Initial commit: Go SRE interview exercises

Includes solved 02-worker-pool and 09-safe-counter (mutex, atomic, sharded).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GbnJj9ZqEx9nUvzkjdwfMq

09-safe-counter/main_test.go

@@ -0,0 +1,129 @@
+package main
+
+import (
+	"fmt"
+	"sync"
+	"testing"
+)
+
+const (
+	goroutines = 16
+	perG       = 1000
+)
+
+func TestMutexCounter(t *testing.T) {
+	c := NewMutexCounter()
+	var wg sync.WaitGroup
+	for g := 0; g < goroutines; g++ {
+		wg.Add(1)
+		go func(g int) {
+			defer wg.Done()
+			for i := 0; i < perG; i++ {
+				c.Inc(fmt.Sprintf("k%d", i%4))
+			}
+		}(g)
+	}
+	wg.Wait()
+	var total int64
+	for i := 0; i < 4; i++ {
+		total += c.Get(fmt.Sprintf("k%d", i))
+	}
+	if want := int64(goroutines * perG); total != want {
+		t.Fatalf("lost updates: want %d got %d", want, total)
+	}
+}
+
+func TestSnapshotIsACopy(t *testing.T) {
+	c := NewMutexCounter()
+	c.Inc("a")
+	snap := c.Snapshot()
+	if snap == nil || snap["a"] != 1 {
+		t.Fatalf("bad snapshot: %v", snap)
+	}
+	snap["a"] = 999 // mutating the snapshot must not affect the counter
+	if c.Get("a") != 1 {
+		t.Fatal("Snapshot returned the internal map, not a copy")
+	}
+	// And writing to the snapshot while counting must not race (-race checks).
+	var wg sync.WaitGroup
+	wg.Add(1)
+	go func() { defer wg.Done(); for i := 0; i < 100; i++ { c.Inc("a") } }()
+	for i := 0; i < 100; i++ {
+		snap["b"] = int64(i)
+	}
+	wg.Wait()
+}
+
+func TestAtomicCounter(t *testing.T) {
+	c := NewAtomicCounter()
+	var wg sync.WaitGroup
+	for g := 0; g < goroutines; g++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			for i := 0; i < perG; i++ {
+				c.Inc()
+			}
+		}()
+	}
+	wg.Wait()
+	if want := int64(goroutines * perG); c.Get() != want {
+		t.Fatalf("lost updates: want %d got %d", want, c.Get())
+	}
+}
+
+func TestShardedCounter(t *testing.T) {
+	c := NewShardedCounter()
+	var wg sync.WaitGroup
+	for g := 0; g < goroutines; g++ {
+		wg.Add(1)
+		go func(g int) {
+			defer wg.Done()
+			for i := 0; i < perG; i++ {
+				c.Inc(fmt.Sprintf("key-%d", i%50))
+			}
+		}(g)
+	}
+	wg.Wait()
+	var total int64
+	for k, v := range c.Snapshot() {
+		_ = k
+		total += v
+	}
+	if want := int64(goroutines * perG); total != want {
+		t.Fatalf("lost updates across shards: want %d got %d", want, total)
+	}
+	if c.Get("key-0") == 0 {
+		t.Fatal("Get by key broken")
+	}
+}
+
+func BenchmarkMutexCounter(b *testing.B) {
+	c := NewMutexCounter()
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			c.Inc("hot")
+		}
+	})
+}
+
+func BenchmarkShardedCounter(b *testing.B) {
+	c := NewShardedCounter()
+	keys := [8]string{"a", "b", "c", "d", "e", "f", "g", "h"}
+	b.RunParallel(func(pb *testing.PB) {
+		i := 0
+		for pb.Next() {
+			c.Inc(keys[i&7])
+			i++
+		}
+	})
+}
+
+func BenchmarkAtomicCounter(b *testing.B) {
+	c := NewAtomicCounter()
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			c.Inc()
+		}
+	})
+}