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<?php
namespace PhpOffice\PhpSpreadsheet\Calculation\Statistical\Distributions;
use PhpOffice\PhpSpreadsheet\Calculation\ArrayEnabled;
use PhpOffice\PhpSpreadsheet\Calculation\Exception;
use PhpOffice\PhpSpreadsheet\Calculation\Information\ExcelError;
class Weibull
{
use ArrayEnabled;
/**
* WEIBULL.
*
* Returns the Weibull distribution. Use this distribution in reliability
* analysis, such as calculating a device's mean time to failure.
*
* @param mixed $value Float value for the distribution
* Or can be an array of values
* @param mixed $alpha Float alpha Parameter
* Or can be an array of values
* @param mixed $beta Float beta Parameter
* Or can be an array of values
* @param mixed $cumulative Boolean value indicating if we want the cdf (true) or the pdf (false)
* Or can be an array of values
*
* @return array|float|string (string if result is an error)
* If an array of numbers is passed as an argument, then the returned result will also be an array
* with the same dimensions
*/
public static function distribution($value, $alpha, $beta, $cumulative)
{
if (is_array($value) || is_array($alpha) || is_array($beta) || is_array($cumulative)) {
return self::evaluateArrayArguments([self::class, __FUNCTION__], $value, $alpha, $beta, $cumulative);
}
try {
$value = DistributionValidations::validateFloat($value);
$alpha = DistributionValidations::validateFloat($alpha);
$beta = DistributionValidations::validateFloat($beta);
$cumulative = DistributionValidations::validateBool($cumulative);
} catch (Exception $e) {
return $e->getMessage();
}
if (($value < 0) || ($alpha <= 0) || ($beta <= 0)) {
return ExcelError::NAN();
}
if ($cumulative) {
return 1 - exp(0 - ($value / $beta) ** $alpha);
}
return ($alpha / $beta ** $alpha) * $value ** ($alpha - 1) * exp(0 - ($value / $beta) ** $alpha);
}
}
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