("received %T, expected %T", msg, resType)
	}
	return res, nil
}

func newMessageOf[T proto.Message]() T {
	var zero T
	t := reflect.TypeOf(zero)
	if t == nil || t.Kind() != reflect.Ptr {
		panic("protobuf type parameter must be a pointer message type")
	}
	return reflect.New(t.Elem()).Interface().(T)
}