SeriesSum.php

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<?php

namespace PhpOffice\PhpSpreadsheet\Calculation\MathTrig;

use PhpOffice\PhpSpreadsheet\Calculation\Exception;
use PhpOffice\PhpSpreadsheet\Calculation\Functions;

class SeriesSum
{
    public static function evaluate($x, $n, $m, ...$args)
    {
        try {
            $x = Helpers::validateNumericNullSubstitution($x, 0);
            $n = Helpers::validateNumericNullSubstitution($n, 0);
            $m = Helpers::validateNumericNullSubstitution($m, 0);

            // Loop through arguments
            $aArgs = Functions::flattenArray($args);
            $returnValue = 0;
            $i = 0;
            foreach ($aArgs as $argx) {
                if ($argx !== null) {
                    $arg = Helpers::validateNumericNullSubstitution($argx, 0);
                    $returnValue += $arg * $x ** ($n + ($m * $i));
                    ++$i;
                }
            }
        } catch (Exception $e) {
            return $e->getMessage();
        }

        return $returnValue;
    }
}