What problems are there in code with a global counter? How to fix it?

Problems with a global counter in code are usually related to shared state across different parts of the program, which leads to:

• Thread safety: race conditions can occur with concurrent access.
• Testing difficulties: global state complicates test isolation.
• Maintenance and scalability issues.

To fix these problems, you can:

1. Use local variables or pass the counter explicitly through function parameters.
2. If a shared counter is needed in a multithreaded environment, apply synchronization, such as mutexes or atomic operations.
3. Encapsulate the counter within a structure with methods controlling access.

Example using an atomic counter in Go:

import (
    "sync/atomic"
)

var counter int64

func increment() int64 {
    return atomic.AddInt64(&counter, 1)
}

This approach ensures safety during concurrent access and improves code readability and maintainability.