<?php
namespace PhpOffice\PhpSpreadsheet\Calculation\Statistical\Distributions;
use PhpOffice\PhpSpreadsheet\Calculation\Exception;
use PhpOffice\PhpSpreadsheet\Calculation\Functions;
use PhpOffice\PhpSpreadsheet\Calculation\MathTrig;
class Poisson
{
/**
* POISSON.
*
* Returns the Poisson distribution. A common application of the Poisson distribution
* is predicting the number of events over a specific time, such as the number of
* cars arriving at a toll plaza in 1 minute.
*
* @param mixed $value Float value for which we want the probability
* @param mixed $mean Mean value as a float
* @param mixed $cumulative Boolean value indicating if we want the cdf (true) or the pdf (false)
*
* @return float|string The result, or a string containing an error
*/
public static function distribution($value, $mean, $cumulative)
{
$value = Functions::flattenSingleValue($value);
$mean = Functions::flattenSingleValue($mean);
try {
$value = DistributionValidations::validateFloat($value);
$mean = DistributionValidations::validateFloat($mean);
$cumulative = DistributionValidations::validateBool($cumulative);
} catch (Exception $e) {
return $e->getMessage();
}
if (($value < 0) || ($mean < 0)) {
return Functions::NAN();
}
if ($cumulative) {
$summer = 0;
$floor = floor($value);
for ($i = 0; $i <= $floor; ++$i) {
$summer += $mean ** $i / MathTrig\Factorial::fact($i);
}
return exp(0 - $mean) * $summer;
}
return (exp(0 - $mean) * $mean ** $value) / MathTrig\Factorial::fact($value);
}
}