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

03-rate-limiter/main.go

@@ -0,0 +1,45 @@
+// Exercise 03 — Token Bucket Rate Limiter
+//
+// ASSIGNMENT
+// ----------
+// Implement a token-bucket limiter:
+//
+//  1. NewLimiter(rate, burst): capacity `burst` tokens, refilled continuously
+//     at `rate` tokens/second. Bucket starts FULL.
+//  2. Allow() consumes one token and returns true, or returns false if no
+//     token is available. Must be non-blocking and safe for concurrent use.
+//  3. Refill must be LAZY — no background goroutine, no ticker. Compute the
+//     accrued tokens from elapsed time inside Allow().
+//  4. Tokens never exceed `burst` (idle time must not build infinite credit).
+//
+// Run the tests:   go test -race ./03-rate-limiter/
+package main
+
+import (
+	"fmt"
+	"time"
+)
+
+type Limiter struct {
+	// TODO: fields (mutex, current tokens, max, rate, last refill time)
+}
+
+// NewLimiter returns a limiter allowing `rate` ops/sec with bursts up to `burst`.
+// TODO: implement me.
+func NewLimiter(rate float64, burst int) *Limiter {
+	return &Limiter{}
+}
+
+// Allow reports whether one operation may proceed now.
+// TODO: implement me.
+func (l *Limiter) Allow() bool {
+	return false
+}
+
+func main() {
+	l := NewLimiter(5, 3) // 5/sec, burst 3
+	for i := 0; i < 10; i++ {
+		fmt.Printf("req %d: allowed=%v\n", i, l.Allow())
+		time.Sleep(100 * time.Millisecond)
+	}
+}