golang密码生成器程序

package main
import (
    "flag"
    "fmt"
    "math/rand"
    "time"
)
var (
    length  int
    charset string
)
const (
    NumStr  = "0123456789"
    CharStr = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
    SpecStr = "+=-@#~,.[]()!%^*$"
)
func parseArgs() {
    flag.IntVar(&length, "l", 16, "-l 生成密码的长度")
    flag.StringVar(&charset, "t", "num",
        `-t 制定密码生成的字符集, 
        num:只使用数字[0-9], 
        char:只使用英文字母[a-zA-Z], 
        mix: 使用数字和字母， 
        advance:使用数字、字母以及特殊字符`)
    flag.Parse()
}
func test1() {
    for i := 0; i < len(CharStr); i++ {
        if CharStr[i] != ' ' {
            fmt.Printf("%c", CharStr[i])
        }
    }
}
func generatePasswd() string {
    var passwd []byte = make([]byte, length, length)
    var sourceStr string
    if charset == "num" {
        sourceStr = NumStr
    } else if charset == "char" {
        sourceStr = CharStr
    } else if charset == "mix" {
        sourceStr = fmt.Sprintf("%s%s", NumStr, CharStr)
    } else if charset == "advance" {
        sourceStr = fmt.Sprintf("%s%s%s", NumStr, CharStr, SpecStr)
    } else {
        sourceStr = NumStr
    }
    //fmt.Println("source:", sourceStr)
    for i := 0; i < length; i++ {
        index := rand.Intn(len(sourceStr))
        passwd[i] = sourceStr[index]
    }
    return string(passwd)
}
func main() {
    rand.Seed(time.Now().UnixNano())
    parseArgs()
    //fmt.Printf("length:%d charset:%s\n", length, charset)
    //test1()
    passwd := generatePasswd()
    fmt.Println(passwd)
}