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

04-log-topn/main_test.go

@@ -0,0 +1,65 @@
+package main
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestTopNBasic(t *testing.T) {
+	logs := strings.Join([]string{
+		"t GET /a 200 1ms",
+		"t GET /b 200 1ms",
+		"t GET /a 200 1ms",
+		"t GET /c 200 1ms",
+		"t GET /a 200 1ms",
+		"t GET /b 200 1ms",
+	}, "\n")
+
+	got, err := TopN(strings.NewReader(logs), 2)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if len(got) != 2 {
+		t.Fatalf("want 2 results, got %d: %v", len(got), got)
+	}
+	if got[0].Endpoint != "/a" || got[0].Count != 3 {
+		t.Errorf("want /a x3 first, got %+v", got[0])
+	}
+	if got[1].Endpoint != "/b" || got[1].Count != 2 {
+		t.Errorf("want /b x2 second, got %+v", got[1])
+	}
+}
+
+func TestMalformedLinesSkipped(t *testing.T) {
+	logs := "t GET /a 200 1ms\n\ngarbage\nonly two\nt GET /a 200 1ms\n"
+	got, err := TopN(strings.NewReader(logs), 5)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if len(got) != 1 || got[0].Count != 2 {
+		t.Fatalf("malformed lines mishandled: %v", got)
+	}
+}
+
+func TestNLargerThanDistinct(t *testing.T) {
+	got, err := TopN(strings.NewReader("t GET /a 200 1ms"), 10)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if len(got) != 1 {
+		t.Fatalf("want 1 result, got %d", len(got))
+	}
+}
+
+func TestLongLines(t *testing.T) {
+	// A single line > 64KB (bufio.Scanner default max token size).
+	long := "t GET /huge?q=" + strings.Repeat("x", 80*1024) + " 200 1ms"
+	logs := long + "\nt GET /small 200 1ms\nt GET /small 200 1ms\n"
+	got, err := TopN(strings.NewReader(logs), 1)
+	if err != nil {
+		t.Fatalf("long line broke the scanner: %v (raise Scanner.Buffer)", err)
+	}
+	if len(got) == 0 || got[0].Endpoint != "/small" {
+		t.Fatalf("want /small on top, got %v", got)
+	}
+}