Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Calculation/MathTrig.php
public static function FACTDOUBLE($factVal)
    {
        $factLoop = Functions::flattenSingleValue($factVal);

        if (is_numeric($factLoop)) {
            $factLoop = floor($factLoop);
            if ($factVal < 0) {
                return Functions::NAN();
            }
            $factorial = 1;
            while ($factLoop > 1) {
                $factorial *= $factLoop--;
                --$factLoop;
            }

            return $factorial;
        }

        return Functions::VALUE();
    }
public static function FACTDOUBLE($factVal)
    {
        $factLoop = Functions::flattenSingleValue($factVal);

        if (is_numeric($factLoop)) {
            $factLoop = floor($factLoop);
            if ($factVal < 0) {
                return Functions::NAN();
            }
            $factorial = 1;
            while ($factLoop > 1) {
                $factorial *= $factLoop--;
                --$factLoop;
            }

            return $factorial;
        }

        return Functions::VALUE();
    }
FACTDOUBLE.

Returns the double factorial of a number.

Excel Function:
FACTDOUBLE(factVal)

@category Mathematical and Trigonometric Functions

@param float $factVal Factorial Value

@return int Double Factorial