Add concurrency_test.go, mutex for Parallel

We need the mutex because we call the go routine from our for loop and when
the first starts, the for-loop has already finished and i == n - 1. We now
lock our mutex, start the function (so no two functions have the same index)
and increment the counter.

concurrency.go

@@ -13,9 +13,21 @@ func Parallel(n int, fn func(int)) {
 	wg.Add(n)
 	defer wg.Wait()
 
+	var m sync.Mutex
+	counter := 0
+
 	for i := 0; i < n; i++ {
 		go func() {
-			fn(n)
+			// Lock mutex to prevent multiple functions with the same index
+			m.Lock()
+
+			// Run function
+			fn(counter)
+
+			// Increment counter
+			counter++
+
+			m.Unlock()
 			wg.Done()
 		}()
 	}

concurrency_test.go

@@ -0,0 +1,70 @@
+package abutil
+
+import (
+	"sync"
+	"testing"
+	"time"
+)
+
+func TestParallel(t *testing.T) {
+	var m sync.Mutex
+	var wg sync.WaitGroup
+
+	counter := 0
+
+	wg.Add(2)
+	go Parallel(2, func(n int) {
+		m.Lock()
+		counter++
+		m.Unlock()
+		wg.Done()
+	})
+
+	wg.Wait()
+
+	if counter != 2 {
+		t.Errorf("Expected counter to be %d, but got %d", 2, counter)
+	}
+}
+
+func TestParallelCounter(t *testing.T) {
+	var m sync.Mutex
+	var wg sync.WaitGroup
+
+	sum := 0
+	wg.Add(4)
+	go Parallel(4, func(n int) {
+		m.Lock()
+		sum += n
+		m.Unlock()
+		wg.Done()
+	})
+
+	wg.Wait()
+
+	if sum != 6 {
+		t.Errorf("Expected sum to be %d, but got %d", 6, sum)
+	}
+}
+
+func TestParallelTiming(t *testing.T) {
+	var m sync.Mutex
+	counter := 0
+
+	go Parallel(4, func(n int) {
+		time.Sleep(time.Duration(n) * time.Millisecond)
+		m.Lock()
+		counter++
+		m.Unlock()
+	})
+
+	done := make(chan bool)
+	time.AfterFunc(2*time.Millisecond, func() {
+		if counter != 2 {
+			t.Errorf("Expected counter to be %d, but got %d", 2, counter)
+		}
+		done <- true
+	})
+
+	<-done
+}