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.go

@@ -0,0 +1,52 @@
+// Exercise 04 — Log Crunching: Top-N Endpoints
+//
+// ASSIGNMENT
+// ----------
+// Log lines look like:
+//
+//     2026-08-28T10:00:01Z GET /api/users 200 123ms
+//
+// Implement TopN(r, n) so that it:
+//
+//  1. Streams the input (must handle logs far larger than memory — no
+//     io.ReadAll).
+//  2. Counts requests per endpoint (3rd whitespace-separated field).
+//  3. Skips malformed lines (fewer than 3 fields) without failing.
+//  4. Handles lines longer than 64KB (hint: bufio.Scanner's default limit).
+//  5. Returns the n most frequent endpoints, most frequent first.
+//     Ties: order among equal counts is unspecified.
+//
+// Run the tests:   go test -race ./04-log-topn/
+package main
+
+import (
+	"fmt"
+	"io"
+	"strings"
+)
+
+type KV struct {
+	Endpoint string
+	Count    int
+}
+
+// TopN returns the n most frequent endpoints in the log stream.
+// TODO: implement me.
+func TopN(r io.Reader, n int) ([]KV, error) {
+	// TODO:
+	//   - bufio.Scanner with an enlarged buffer
+	//   - map[string]int for counting
+	//   - copy map into []KV and sort by count desc (maps can't be sorted)
+	//   - truncate to n
+	return nil, nil
+}
+
+func main() {
+	logs := `2026-08-28T10:00:01Z GET /api/users 200 12ms
+2026-08-28T10:00:02Z GET /api/orders 200 40ms
+2026-08-28T10:00:03Z GET /api/users 500 3ms
+garbage line
+2026-08-28T10:00:04Z POST /api/users 201 88ms`
+	top, err := TopN(strings.NewReader(logs), 2)
+	fmt.Println(top, err)
+}