package base58

// 20% of AI generated code
// human made; AI enhanced

import (
	"errors"
	"math"
	"math/big"
)

var (
	ErrInvalidBase58Character = errors.New("invalid character in base58 string")
)

const alphabet = "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz"

var revAlphabet = func() map[byte]int {
	m := make(map[byte]int)
	for i := range len(alphabet) {
		m[alphabet[i]] = i
	}
	return m
}()

var b58 = big.NewInt(58)
var b256 = big.NewInt(256)
var estimate = math.Log(256) / math.Log(58)

func Encode(input []byte) string {
	number := new(big.Int).SetBytes(input)
	if number.Sign() == 0 {
		return string(alphabet[0])
	}

	bufCap := int(float64(len(input))*estimate) + 8
	buf := make([]byte, 0, bufCap)

	mod := new(big.Int)
	for number.Sign() > 0 {
		number.DivMod(number, b58, mod)
		if cap(buf) > len(buf) {
			buf = buf[:len(buf)+1]
			copy(buf[1:], buf[:len(buf)-1+1])
			buf[0] = alphabet[mod.Int64()]
		} else {
			tmp := make([]byte, 1, cap(buf)+1)
			tmp[0] = alphabet[mod.Int64()]
			buf = append(tmp, buf...)
		}
	}

	return string(buf)
}

func Decode(input string) ([]byte, error) {
	number := big.NewInt(0)
	for i := 0; i < len(input); i++ {
		c := input[i]
		val, ok := revAlphabet[c]
		if !ok {
			return nil, ErrInvalidBase58Character
		}
		number.Mul(number, b58)
		number.Add(number, big.NewInt(int64(val)))
	}

	return number.Bytes(), nil
}