private function buildConversionTable()
    {
        if ($this->source->getRadix() > $this->target->getRadix()) {
            return $this->createTable($this->source->getDigitList(), $this->target->getDigitList());
        }

        return array_flip($this->createTable($this->target->getDigitList(), $this->source->getDigitList()));
    }