golang interview task

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Golang Interview Task: Event Bus with Middleware Support

Problem Statement

Design and implement an event-driven messaging system in Go that supports middleware processing, similar to middleware patterns found in web frameworks like Express.js or Gin.

Core Requirements

Build an event bus system that allows:

1. Event Registration: Subscribe handlers to named events
2. Middleware Pipeline: Process events through middleware before reaching handlers
3. Event Emission: Trigger events with associated data
4. One-time Handlers: Support handlers that automatically unsubscribe after first execution

Expected Functionality

Your implementation should support the following usage pattern:

func main() {
    // Create event bus
    bus := NewEventBus()
    
    // Register middleware that runs for ALL events
    bus.Use(func(event string, data EventData, next func()) {
        log.Printf("[Middleware] Processing event: %s", event)
        data["timestamp"] = time.Now().Unix()
        next() // Call next to continue the middleware chain
    })
    
    // Register multiple handlers for the same event
    bus.On("user:login", func(data EventData) {
        fmt.Printf("User %v logged in at %v\n", data["username"], data["timestamp"])
    })
    
    bus.On("user:login", func(data EventData) {
        fmt.Printf("Send welcome email to %v\n", data["username"])
    })
    
    // Register a one-time handler
    bus.Once("app:init", func(data EventData) {
        fmt.Println("Application initialized")
    })
    
    // Emit events
    bus.Emit("user:login", EventData{"username": "alice", "ip": "192.168.1.1"})
    bus.Emit("app:init", EventData{})
    bus.Emit("app:init", EventData{}) // Should not trigger the Once handler again
}

Technical Specifications

• Design a flexible approach for event data (consider generics, interfaces, or other Go patterns)
• Each middleware must call next() to continue the chain
• Middleware can modify event data before it reaches handlers
• If middleware doesn't call next(), the chain stops

Focus on getting a working solution first, then optimize. Good luck!