package mxgateway

import (
	"context"
	"crypto/rand"
	"encoding/hex"
	"errors"
	"io"
	"sync"

	pb "gitea.dohertylan.com/dohertj2/mxaccessgw/clients/go/internal/generated"
	"google.golang.org/grpc/codes"
	"google.golang.org/grpc/status"
)

// EventResult carries either the next ordered event or a terminal stream error.
type EventResult struct {
	Event *MxEvent
	Err   error
}

// Session represents one gateway-backed MXAccess session.
type Session struct {
	client     *Client
	openReply  *OpenSessionReply
	closeMu    sync.Mutex
	closeReply *CloseSessionReply
}

func newSession(client *Client, openReply *OpenSessionReply) *Session {
	return &Session{
		client:    client,
		openReply: openReply,
	}
}

// NewSessionForID creates a session wrapper for commands against an existing
// gateway session id.
func NewSessionForID(client *Client, sessionID string) *Session {
	return newSession(client, &pb.OpenSessionReply{SessionId: sessionID})
}

// ID returns the gateway session identifier.
func (s *Session) ID() string {
	return s.openReply.GetSessionId()
}

// OpenReply returns the raw OpenSession reply.
func (s *Session) OpenReply() *OpenSessionReply {
	return s.openReply
}

// Close closes the gateway session once and returns the raw close reply.
func (s *Session) Close(ctx context.Context) (*CloseSessionReply, error) {
	s.closeMu.Lock()
	defer s.closeMu.Unlock()

	if s.closeReply != nil {
		return s.closeReply, nil
	}

	reply, err := s.client.CloseSessionRaw(ctx, &pb.CloseSessionRequest{SessionId: s.ID()})
	if err != nil {
		return reply, err
	}
	s.closeReply = reply
	return reply, nil
}

// Register invokes MXAccess Register and returns the server handle.
func (s *Session) Register(ctx context.Context, clientName string) (int32, error) {
	reply, err := s.RegisterRaw(ctx, clientName)
	if err != nil {
		return 0, err
	}
	if reply.GetRegister() != nil {
		return reply.GetRegister().GetServerHandle(), nil
	}
	return reply.GetReturnValue().GetInt32Value(), nil
}

// RegisterRaw invokes MXAccess Register and returns the raw reply.
func (s *Session) RegisterRaw(ctx context.Context, clientName string) (*MxCommandReply, error) {
	if clientName == "" {
		return nil, errors.New("mxgateway: client name is required")
	}

	return s.invokeCommand(ctx, &pb.MxCommand{
		Kind: pb.MxCommandKind_MX_COMMAND_KIND_REGISTER,
		Payload: &pb.MxCommand_Register{
			Register: &pb.RegisterCommand{ClientName: clientName},
		},
	})
}

// Unregister invokes MXAccess Unregister.
func (s *Session) Unregister(ctx context.Context, serverHandle int32) error {
	_, err := s.invokeCommand(ctx, &pb.MxCommand{
		Kind: pb.MxCommandKind_MX_COMMAND_KIND_UNREGISTER,
		Payload: &pb.MxCommand_Unregister{
			Unregister: &pb.UnregisterCommand{ServerHandle: serverHandle},
		},
	})
	return err
}

// AddItem invokes MXAccess AddItem and returns the item handle.
func (s *Session) AddItem(ctx context.Context, serverHandle int32, itemDefinition string) (int32, error) {
	reply, err := s.AddItemRaw(ctx, serverHandle, itemDefinition)
	if err != nil {
		return 0, err
	}
	if reply.GetAddItem() != nil {
		return reply.GetAddItem().GetItemHandle(), nil
	}
	return reply.GetReturnValue().GetInt32Value(), nil
}

// AddItemRaw invokes MXAccess AddItem and returns the raw reply.
func (s *Session) AddItemRaw(ctx context.Context, serverHandle int32, itemDefinition string) (*MxCommandReply, error) {
	if itemDefinition == "" {
		return nil, errors.New("mxgateway: item definition is required")
	}

	return s.invokeCommand(ctx, &pb.MxCommand{
		Kind: pb.MxCommandKind_MX_COMMAND_KIND_ADD_ITEM,
		Payload: &pb.MxCommand_AddItem{
			AddItem: &pb.AddItemCommand{
				ServerHandle:   serverHandle,
				ItemDefinition: itemDefinition,
			},
		},
	})
}

// AddItem2 invokes MXAccess AddItem2 and returns the item handle.
func (s *Session) AddItem2(ctx context.Context, serverHandle int32, itemDefinition, itemContext string) (int32, error) {
	reply, err := s.AddItem2Raw(ctx, serverHandle, itemDefinition, itemContext)
	if err != nil {
		return 0, err
	}
	if reply.GetAddItem2() != nil {
		return reply.GetAddItem2().GetItemHandle(), nil
	}
	return reply.GetReturnValue().GetInt32Value(), nil
}

// AddItem2Raw invokes MXAccess AddItem2 and returns the raw reply.
func (s *Session) AddItem2Raw(ctx context.Context, serverHandle int32, itemDefinition, itemContext string) (*MxCommandReply, error) {
	if itemDefinition == "" {
		return nil, errors.New("mxgateway: item definition is required")
	}

	return s.invokeCommand(ctx, &pb.MxCommand{
		Kind: pb.MxCommandKind_MX_COMMAND_KIND_ADD_ITEM2,
		Payload: &pb.MxCommand_AddItem2{
			AddItem2: &pb.AddItem2Command{
				ServerHandle:   serverHandle,
				ItemDefinition: itemDefinition,
				ItemContext:    itemContext,
			},
		},
	})
}

// Advise invokes MXAccess Advise.
func (s *Session) Advise(ctx context.Context, serverHandle, itemHandle int32) error {
	_, err := s.AdviseRaw(ctx, serverHandle, itemHandle)
	return err
}

// AdviseRaw invokes MXAccess Advise and returns the raw reply.
func (s *Session) AdviseRaw(ctx context.Context, serverHandle, itemHandle int32) (*MxCommandReply, error) {
	return s.invokeCommand(ctx, &pb.MxCommand{
		Kind: pb.MxCommandKind_MX_COMMAND_KIND_ADVISE,
		Payload: &pb.MxCommand_Advise{
			Advise: &pb.AdviseCommand{
				ServerHandle: serverHandle,
				ItemHandle:   itemHandle,
			},
		},
	})
}

// Write invokes MXAccess Write.
func (s *Session) Write(ctx context.Context, serverHandle, itemHandle int32, value *MxValue, userID int32) error {
	_, err := s.WriteRaw(ctx, serverHandle, itemHandle, value, userID)
	return err
}

// WriteRaw invokes MXAccess Write and returns the raw reply.
func (s *Session) WriteRaw(ctx context.Context, serverHandle, itemHandle int32, value *MxValue, userID int32) (*MxCommandReply, error) {
	if value == nil {
		return nil, errors.New("mxgateway: write value is required")
	}

	return s.invokeCommand(ctx, &pb.MxCommand{
		Kind: pb.MxCommandKind_MX_COMMAND_KIND_WRITE,
		Payload: &pb.MxCommand_Write{
			Write: &pb.WriteCommand{
				ServerHandle: serverHandle,
				ItemHandle:   itemHandle,
				Value:        value,
				UserId:       userID,
			},
		},
	})
}

// Events streams ordered session events until the server ends the stream,
// context cancellation stops Recv, or a terminal error is sent.
func (s *Session) Events(ctx context.Context) (<-chan EventResult, error) {
	return s.EventsAfter(ctx, 0)
}

// EventsAfter streams ordered session events after the given worker sequence.
func (s *Session) EventsAfter(ctx context.Context, afterWorkerSequence uint64) (<-chan EventResult, error) {
	stream, err := s.client.StreamEventsRaw(ctx, &pb.StreamEventsRequest{
		SessionId:           s.ID(),
		AfterWorkerSequence: afterWorkerSequence,
	})
	if err != nil {
		return nil, err
	}

	results := make(chan EventResult, 16)
	go func() {
		defer close(results)
		for {
			event, err := stream.Recv()
			if err == nil {
				results <- EventResult{Event: event}
				continue
			}
			if err == io.EOF || status.Code(err) == codes.Canceled || ctx.Err() != nil {
				return
			}
			results <- EventResult{Err: &GatewayError{Op: "stream events", Err: err}}
			return
		}
	}()

	return results, nil
}

func (s *Session) invokeCommand(ctx context.Context, command *MxCommand) (*MxCommandReply, error) {
	return s.client.Invoke(ctx, &pb.MxCommandRequest{
		SessionId:           s.ID(),
		ClientCorrelationId: newCorrelationID(),
		Command:             command,
	})
}

func newCorrelationID() string {
	var buffer [16]byte
	if _, err := rand.Read(buffer[:]); err != nil {
		return ""
	}
	return hex.EncodeToString(buffer[:])
}