package main

import (
	"fmt"
	"sync"
)

// without using mutex the following error can happen: "fatal error: concurrent map writes" while changing the map in multiple goroutines
type Storage struct {
	sync.Mutex
	storage map[string]int // map is not threade-safe
}

func (s *Storage) Increment(name string) {
	s.Lock() // lock the mutex before accessing counters
	defer s.Unlock() // unlock the mutex at the end of the function using a defer statement
    s.storage[name]++
}

func (s *Storage) Decrement(name string) {
	s.Lock()
    defer s.Unlock()
	s.storage[name]--
}

func main() {
	s := &Storage{
		storage: make(map[string]int),
	}

	wg := sync.WaitGroup{}
	wg.Add(5)

    // increment a named counter in a loop
	increment := func(name string, count uint) {
		for i := 0; i < int(count); i++ {
			s.Increment(name)
		}

		wg.Done()
	}

    // decrement a named counter in a loop
	decrement := func(name string, count uint) {
		for i := 0; i < int(count); i++ {
			s.Decrement(name)
		}

		wg.Done()
	}

    // run 5 goroutines concurrently
	go increment("a", 1000)
	go decrement("a", 500)

	go increment("b", 1000)
	go increment("b", 1000)
	go decrement("b", 1500)

	wg.Wait() // wait for the goroutines to finish

	fmt.Println(s.storage)
}

// $ go run main.go  
// map[a:500 b:500]

// without mutex the following error can happen
// $ go run main.go
// fatal error: concurrent map writes