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lib/PhpSpreadsheet/Calculation/Engineering/BesselK.php

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+<?php
+
+namespace PhpOffice\PhpSpreadsheet\Calculation\Engineering;
+
+use PhpOffice\PhpSpreadsheet\Calculation\ArrayEnabled;
+use PhpOffice\PhpSpreadsheet\Calculation\Exception;
+use 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|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(mixed $x, mixed $ord): array|string|float
+    {
+        if (is_array($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 $ord): float
+    {
+        return match ($ord) {
+            0 => self::besselK0($x),
+            1 => self::besselK1($x),
+            default => 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;
+    }
+}