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base58.go
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base58.go
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package traddr
import (
"crypto/sha256"
"math/big"
)
const (
// alphabet is the modified base58 alphabet used by Bitcoin.
alphabet = "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz"
alphabetIdx0 = '1'
)
var b58 = [256]byte{
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 0, 1, 2, 3, 4, 5, 6,
7, 8, 255, 255, 255, 255, 255, 255,
255, 9, 10, 11, 12, 13, 14, 15,
16, 255, 17, 18, 19, 20, 21, 255,
22, 23, 24, 25, 26, 27, 28, 29,
30, 31, 32, 255, 255, 255, 255, 255,
255, 33, 34, 35, 36, 37, 38, 39,
40, 41, 42, 43, 255, 44, 45, 46,
47, 48, 49, 50, 51, 52, 53, 54,
55, 56, 57, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
255, 255, 255, 255, 255, 255, 255, 255,
}
var bigRadix = [...]*big.Int{
big.NewInt(0),
big.NewInt(58),
big.NewInt(58 * 58),
big.NewInt(58 * 58 * 58),
big.NewInt(58 * 58 * 58 * 58),
big.NewInt(58 * 58 * 58 * 58 * 58),
big.NewInt(58 * 58 * 58 * 58 * 58 * 58),
big.NewInt(58 * 58 * 58 * 58 * 58 * 58 * 58),
big.NewInt(58 * 58 * 58 * 58 * 58 * 58 * 58 * 58),
big.NewInt(58 * 58 * 58 * 58 * 58 * 58 * 58 * 58 * 58),
bigRadix10,
}
var bigRadix10 = big.NewInt(58 * 58 * 58 * 58 * 58 * 58 * 58 * 58 * 58 * 58) // 58^10
// checkDecode decodes a string that was encoded with CheckEncode and verifies the checksum.
func checkDecode(input string) (result []byte, err error) {
decoded := decode(input)
if len(decoded) < 5 {
return nil, ErrInvalidFormat
}
var cksum [4]byte
copy(cksum[:], decoded[len(decoded)-4:])
if checksum(decoded[:len(decoded)-4]) != cksum {
return nil, ErrChecksum
}
payload := decoded[0 : len(decoded)-4]
result = append(result, payload...)
return
}
// decode decodes a modified base58 string to a byte slice.
func decode(b string) []byte {
answer := big.NewInt(0)
scratch := new(big.Int)
// Calculating with big.Int is slow for each iteration.
// x += b58[b[i]] * j
// j *= 58
//
// Instead we can try to do as much calculations on int64.
// We can represent a 10 digit base58 number using an int64.
//
// Hence we'll try to convert 10, base58 digits at a time.
// The rough idea is to calculate `t`, such that:
//
// t := b58[b[i+9]] * 58^9 ... + b58[b[i+1]] * 58^1 + b58[b[i]] * 58^0
// x *= 58^10
// x += t
//
// Of course, in addition, we'll need to handle boundary condition when `b` is not multiple of 58^10.
// In that case we'll use the bigRadix[n] lookup for the appropriate power.
for t := b; len(t) > 0; {
n := len(t)
if n > 10 {
n = 10
}
total := uint64(0)
for _, v := range t[:n] {
if v > 255 {
return []byte("")
}
tmp := b58[v]
if tmp == 255 {
return []byte("")
}
total = total*58 + uint64(tmp)
}
answer.Mul(answer, bigRadix[n])
scratch.SetUint64(total)
answer.Add(answer, scratch)
t = t[n:]
}
tmpval := answer.Bytes()
var numZeros int
for numZeros = 0; numZeros < len(b); numZeros++ {
if b[numZeros] != alphabetIdx0 {
break
}
}
flen := numZeros + len(tmpval)
val := make([]byte, flen)
copy(val[numZeros:], tmpval)
return val
}
// checkEncode prepends a version byte and appends a four byte checksum.
func checkEncode(input []byte) string {
b := make([]byte, 0, len(input)+4)
b = append(b, input...)
cksum := checksum(b)
b = append(b, cksum[:]...)
return encode(b)
}
// encode encodes a byte slice to a modified base58 string.
func encode(b []byte) string {
x := new(big.Int)
x.SetBytes(b)
// maximum length of output is log58(2^(8*len(b))) == len(b) * 8 / log(58)
maxlen := int(float64(len(b))*1.365658237309761) + 1
answer := make([]byte, 0, maxlen)
mod := new(big.Int)
for x.Sign() > 0 {
// Calculating with big.Int is slow for each iteration.
// x, mod = x / 58, x % 58
//
// Instead we can try to do as much calculations on int64.
// x, mod = x / 58^10, x % 58^10
//
// Which will give us mod, which is 10 digit base58 number.
// We'll loop that 10 times to convert to the answer.
x.DivMod(x, bigRadix10, mod)
if x.Sign() == 0 {
// When x = 0, we need to ensure we don't add any extra zeros.
m := mod.Int64()
for m > 0 {
answer = append(answer, alphabet[m%58])
m /= 58
}
} else {
m := mod.Int64()
for i := 0; i < 10; i++ {
answer = append(answer, alphabet[m%58])
m /= 58
}
}
}
// leading zero bytes
for _, i := range b {
if i != 0 {
break
}
answer = append(answer, alphabetIdx0)
}
// reverse
alen := len(answer)
for i := 0; i < alen/2; i++ {
answer[i], answer[alen-1-i] = answer[alen-1-i], answer[i]
}
return string(answer)
}
// checksum: first four bytes of sha256^2
func checksum(input []byte) (cksum [4]byte) {
h := sha256.Sum256(input)
h2 := sha256.Sum256(h[:])
copy(cksum[:], h2[:4])
return
}