# tablepress/3.4/libraries/vendor/PhpSpreadsheet/Calculation/Engineering/BesselK.php

TablePress – Tables in WordPress made easy, version 3.4. 134 lines.

- Page: https://pluginprobe.com/plugins/tablepress/3.4/code/libraries/vendor/PhpSpreadsheet/Calculation/Engineering/BesselK.php
- Raw: https://pluginprobe.com/plugins/tablepress/3.4/raw/libraries/vendor/PhpSpreadsheet/Calculation/Engineering/BesselK.php
- Modified: 2025-08-26T04:06:14+00:00

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```php
<?php

namespace TablePress\PhpOffice\PhpSpreadsheet\Calculation\Engineering;

use TablePress\PhpOffice\PhpSpreadsheet\Calculation\ArrayEnabled;
use TablePress\PhpOffice\PhpSpreadsheet\Calculation\Exception;
use TablePress\PhpOffice\PhpSpreadsheet\Calculation\Information\ExcelError;

class BesselK
{
	use ArrayEnabled;

	/**
	 * BESSELK.
	 *
	 *    Returns the modified Bessel function Kn(x), which is equivalent to the Bessel functions evaluated
	 *        for purely imaginary arguments.
	 *
	 *    Excel Function:
	 *        BESSELK(x,ord)
	 *
	 * @param mixed $x A float value at which to evaluate the function.
	 *                                If x is nonnumeric, BESSELK returns the #VALUE! error value.
	 *                      Or can be an array of values
	 * @param mixed $ord The integer order of the Bessel function.
	 *                       If ord is not an integer, it is truncated.
	 *                                If $ord is nonnumeric, BESSELK returns the #VALUE! error value.
	 *                       If $ord < 0, BESSELKI returns the #NUM! error value.
	 *                      Or can be an array of values
	 *
	 * @return array<mixed>|float|string Result, or a string containing an error
	 *         If an array of numbers is passed as an argument, then the returned result will also be an array
	 *            with the same dimensions
	 */
	public static function BESSELK($x, $ord)
	{
		if (is_array($x) || is_array($ord)) {
			return self::evaluateArrayArguments([self::class, __FUNCTION__], $x, $ord);
		}

		try {
			$x = EngineeringValidations::validateFloat($x);
			$ord = EngineeringValidations::validateInt($ord);
		} catch (Exception $e) {
			return $e->getMessage();
		}

		if (($ord < 0) || ($x <= 0.0)) {
			return ExcelError::NAN();
		}

		$fBk = self::calculate($x, $ord);

		return (is_nan($fBk)) ? ExcelError::NAN() : $fBk;
	}

	private static function calculate(float $x, int $ord): float
	{
		switch ($ord) {
									case 0:
										return self::besselK0($x);
									case 1:
										return self::besselK1($x);
									default:
										return self::besselK2($x, $ord);
								}
	}

	/**
	 * Mollify Phpstan.
	 *
	 * @codeCoverageIgnore
	 */
	private static function callBesselI(float $x, int $ord): float
	{
		$rslt = BesselI::BESSELI($x, $ord);
		if (!is_float($rslt)) {
			throw new Exception('Unexpected array or string');
		}

		return $rslt;
	}

	private static function besselK0(float $x): float
	{
		if ($x <= 2) {
			$fNum2 = $x * 0.5;
			$y = ($fNum2 * $fNum2);

			return -log($fNum2) * self::callBesselI($x, 0)
				+ (-0.57721566 + $y * (0.42278420 + $y * (0.23069756 + $y * (0.3488590e-1 + $y * (0.262698e-2 + $y
									* (0.10750e-3 + $y * 0.74e-5))))));
		}

		$y = 2 / $x;

		return exp(-$x) / sqrt($x)
			* (1.25331414 + $y * (-0.7832358e-1 + $y * (0.2189568e-1 + $y * (-0.1062446e-1 + $y
							* (0.587872e-2 + $y * (-0.251540e-2 + $y * 0.53208e-3))))));
	}

	private static function besselK1(float $x): float
	{
		if ($x <= 2) {
			$fNum2 = $x * 0.5;
			$y = ($fNum2 * $fNum2);

			return log($fNum2) * self::callBesselI($x, 1)
				+ (1 + $y * (0.15443144 + $y * (-0.67278579 + $y * (-0.18156897 + $y * (-0.1919402e-1 + $y
									* (-0.110404e-2 + $y * (-0.4686e-4))))))) / $x;
		}

		$y = 2 / $x;

		return exp(-$x) / sqrt($x)
			* (1.25331414 + $y * (0.23498619 + $y * (-0.3655620e-1 + $y * (0.1504268e-1 + $y * (-0.780353e-2 + $y
								* (0.325614e-2 + $y * (-0.68245e-3)))))));
	}

	private static function besselK2(float $x, int $ord): float
	{
		$fTox = 2 / $x;
		$fBkm = self::besselK0($x);
		$fBk = self::besselK1($x);
		for ($n = 1; $n < $ord; ++$n) {
			$fBkp = $fBkm + $n * $fTox * $fBk;
			$fBkm = $fBk;
			$fBk = $fBkp;
		}

		return $fBk;
	}
}

```