Add semaphore example. (#89)

Author: petersondmg

advanced/semaphore.go

@@ -0,0 +1,43 @@
+package main
+
+// This example shows how easy it is to implement a semaphore using channels.
+// A semaphore can be used to limit the number of concurrent go routines executing a task.
+
+import (
+	"fmt"
+	"sync"
+	"time"
+)
+
+func main() {
+	const maxConcurrent = 2
+
+	const totalTasks = 10
+
+	semaphore := make(chan struct{}, maxConcurrent)
+
+	wg := sync.WaitGroup{}
+
+	for i := range make([]int, totalTasks) {
+		// blocks until semaphore is released
+		semaphore <- struct{}{}
+
+		taskNumber := i
+		// executes task async
+		wg.Add(1)
+		go func() {
+			defer func() {
+				wg.Done()
+				// release semaphore
+				<-semaphore
+			}()
+
+			fmt.Println("executing task: ", taskNumber)
+			time.Sleep(time.Second)
+		}()
+	}
+
+	wg.Wait()
+
+	fmt.Println("done")
+}