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lib/PhpSpreadsheet/Calculation/Statistical/Distributions/Poisson.php

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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;
+use PhpOffice\PhpSpreadsheet\Calculation\MathTrig;
+
+class Poisson
+{
+    use ArrayEnabled;
+
+    /**
+     * 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
+     *                      Or can be an array of values
+     * @param mixed $mean Mean value as a float
+     *                      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 The result, or a string containing 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(mixed $value, mixed $mean, mixed $cumulative): array|string|float
+    {
+        if (is_array($value) || is_array($mean) || is_array($cumulative)) {
+            return self::evaluateArrayArguments([self::class, __FUNCTION__], $value, $mean, $cumulative);
+        }
+
+        try {
+            $value = DistributionValidations::validateFloat($value);
+            $mean = DistributionValidations::validateFloat($mean);
+            $cumulative = DistributionValidations::validateBool($cumulative);
+        } catch (Exception $e) {
+            return $e->getMessage();
+        }
+
+        if (($value < 0) || ($mean < 0)) {
+            return ExcelError::NAN();
+        }
+
+        if ($cumulative) {
+            $summer = 0;
+            $floor = floor($value);
+            for ($i = 0; $i <= $floor; ++$i) {
+                /** @var float $fact */
+                $fact = MathTrig\Factorial::fact($i);
+                $summer += $mean ** $i / $fact;
+            }
+
+            return exp(0 - $mean) * $summer;
+        }
+        /** @var float $fact */
+        $fact = MathTrig\Factorial::fact($value);
+
+        return (exp(0 - $mean) * $mean ** $value) / $fact;
+    }
+}