Semaphore using buffered channel.

sema.go

package main

import (
	"fmt"
	"log"
	"math/rand/v2"
	"sync"
	"sync/atomic"
	"time"
)

// Semaphore is a channel based semaphore.
type Semaphore chan struct{}

// New returns semaphore of size `size`.
func New(size int) (Semaphore, error) {
	if size <= 0 {
		return nil, fmt.Errorf("size (%d) <= 0", size)
	}

	s := make(Semaphore, size)
	return s, nil
}

// Enter enters the semaphore.
func (s Semaphore) Enter() {
	s <- struct{}{}
}

// Leave leaves the semaphore.
func (s Semaphore) Leave() {
	<-s
}

func main() {
	s, err := New(3)
	if err != nil {
		log.Fatal(err)
	}

	count := int64(0)
	var wg sync.WaitGroup

	for i := range 7 {
		wg.Add(1)
		go func() {
			defer wg.Done()
			for range 100 {
				s.Enter()
				v := atomic.AddInt64(&count, 1)
				fmt.Printf("[%d] count=%d\n", i, v)
				n := time.Duration(rand.IntN(100))
				time.Sleep(n)
				atomic.AddInt64(&count, -1)
				s.Leave()
			}
		}()
	}

	wg.Wait()
}