package security

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

// RandomString generates a random string with the specified length.
//
// The generated string is cryptographically random and matches
// [A-Za-z0-9]+ (aka. it's transparent to URL-encoding).
func RandomString(length int) string {
	const alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789"

	return RandomStringWithAlphabet(length, alphabet)
}

// RandomStringWithAlphabet generates a cryptographically random string
// with the specified length and characters set.
//
// It panics if for some reason rand.Int returns a non-nil error.
func RandomStringWithAlphabet(length int, alphabet string) string {
	b := make([]byte, length)
	max := big.NewInt(int64(len(alphabet)))

	for i := range b {
		n, err := rand.Int(rand.Reader, max)
		if err != nil {
			panic(err)
		}
		b[i] = alphabet[n.Int64()]
	}

	return string(b)
}