counter: add baisics

counter/counter.go

@@ -0,0 +1,13 @@
+package counter
+
+type Counter struct {
+	count int
+}
+
+func (c *Counter) Inc() {
+	c.count++
+}
+
+func (c *Counter) Value() int {
+	return c.count
+}

counter/counter_test.go

@@ -0,0 +1,20 @@
+package counter
+
+import "testing"
+
+func TestCounter(t *testing.T) {
+	t.Run("incrementing the counter 3 times leaves it at 3", func(t *testing.T) {
+		counter := Counter{}
+
+		counter.Inc()
+		counter.Inc()
+		counter.Inc()
+		assertCounter(t, counter, 3)
+	})
+}
+
+func assertCounter(t testing.TB, got Counter, want int) {
+	if got.Value() != 3 {
+		t.Errorf("got %d, want %d", got.Value(), want)
+	}
+}