Exponential.php

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<?php

namespace PhpOffice\PhpSpreadsheet\Calculation\Statistical\Distributions;

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

class Exponential
{
    public static function distribution($value, $lambda, $cumulative)
    {
        $value = Functions::flattenSingleValue($value);
        $lambda = Functions::flattenSingleValue($lambda);
        $cumulative = Functions::flattenSingleValue($cumulative);

        try {
            $value = DistributionValidations::validateFloat($value);
            $lambda = DistributionValidations::validateFloat($lambda);
            $cumulative = DistributionValidations::validateBool($cumulative);
        } catch (Exception $e) {
            return $e->getMessage();
        }

        if (($value < 0) || ($lambda < 0)) {
            return Functions::NAN();
        }

        if ($cumulative === true) {
            return 1 - exp(0 - $value * $lambda);
        }

        return $lambda * exp(0 - $value * $lambda);
    }
}