interests.ergo

/*
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

namespace org.accordproject.loan.interests

import org.accordproject.time.*
import org.accordproject.money.*

define function round(x:Double) {
    return floor(x + 0.5)
}
define function roundn(x:Double, n:Integer) {
    let nd = integerToDouble(n);
    let e = 10.0 ^ nd;
    return round(x * e) / e
}

// Formula taken from https://en.wikipedia.org/wiki/Mortgage_calculator#Monthly_payment_formula
define function monthlyPaymentFormula(loanAmount: MonetaryAmount, rate: Double, loanDuration: Integer) : MonetaryAmount {
	let currency = loanAmount.currencyCode;
	let amount = loanAmount.doubleValue;
  let term = longToDouble(loanDuration * 12);
  if (rate = 0.0) then return MonetaryAmount{ doubleValue: (amount / term), currencyCode: currency }
  else
    let monthlyRate = (rate / 12.0) / 100.0;
    let monthlyPayment =
      (monthlyRate * amount)
      / (1.0 - ((1.0 + monthlyRate) ^ (-term)));
    return MonetaryAmount{ doubleValue: roundn(monthlyPayment, 0), currencyCode: currency }
}

define function cummulativeInterests(loanAmount: MonetaryAmount, rate: Double, loanDuration: Integer) : MonetaryAmount {
  let term = longToDouble(loanDuration * 12);
  let monthly = monthlyPaymentFormula(loanAmount, rate, loanDuration);
  return MonetaryAmount{
    doubleValue: term * (monthly.doubleValue - loanAmount.doubleValue),
    currencyCode: monthly.currencyCode
  }
}

// Try it also on NYT calculator: https://www.nytimes.com/real-estate/mortgage-calculator
// test: return monthlyPaymentFormula(100000.00, 2.5, 15)
// result: 667.0
// test (: return monthlyPaymentFormula(200000.00, 6.5, 30)
// result: 1264.0