exercism/go/trinary/trinary.go

package trinary

import (
	"errors"
	"math/big"
)

// ParseTrinary takes a trinary string and returns the
// decimal value
func ParseTrinary(in string) (int64, error) {
	return convertNumStringBase(in, int64(3))
}

// convertNumStringBase takes a numeric string and the base of that string
// and returns an int64 of the decimal representation
func convertNumStringBase(in string, oldBase int64) (int64, error) {
	base := big.NewInt(oldBase)
	var sum big.Int
	for i := range in {
		x := big.NewInt(int64(in[i] - '0'))
		pow := big.NewInt(int64(len(in) - i - 1))

		var n big.Int
		n.Exp(base, pow, nil)
		n.Mul(x, &n)

		sum.Add(&sum, &n)
		if sum.Int64() < 0 {
			return 0, errors.New("Integer Overflow")
		}
	}
	return sum.Int64(), nil
}