util.go

package spotcontrol

import (
	"math/big"
	"strings"
)

const alphabet = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"

func convert62(id string) []byte {
	base := big.NewInt(62)

	n := &big.Int{}
	for _, c := range []byte(id) {
		d := big.NewInt(int64(strings.IndexByte(alphabet, c)))
		n = n.Mul(n, base)
		n = n.Add(n, d)
	}

	return n.Bytes()
}

func reverse(s string) string {
	r := []rune(s)
	for i, j := 0, len(r)-1; i < len(r)/2; i, j = i+1, j-1 {
		r[i], r[j] = r[j], r[i]
	}
	return string(r)
}

func ConvertTo62(raw []byte) string {
	bi := big.Int{}
	bi.SetBytes(raw)
	rem := big.NewInt(0)
	base := big.NewInt(62)
	zero := big.NewInt(0)
	result := ""

	for bi.Cmp(zero) > 0 {
		_, rem = bi.DivMod(&bi, base, rem)
		result += string(alphabet[int(rem.Uint64())])
	}

	for len(result) < 22 {
		result += "0"
	}
	return reverse(result)
}