Generate random string in Go

gistfile2.txt

BenchmarkRandomStringA-4   	 2000000	       702 ns/op
BenchmarkRandomStringB-4   	10000000	       120 ns/op
BenchmarkRandomStringC-4   	20000000	       115 ns/op
BenchmarkRandomStringD-4   	10000000	       121 ns/op

random_string.go

func RandomStringA(n int) string {
	const letters = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_-"
	b := make([]byte, n)
	lenLetters := len(letters)
	for i := 0; i < n; i++ {
		b[i] = letters[rand.Intn(lenLetters)]
	}
	return string(b)
}

func RandomStringB(rng *rand.Rand, n int) string {
	const letters = `0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_-`
	b := make([]byte, ((n+9)/10)*10)
	for i := 0; i < n; i += 10 {
		rn := rng.Uint64()
		b[i+0] = letters[(rn&0x0FC0000000000000)>>54]
		b[i+1] = letters[(rn&0x003F000000000000)>>48]
		b[i+2] = letters[(rn&0x0000FC0000000000)>>42]
		b[i+3] = letters[(rn&0x000003F000000000)>>36]
		b[i+4] = letters[(rn&0x0000000FC0000000)>>30]
		b[i+5] = letters[(rn&0x000000003F000000)>>24]
		b[i+6] = letters[(rn&0x0000000000FC0000)>>18]
		b[i+7] = letters[(rn&0x000000000003F000)>>12]
		b[i+8] = letters[(rn&0x0000000000000FC0)>>6]
		b[i+9] = letters[(rn&0x000000000000003F)>>0]
	}
	return string(b[:n])
}

func RandomStringC(rng *rand.Rand, n int) string {
	// extract 10 letters — 60 bits of entropy — for each pseudo-random uint64
	const K = 10
	const L = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_-"
	b := make([]byte, ((n+K-1)/K)*K)
	for i := 0; i < n; i += K {
		rn := rng.Uint64()
		for j := 0; j < K; j++ {
			b[i+j] = L[rn&0x3F]
			rn >>= 6
		}
	}
	return string(b[:n])
}

func RandomStringD(rng *rand.Rand, n int) string {
	const letterBytes = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_-"
	const (
		letterIdxBits = 6                    // 6 bits to represent a letter index
		letterIdxMask = 1<<letterIdxBits - 1 // All 1-bits, as many as letterIdxBits
		letterIdxMax  = 63 / letterIdxBits   // # of letter indices fitting in 63 bits
	)
	b := make([]byte, n)
	// A src.Int63() generates 63 random bits, enough for letterIdxMax characters!
	for i, cache, remain := n-1, rng.Int63(), letterIdxMax; i >= 0; {
		if remain == 0 {
			cache, remain = rng.Int63(), letterIdxMax
		}
		idx := int(cache & letterIdxMask)
		b[i] = letterBytes[idx]
		i--
		cache >>= letterIdxBits
		remain--
	}

	return string(b)
}