/**
 * Microservices Communication System
 * TODO: Implement event-driven microservices architecture using GitHub Copilot
 * 
 * Services: Go User Service, Java Order Service, Python Payment Service, Node.js Notification Service
 * Event Bus: Apache Kafka
 * Pattern: Event Sourcing with CQRS
 */

package main

import (
	"context"
	"encoding/json"
	"log"
	"net/http"
	"time"

	"github.com/gin-gonic/gin"
	"github.com/segmentio/kafka-go"
	"gorm.io/driver/postgres"
	"gorm.io/gorm"
)

// Domain models for User Service
type User struct {
	ID        string    `json:"id" gorm:"primaryKey"`
	Email     string    `json:"email" gorm:"uniqueIndex"`
	Username  string    `json:"username" gorm:"uniqueIndex"`
	Profile   Profile   `json:"profile" gorm:"embedded"`
	CreatedAt time.Time `json:"created_at"`
	UpdatedAt time.Time `json:"updated_at"`
}

type Profile struct {
	FirstName   string            `json:"first_name"`
	LastName    string            `json:"last_name"`
	Avatar      string            `json:"avatar"`
	Preferences map[string]string `json:"preferences" gorm:"serializer:json"`
}

// Event structures for inter-service communication
type UserEvent struct {
	EventID     string                 `json:"event_id"`
	EventType   string                 `json:"event_type"`
	AggregateID string                 `json:"aggregate_id"`
	Version     int                    `json:"version"`
	Timestamp   time.Time              `json:"timestamp"`
	Data        map[string]interface{} `json:"data"`
	Metadata    map[string]string      `json:"metadata"`
}

// User Service implementation
type UserService struct {
	db           *gorm.DB
	router       *gin.Engine
	eventBus     *kafka.Writer
	eventStore   EventStore
	queryHandler QueryHandler
}

// TODO: Implement Event Store for event sourcing
type EventStore interface {
	AppendEvents(aggregateID string, events []UserEvent, expectedVersion int) error
	GetEvents(aggregateID string, fromVersion int) ([]UserEvent, error)
	GetAllEvents(fromTimestamp time.Time) ([]UserEvent, error)
}

// TODO: Implement Query Handler for CQRS read models
type QueryHandler interface {
	GetUser(userID string) (*User, error)
	GetUserByEmail(email string) (*User, error)
	SearchUsers(query string, limit int) ([]User, error)
	GetUserStatistics() (map[string]interface{}, error)
}

// TODO: Implement Event Store implementation
type PostgresEventStore struct {
	db *gorm.DB
}

func (es *PostgresEventStore) AppendEvents(aggregateID string, events []UserEvent, expectedVersion int) error {
	// TODO: Implement event storage with optimistic concurrency control
	// 1. Check current version matches expected version
	// 2. Store events in order
	// 3. Update aggregate version
	// 4. Ensure atomicity with transaction
	return nil
}

func (es *PostgresEventStore) GetEvents(aggregateID string, fromVersion int) ([]UserEvent, error) {
	// TODO: Implement event retrieval
	// 1. Query events for aggregate ID
	// 2. Order by version
	// 3. Filter by from version
	return nil, nil
}

func (es *PostgresEventStore) GetAllEvents(fromTimestamp time.Time) ([]UserEvent, error) {
	// TODO: Implement global event stream
	// Used for event replay and projection rebuilding
	return nil, nil
}

// TODO: Implement Query Handler with read models
type UserQueryHandler struct {
	db *gorm.DB
}

func (qh *UserQueryHandler) GetUser(userID string) (*User, error) {
	// TODO: Query read model (denormalized view)
	var user User
	// Implement efficient user lookup
	return &user, nil
}

func (qh *UserQueryHandler) SearchUsers(query string, limit int) ([]User, error) {
	// TODO: Implement full-text search
	// Use PostgreSQL full-text search or Elasticsearch
	return nil, nil
}

// TODO: Implement User aggregate for business logic
type UserAggregate struct {
	ID      string
	Version int
	State   User
	Changes []UserEvent
}

func (ua *UserAggregate) CreateUser(email, username string, profile Profile) error {
	// TODO: Implement user creation business logic
	// 1. Validate email format
	// 2. Check username constraints
	// 3. Generate user ID
	// 4. Create UserCreated event
	// 5. Apply event to state
	return nil
}

func (ua *UserAggregate) UpdateProfile(profile Profile) error {
	// TODO: Implement profile update logic
	// 1. Validate profile data
	// 2. Create ProfileUpdated event
	// 3. Apply event to state
	return nil
}

func (ua *UserAggregate) ChangeEmail(newEmail string) error {
	// TODO: Implement email change logic
	// 1. Validate email format
	// 2. Check email uniqueness
	// 3. Create EmailChanged event
	// 4. Apply event to state
	return nil
}

// TODO: Implement event application (for event sourcing)
func (ua *UserAggregate) ApplyEvent(event UserEvent) error {
	// TODO: Apply event to aggregate state
	switch event.EventType {
	case "UserCreated":
		// Apply user creation
	case "ProfileUpdated":
		// Apply profile update
	case "EmailChanged":
		// Apply email change
	}
	ua.Version++
	return nil
}

// TODO: Implement Kafka event publishing
func (us *UserService) PublishEvent(event UserEvent) error {
	// TODO: Publish event to Kafka topic
	// 1. Serialize event to JSON
	// 2. Send to appropriate topic
	// 3. Handle publishing errors
	// 4. Implement retry logic
	
	eventData, err := json.Marshal(event)
	if err != nil {
		return err
	}

	return us.eventBus.WriteMessages(context.Background(),
		kafka.Message{
			Key:   []byte(event.AggregateID),
			Value: eventData,
		},
	)
}

// TODO: Implement HTTP handlers
func (us *UserService) CreateUserHandler(c *gin.Context) {
	// TODO: Handle user creation HTTP request
	// 1. Parse request body
	// 2. Validate input
	// 3. Load or create aggregate
	// 4. Execute business logic
	// 5. Store events
	// 6. Publish events
	// 7. Return response
	
	var request struct {
		Email    string  `json:"email" binding:"required,email"`
		Username string  `json:"username" binding:"required"`
		Profile  Profile `json:"profile"`
	}

	if err := c.ShouldBindJSON(&request); err != nil {
		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
		return
	}

	// TODO: Implement the rest of the handler
	c.JSON(http.StatusCreated, gin.H{"status": "created"})
}

func (us *UserService) GetUserHandler(c *gin.Context) {
	// TODO: Handle user retrieval
	userID := c.Param("id")
	// Use query handler to get user from read model
	user, err := us.queryHandler.GetUser(userID)
	if err != nil {
		c.JSON(http.StatusNotFound, gin.H{"error": "User not found"})
		return
	}
	c.JSON(http.StatusOK, user)
}

func (us *UserService) UpdateUserHandler(c *gin.Context) {
	// TODO: Handle user updates
	// Similar pattern to CreateUserHandler
	c.JSON(http.StatusOK, gin.H{"status": "updated"})
}

// TODO: Implement event consumer for projections
func (us *UserService) StartEventConsumer() {
	// TODO: Consume events to update read models
	// 1. Subscribe to user events topic
	// 2. Process events to update projections
	// 3. Handle event replay for new projections
	// 4. Implement idempotent event processing
	
	reader := kafka.NewReader(kafka.ReaderConfig{
		Brokers: []string{"localhost:9092"},
		Topic:   "user-events",
		GroupID: "user-service-projections",
	})

	for {
		message, err := reader.ReadMessage(context.Background())
		if err != nil {
			log.Printf("Error reading message: %v", err)
			continue
		}

		var event UserEvent
		if err := json.Unmarshal(message.Value, &event); err != nil {
			log.Printf("Error unmarshaling event: %v", err)
			continue
		}

		// TODO: Process event for read model updates
		us.processEventForProjections(event)
	}
}

func (us *UserService) processEventForProjections(event UserEvent) {
	// TODO: Update read models based on event
	// 1. Route event to appropriate projection handlers
	// 2. Update denormalized views
	// 3. Update search indices
	// 4. Handle projection errors
}

// TODO: Implement service initialization
func NewUserService() *UserService {
	// TODO: Initialize all components
	// 1. Connect to PostgreSQL
	// 2. Set up Kafka writer
	// 3. Initialize event store
	// 4. Initialize query handler
	// 5. Set up HTTP routes
	
	db, err := gorm.Open(postgres.Open("postgres://user:pass@localhost/userdb?sslmode=disable"), &gorm.Config{})
	if err != nil {
		log.Fatal("Failed to connect to database")
	}

	// Auto-migrate tables
	db.AutoMigrate(&User{})

	kafkaWriter := &kafka.Writer{
		Addr:     kafka.TCP("localhost:9092"),
		Topic:    "user-events",
		Balancer: &kafka.LeastBytes{},
	}

	eventStore := &PostgresEventStore{db: db}
	queryHandler := &UserQueryHandler{db: db}

	router := gin.Default()
	
	service := &UserService{
		db:           db,
		router:       router,
		eventBus:     kafkaWriter,
		eventStore:   eventStore,
		queryHandler: queryHandler,
	}

	// TODO: Set up routes
	service.setupRoutes()
	
	return service
}

func (us *UserService) setupRoutes() {
	// TODO: Define HTTP routes
	api := us.router.Group("/api/v1")
	{
		api.POST("/users", us.CreateUserHandler)
		api.GET("/users/:id", us.GetUserHandler)
		api.PUT("/users/:id", us.UpdateUserHandler)
		api.DELETE("/users/:id", us.DeleteUserHandler)
		api.GET("/users/search", us.SearchUsersHandler)
	}

	// TODO: Add health check and metrics endpoints
	us.router.GET("/health", us.HealthCheckHandler)
	us.router.GET("/metrics", us.MetricsHandler)
}

// TODO: Implement additional handlers
func (us *UserService) DeleteUserHandler(c *gin.Context) {
	// TODO: Handle user deletion (soft delete)
	c.JSON(http.StatusOK, gin.H{"status": "deleted"})
}

func (us *UserService) SearchUsersHandler(c *gin.Context) {
	// TODO: Handle user search
	c.JSON(http.StatusOK, gin.H{"users": []User{}})
}

func (us *UserService) HealthCheckHandler(c *gin.Context) {
	// TODO: Check service health
	// 1. Check database connection
	// 2. Check Kafka connection
	// 3. Return health status
	c.JSON(http.StatusOK, gin.H{
		"status":    "healthy",
		"timestamp": time.Now(),
		"version":   "1.0.0",
	})
}

func (us *UserService) MetricsHandler(c *gin.Context) {
	// TODO: Return service metrics
	// 1. Request counts
	// 2. Response times
	// 3. Error rates
	// 4. Business metrics
	c.JSON(http.StatusOK, gin.H{
		"requests_total":   1000,
		"errors_total":     10,
		"avg_response_ms":  25,
		"active_users":     500,
	})
}

// TODO: Implement graceful shutdown
func (us *UserService) Shutdown() {
	// TODO: Gracefully shutdown service
	// 1. Stop accepting new requests
	// 2. Finish processing current requests
	// 3. Close database connections
	// 4. Close Kafka connections
	
	us.eventBus.Close()
}

// Main function
func main() {
	// TODO: Initialize and start the user service
	service := NewUserService()
	
	// Start event consumer in background
	go service.StartEventConsumer()
	
	// TODO: Start HTTP server
	log.Println("Starting User Service on :8080")
	if err := service.router.Run(":8080"); err != nil {
		log.Fatal("Failed to start server:", err)
	}
}

/*
Expected Implementation Areas for GitHub Copilot:

1. Event Sourcing Implementation:
   - Event store with optimistic concurrency
   - Event replay capabilities
   - Snapshot mechanisms for performance

2. CQRS Pattern:
   - Separate read and write models
   - Projection updates from events
   - Query optimization

3. Microservice Communication:
   - Kafka producer/consumer
   - Event schema evolution
   - Message ordering and idempotency

4. Business Logic:
   - Domain aggregates
   - Business rule validation
   - State transitions

5. Infrastructure:
   - Database connections and migrations
   - HTTP server setup
   - Monitoring and health checks

6. Testing:
   - Unit tests for aggregates
   - Integration tests for event flow
   - Contract tests between services

Example Usage:
go mod init user-service
go mod tidy
go run main.go

This should demonstrate Copilot's ability to:
- Implement complex architectural patterns
- Handle distributed system concerns
- Generate production-ready microservice code
- Understand event-driven architecture
- Implement proper error handling and monitoring
*/
