rand.go

package utils

import (
	cryptoRand "crypto/rand"
	"encoding/base64"
	mathRand "math/rand"
	"strings"
	"time"
)

const (
	letterBytes   = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
	letterIdxBits = 6
	letterIdxMask = 1<<letterIdxBits - 1
	letterIdxMax  = 63 / letterIdxBits
)

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

// GenerateRandomBytes adapted from https://elithrar.github.io/article/generating-secure-random-numbers-crypto-rand/
// GenerateRandomBytes returns securely generated random bytes.
// It will return an error if the system's secure random
// number generator fails to function correctly, in which
// case the caller should not continue.
func GenerateRandomBytes(n int) ([]byte, error) {
	b := make([]byte, n)
	_, err := cryptoRand.Read(b)
	// Note that err == nil only if we read len(b) bytes.
	if err != nil {
		return nil, err
	}

	return b, nil
}

// GenerateRandomString returns a securely generated random string.
// It will return an error if the system's secure random
// number generator fails to function correctly, in which
// case the caller should not continue.
func GenerateRandomString(n int) (string, error) {
	const letters = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz-"
	bytes, err := GenerateRandomBytes(n)
	if err != nil {
		return "", err
	}
	for i, b := range bytes {
		bytes[i] = letters[b%byte(len(letters))]
	}
	return string(bytes), nil
}

// GenerateRandomStringURLSafe returns a URL-safe, base64 encoded
// securely generated random string.
// It will return an error if the system's secure random
// number generator fails to function correctly, in which
// case the caller should not continue.
func GenerateRandomStringURLSafe(n int) (string, error) {
	b, err := GenerateRandomBytes(n)
	return base64.URLEncoding.EncodeToString(b), err
}

// GenerateRandomAlphanumeric Generate random alphanumeric character adapted from
// https://stackoverflow.com/questions/22892120/how-to-generate-a-random-string-of-a-fixed-length-in-go
func GenerateRandomAlphanumeric(n int) string {
	sb := strings.Builder{}
	sb.Grow(n)
	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) {
			sb.WriteByte(letterBytes[idx])
			i--
		}
		cache >>= letterIdxBits
		remain--
	}

	return sb.String()
}