//++++++++++++++++++++++++++++++++++++++++
// 《Go语言高级开发与实战》源码
//++++++++++++++++++++++++++++++++++++++++
// Author:廖显东（ShirDon）
// Blog:https://www.shirdon.com/
// 知乎：https://www.zhihu.com/people/shirdonl
// 公众号:源码大数据
// 仓库地址：https://gitee.com/shirdonl/goAdvanced
// 仓库地址：https://github.com/shirdonl/goAdvanced
//++++++++++++++++++++++++++++++++++++++++

package main

import (
	"crypto/rand"
	"fmt"
)

const StdLen = 16

var StdChars = []byte("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789")

func main() {
	fmt.Printf("%s\n", New())
	fmt.Printf("%s\n", New())

	fmt.Printf("%s\n", NewLen(6))
	fmt.Printf("%s\n", NewLen(6))
}

func New() string {
	return NewLenChars(StdLen, StdChars)
}

func NewLen(length int) string {
	return NewLenChars(length, StdChars)
}

//按照指定长度返回随机字符串
func NewLenChars(length int, chars []byte) string {
	if length == 0 {
		return ""
	}
	clen := len(chars)
	if clen < 2 || clen > 256 {
		panic("Wrong charset length for NewLenChars()")
	}
	maxrb := 255 - (256 % clen)
	b := make([]byte, length)
	r := make([]byte, length+(length/4)) //存储随机字节
	i := 0
	for {
		if _, err := rand.Read(r); err != nil {
			panic("Error reading random bytes: " + err.Error())
		}
		for _, rb := range r {
			c := int(rb)
			if c > maxrb {
				continue // 跳过此数字以避免偏差
			}
			b[i] = chars[c%clen]
			i++
			if i == length {
				return string(b)
			}
		}
	}
}