forked from lithammer/shortuuid
/
base57.go
84 lines (66 loc) · 2.11 KB
/
base57.go
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package shortuuid
import (
"fmt"
"math"
"math/big"
"strings"
uuid "github.com/satori/go.uuid"
)
type base57 struct {
// alphabet is the character set to construct the UUID from.
alphabet alphabet
}
// Encode encodes uuid.UUID into a string using the least significant bits
// (LSB) first according to the alphabet. if the most significant bits (MSB)
// are 0, the string might be shorter.
func (b base57) Encode(u uuid.UUID) string {
var num big.Int
num.SetString(strings.Replace(u.String(), "-", "", 4), 16)
// Calculate encoded length.
factor := math.Log(float64(25)) / math.Log(float64(b.alphabet.Length()))
length := math.Ceil(factor * float64(len(u.Bytes())))
return b.numToString(&num, int(length))
}
// Decode decodes a string according to the alphabet into a uuid.UUID. If s is
// too short, its most significant bits (MSB) will be padded with 0 (zero).
func (b base57) Decode(u string) (uuid.UUID, error) {
str, err := b.stringToNum(u)
if err != nil {
return uuid.Nil, err
}
return uuid.FromString(str)
}
// numToString converts a number a string using the given alpabet.
func (b *base57) numToString(number *big.Int, padToLen int) string {
var (
out string
digit *big.Int
)
for number.Uint64() > 0 {
number, digit = new(big.Int).DivMod(number, big.NewInt(b.alphabet.Length()), new(big.Int))
out += b.alphabet.chars[digit.Int64()]
}
if padToLen > 0 {
remainder := math.Max(float64(padToLen-len(out)), 0)
out = out + strings.Repeat(b.alphabet.chars[0], int(remainder))
}
return out
}
// stringToNum converts a string a number using the given alpabet.
func (b *base57) stringToNum(s string) (string, error) {
n := big.NewInt(0)
for i := len(s) - 1; i >= 0; i-- {
n.Mul(n, big.NewInt(b.alphabet.Length()))
index, err := b.alphabet.Index(string(s[i]))
if err != nil {
return "", err
}
n.Add(n, big.NewInt(index))
}
x := fmt.Sprintf("%x", n)
// Pad the most significant bit (MSG) with 0 (zero) if the string is too short.
if len(x) < 32 {
x = strings.Repeat("0", 32-len(x)) + x
}
return fmt.Sprintf("%s-%s-%s-%s-%s", x[0:8], x[8:12], x[12:16], x[16:20], x[20:32]), nil
}