counter: Use a constructor

counter/counter.go

@@ -0,0 +1,22 @@
+package counter
+
+import "sync"
+
+type Counter struct {
+	count int
+	mutex sync.Mutex
+}
+
+func NewCounter() *Counter {
+	return &Counter{}
+}
+
+func (c *Counter) Inc() {
+	c.mutex.Lock()
+	defer c.mutex.Unlock()
+	c.count++
+}
+
+func (c *Counter) Value() int {
+	return c.count
+}

counter/counter_test.go

@@ -0,0 +1,44 @@
+package counter
+
+import (
+	"sync"
+	"testing"
+)
+
+// assertCounter
+// XXX: a copy of Counter is created, but we shouldn't copy a mutex. Use pointer instead.
+func assertCounter(t testing.TB, got *Counter, want int) {
+	t.Helper()
+	if got.Value() != want {
+		t.Errorf("got %d, want %d", got.Value(), want)
+	}
+}
+
+func TestCounter(t *testing.T) {
+	t.Run("incrementing the counter 3 times leaves it at 3", func(t *testing.T) {
+		counter := NewCounter()
+
+		counter.Inc()
+		counter.Inc()
+		counter.Inc()
+
+		assertCounter(t, counter, 3)
+	})
+
+	t.Run("it runs safely concurrently", func(t *testing.T) {
+		wantedCount := 1000
+		counter := NewCounter()
+
+		var wg sync.WaitGroup
+		wg.Add(wantedCount)
+
+		for i := 0; i < wantedCount; i++ {
+			go func() {
+				counter.Inc()
+				wg.Done()
+			}()
+		}
+		wg.Wait()
+		assertCounter(t, counter, wantedCount)
+	})
+}