package tool

import (
	"crypto/rand"
	"math/big"
)

const (
	letters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
	digits  = "0123456789"
)

// RandString 生成指定长度的随机字母数字串
func RandString(n int) string {
	return randFrom(letters, n)
}

// RandDigit 生成指定长度的随机数字串（短信验证码等）
func RandDigit(n int) string {
	return randFrom(digits, n)
}

func randFrom(charset string, n int) string {
	b := make([]byte, n)
	max := big.NewInt(int64(len(charset)))
	for i := range b {
		idx, err := rand.Int(rand.Reader, max)
		if err != nil {
			b[i] = charset[0]
			continue
		}
		b[i] = charset[idx.Int64()]
	}
	return string(b)
}