type lockedRandSource struct {
	lk  sync.Mutex
	src rand.Source
}

// Int63 returns a non-negative pseudo-random 63-bit integer as an int64.
func (r *lockedRandSource) Int63() (n int64) {
	r.lk.Lock()
	n = r.src.Int63()
	r.lk.Unlock()
	return n
}

// Seed uses the provided seed value to initialize the generator to a
// deterministic state.

var src = rand.NewSource(time.Now().UnixNano())

func randString(n int) string {
	b := make([]byte, n)
	// A src.Int63() generates 63 random bits, enough for letterIdxMax characters!
	for i, cache, remain := n-1, src.Int63(), letterIdxMax; i >= 0; {
		if remain == 0 {
			cache, remain = src.Int63(), letterIdxMax
		}
		if idx := int(cache & letterIdxMask); idx < len(letterBytes) {
			b[i] = letterBytes[idx]
			i--

	// 4 MB block.
	// Approx runtime ~30ms
	data := make([]byte, 4<<20)
	rng := rand.New(rand.NewSource(0))
	for i := range data {
		// Generate compressible stream...
		data[i] = byte(rng.Int63() & 3)
	}
	failOnErr := func(err error) {
		if err != nil {
			logger.Fatal(errSelfTestFailure, "compress: error on self-test: %v", err)
		}
	}
	const skip = 2<<20 + 511