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

05-lru-cache/main_test.go

@@ -0,0 +1,84 @@
+package main
+
+import (
+	"fmt"
+	"sync"
+	"testing"
+)
+
+func TestBasicPutGet(t *testing.T) {
+	c := NewLRU(2)
+	c.Put("a", 1)
+	if v, ok := c.Get("a"); !ok || v.(int) != 1 {
+		t.Fatalf("want (1,true), got (%v,%v)", v, ok)
+	}
+	if _, ok := c.Get("missing"); ok {
+		t.Fatal("miss reported as hit")
+	}
+}
+
+func TestEvictionOrder(t *testing.T) {
+	c := NewLRU(2)
+	c.Put("a", 1)
+	c.Put("b", 2)
+	c.Put("c", 3) // evicts a (LRU)
+	if _, ok := c.Get("a"); ok {
+		t.Fatal("a should have been evicted")
+	}
+	for _, k := range []string{"b", "c"} {
+		if _, ok := c.Get(k); !ok {
+			t.Fatalf("%s should still be present", k)
+		}
+	}
+}
+
+func TestGetRefreshesRecency(t *testing.T) {
+	c := NewLRU(2)
+	c.Put("a", 1)
+	c.Put("b", 2)
+	c.Get("a")    // a becomes MRU
+	c.Put("c", 3) // must evict b, not a
+	if _, ok := c.Get("a"); !ok {
+		t.Fatal("a was evicted despite being MRU — Get must refresh recency")
+	}
+	if _, ok := c.Get("b"); ok {
+		t.Fatal("b should have been evicted")
+	}
+}
+
+func TestUpdateExistingKey(t *testing.T) {
+	c := NewLRU(2)
+	c.Put("a", 1)
+	c.Put("b", 2)
+	c.Put("a", 99) // update, not insert — must NOT evict
+	if c.Len() != 2 {
+		t.Fatalf("update inflated size: len=%d", c.Len())
+	}
+	if v, _ := c.Get("a"); v.(int) != 99 {
+		t.Fatalf("update lost: got %v", v)
+	}
+	c.Put("c", 3) // evicts b (a was refreshed by the update)
+	if _, ok := c.Get("b"); ok {
+		t.Fatal("b should have been evicted after a's update refreshed it")
+	}
+}
+
+func TestConcurrentAccess(t *testing.T) {
+	c := NewLRU(64)
+	var wg sync.WaitGroup
+	for g := 0; g < 8; g++ {
+		wg.Add(1)
+		go func(g int) {
+			defer wg.Done()
+			for i := 0; i < 500; i++ {
+				k := fmt.Sprintf("k%d", i%100)
+				c.Put(k, i)
+				c.Get(k)
+			}
+		}(g)
+	}
+	wg.Wait()
+	if c.Len() > 64 {
+		t.Fatalf("capacity exceeded under concurrency: %d", c.Len())
+	}
+}