feat: add wait grp exercise

internal/exercises/catalog.yaml

@@ -251,3 +251,14 @@ projects:
       - "Use time.Parse() to parse time strings with known layouts"
       - "Use time.LoadLocation() to work with different timezones"
       - "Extract time components using .Date() and .Clock() methods"
+
+- slug: 39_waitgroup
+  title: "WaitGroups"
+  difficulty: beginner
+  topics: ["concurrency", "sync", "goroutines", "waitgroup"]
+  hints:
+    - "Use sync.WaitGroup to wait for goroutines."
+    - "Call wg.Add(n) before starting n goroutines; each must call wg.Done()."
+    - "Call wg.Wait() in the parent to block until completion."
+
+

internal/exercises/solutions/39_waitgroup/waitGroup.go

@@ -0,0 +1,21 @@
+package waitgroup
+
+import (
+	"sync"
+)
+
+// The waitGroup in Go is used to wait for a collection of goroutines to finish executing
+
+func worker(wg *sync.WaitGroup, result *string) {
+	defer wg.Done()
+	*result = "Worker done"
+}
+
+func waitGroup() string {
+	var wg sync.WaitGroup
+	result := ""
+	wg.Add(1)
+	go worker(&wg, &result)
+	wg.Wait()
+	return result
+}

internal/exercises/templates/39_waitgroup/waitgroup.go

@@ -0,0 +1,18 @@
+package waitgroup
+
+import (
+	"sync"
+)
+
+// TODO: Implement these functions so tests pass
+
+func worker(wg *sync.WaitGroup, result *string) {
+	*result = "Worker done"
+}
+
+func waitGroup() string {
+	var wg sync.WaitGroup
+	result := ""
+	go worker(&wg, &result)
+	return result
+}

internal/exercises/templates/39_waitgroup/waitgroup_test.go

@@ -0,0 +1,11 @@
+package waitgroup
+
+import "testing"
+
+func TestWaitGroup(t *testing.T) {
+	got := waitGroup()
+	want := "Worker done"
+	if got != want {
+		t.Fatalf("waitGroup() = %q, want %q", got, want)
+	}
+}