package generation

import (
	"errors"
)

const alphabet = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"

var (
	base = len(alphabet)
	rev  [256]int
)

func init() {
	for i := range rev {
		rev[i] = -1
	}
	for i, c := range alphabet {
		rev[c] = i
	}
}

// EncodeBase62 encodes a byte slice into a base62 string
func EncodeBase62(b []byte) string {
	var n uint64
	for i := 0; i < len(b); i++ {
		n = n<<8 + uint64(b[i])
	}

	if n == 0 {
		return string(alphabet[0])
	}

	var s []byte
	for n > 0 {
		r := n % uint64(base)
		s = append([]byte{alphabet[r]}, s...)
		n = n / uint64(base)
	}
	return string(s)
}

// DecodeBase62 decodes a base62 string into a byte slice
func DecodeBase62(s string) ([]byte, error) {
	var n uint64
	for i := 0; i < len(s); i++ {
		c := s[i]
		val := rev[c]
		if val == -1 {
			return nil, errors.New("invalid base62 character")
		}
		n = n*uint64(base) + uint64(val)
	}

	// Recover byte slice from integer
	b := make([]byte, rawIDLength)
	for i := rawIDLength - 1; i >= 0; i-- {
		b[i] = byte(n & 0xff)
		n >>= 8
	}
	return b, nil
}