nonce create math/rand vs crypt/rand

gistfile1.go

package main

import (
	"encoding/binary"
	"fmt"
	"math/rand"

	crand "crypto/rand"
	"sync"
	"time"
)

var (
	rsLetterIdxBits       = 6
	rsLetterIdxMask int64 = 1<<rsLetterIdxBits - 1
	rsLetterIdxMax        = 63 / rsLetterIdxBits
	randSrc               = rand.NewSource(time.Now().UnixNano())
)

func main() {

	check := map[string]bool{}
	for i := 0; i < 100; i++ {
		gen(check)
	}
}

func gen(check map[string]bool) {
	var mu sync.RWMutex
	var wg sync.WaitGroup
	sem := make(chan struct{}, 4)
	for i := 0; i < 13; i++ {
		wg.Add(1)
		go func(index int) {
			sem <- struct{}{}
			defer wg.Done()
			defer mu.Unlock()
			defer func() { <-sem }()

			now := NowMsec()
			nonce := GenerateNonce(now)
			mu.Lock()
			if check[nonce] {
				fmt.Printf("already create %d %v %v\n", index, now, nonce)
			}
			//	fmt.Printf("%d %v %v\n", index, now, nonce)
			check[nonce] = true

		}(i)
	}
	wg.Wait()

}

func NowMsec() int64 {
	return time.Now().UnixNano() / 1000000
}

func GenerateNonce(timestampMsec int64) string {
	// TODO The same nonce can’t be reused per service-api-key within 20 seconds.
	// An error is returned when the nonce of the successful request is reused within 20 seconds.

	// math/rand
	return randString(8)
	// crypt/rand
	//return crandString(8)

}
func randInt() int64 {
	return Int63()
}

func Uint64() uint64 {
	b := [8]byte{}
	ct, _ := crand.Read(b[:])
	return binary.BigEndian.Uint64(b[:ct])
}

// Int63 method generates a random number in the range [0, 1<<63).
func Int63() int64 {
	return (int64)(Uint64() >> 1)
}

func randString(l int) string {
	rsLetters := "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
	b := make([]byte, l)
	cache, remain := randSrc.Int63(), rsLetterIdxMax
	for i := l - 1; i >= 0; {
		if remain == 0 {
			cache, remain = randSrc.Int63(), rsLetterIdxMax
		}
		idx := int(cache & rsLetterIdxMask)
		if idx < len(rsLetters) {
			b[i] = rsLetters[idx]
			i--
		}
		cache >>= rsLetterIdxBits
		remain--
	}
	return string(b)
}

func crandString(l int) string {
	rsLetters := "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
	b := make([]byte, l)
	cache, remain := randInt(), rsLetterIdxMax
	for i := l - 1; i >= 0; {
		if remain == 0 {
			cache, remain = randInt(), rsLetterIdxMax
		}
		idx := int(cache & rsLetterIdxMask)
		if idx < len(rsLetters) {
			b[i] = rsLetters[idx]
			i--
		}
		cache >>= rsLetterIdxBits
		remain--
	}
	return string(b)
}