Complex number multiply

complex_mul.php

<?php

    public function multiply($num)
    {

        [
            $thatRealPart,
            $thatImaginaryPart,
            $thisRealPart,
            $thisImaginaryPart,
            $num
        ] = $this->translateToParts($this, $num, 1);

        if ($num->isComplex()) {
            $foiled = PolynomialFunction::createFromFoil([
                $thisRealPart,
                $thisImaginaryPart
            ], [
                $thatRealPart,
                $thatImaginaryPart
            ]);

            $parts = $foiled->describeShape();

            $newRealPart = new ImmutableDecimal($parts[0]);
            $newImaginaryPart = new ImmutableDecimal($parts[1]);
            /** @var ImmutableDecimal $newRealPart */
            $newRealPart = $newRealPart->add($parts[2]);
        } else {
            $value1 = $thisRealPart->multiply($num);
            $value2 = $thisImaginaryPart->multiply($num);

            $newRealPart = $value1->isReal() ? $value1 : $value2;
            $newImaginaryPart = $value1->isImaginary() ? $value1 : $value2;
        }

        if (!$newRealPart->isEqual(0) && !$newImaginaryPart->isEqual(0)) {
            return $this->setValue($newRealPart, $newImaginaryPart);
        }

        $value = $newRealPart->isEqual(0) ? $newImaginaryPart : $newRealPart;

        return new ImmutableDecimal($value->getValue());

    }