package back

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

var chars = []byte("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ")

func SecureGenString(length int) (string, error) {
	charsLength := big.NewInt(int64(len(chars)))
	var maxN big.Int
	maxN.Exp(charsLength, big.NewInt(int64(length)), nil)
	n, err := rand.Int(rand.Reader, &maxN)
	if err != nil {
		return "", err
	}

	var buf bytes.Buffer
	for n.Cmp(&big.Int{}) == 1 {
		var charIdx big.Int
		n.DivMod(n, charsLength, &charIdx)
		_ = buf.WriteByte(chars[charIdx.Int64()])
	}
	return buf.String(), nil
}