feature #818 Implement loan calculator.

loan/src/main/java/jp/toastkid/loan/Calculator.kt

@@ -0,0 +1,34 @@
+/*
+ * Copyright (c) 2021 toastkidjp.
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License v1.0
+ * which accompany this distribution.
+ * The Eclipse Public License is available at http://www.eclipse.org/legal/epl-v10.html.
+ */
+
+package jp.toastkid.loan
+
+import kotlin.math.max
+import kotlin.math.pow
+
+class Calculator {
+
+    operator fun invoke(
+        amount: Int,
+        term: Int,
+        interestRate: Double,
+        downPayment: Int,
+        managementFee: Int,
+        renovationReserves: Int
+    ): Int {
+        val paymentCount = (term * 12).toDouble()
+        val convertedRate = interestRate / 100.0
+        val poweredMonthlyInterestRate = (1 + convertedRate / 12).pow(paymentCount)
+
+        val numerator = ((amount - downPayment) * convertedRate) / 12 * poweredMonthlyInterestRate
+        val denominator = poweredMonthlyInterestRate - 1
+
+        return (max(numerator / denominator, 0.0)).toInt() + managementFee + renovationReserves
+    }
+
+}