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lib/PhpSpreadsheet/Calculation/Financial/InterestRate.php

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+<?php
+
+namespace PhpOffice\PhpSpreadsheet\Calculation\Financial;
+
+use PhpOffice\PhpSpreadsheet\Calculation\Exception;
+use PhpOffice\PhpSpreadsheet\Calculation\Functions;
+use PhpOffice\PhpSpreadsheet\Calculation\Information\ExcelError;
+
+class InterestRate
+{
+    /**
+     * EFFECT.
+     *
+     * Returns the effective interest rate given the nominal rate and the number of
+     *        compounding payments per year.
+     *
+     * Excel Function:
+     *        EFFECT(nominal_rate,npery)
+     *
+     * @param mixed $nominalRate Nominal interest rate as a float
+     * @param mixed $periodsPerYear Integer number of compounding payments per year
+     */
+    public static function effective(mixed $nominalRate = 0, mixed $periodsPerYear = 0): string|float
+    {
+        $nominalRate = Functions::flattenSingleValue($nominalRate);
+        $periodsPerYear = Functions::flattenSingleValue($periodsPerYear);
+
+        try {
+            $nominalRate = FinancialValidations::validateFloat($nominalRate);
+            $periodsPerYear = FinancialValidations::validateInt($periodsPerYear);
+        } catch (Exception $e) {
+            return $e->getMessage();
+        }
+
+        if ($nominalRate <= 0 || $periodsPerYear < 1) {
+            return ExcelError::NAN();
+        }
+
+        return ((1 + $nominalRate / $periodsPerYear) ** $periodsPerYear) - 1;
+    }
+
+    /**
+     * NOMINAL.
+     *
+     * Returns the nominal interest rate given the effective rate and the number of compounding payments per year.
+     *
+     * @param mixed $effectiveRate Effective interest rate as a float
+     * @param mixed $periodsPerYear Integer number of compounding payments per year
+     *
+     * @return float|string Result, or a string containing an error
+     */
+    public static function nominal(mixed $effectiveRate = 0, mixed $periodsPerYear = 0): string|float
+    {
+        $effectiveRate = Functions::flattenSingleValue($effectiveRate);
+        $periodsPerYear = Functions::flattenSingleValue($periodsPerYear);
+
+        try {
+            $effectiveRate = FinancialValidations::validateFloat($effectiveRate);
+            $periodsPerYear = FinancialValidations::validateInt($periodsPerYear);
+        } catch (Exception $e) {
+            return $e->getMessage();
+        }
+
+        if ($effectiveRate <= 0 || $periodsPerYear < 1) {
+            return ExcelError::NAN();
+        }
+
+        // Calculate
+        return $periodsPerYear * (($effectiveRate + 1) ** (1 / $periodsPerYear) - 1);
+    }
+}