loan_schedule.ex

defmodule LoanSchedule do
  def perform(apr, monthly_payment, balance) do
    schedule = calculate(apr, monthly_payment, balance, 0, %{})
    total_payments = Map.keys(schedule) |> Enum.max
    total_paid = Map.values(schedule) |> Enum.reduce(0, fn(loan, acc) -> loan[:monthly_payment] + acc end)
    [
      payment_schedule: schedule,
      loan_details: %{
        apr: apr,
        monthly_payment: monthly_payment,
        initial_balance: balance,
        total_payments: total_payments,
        total_paid: total_paid
      }
    ]
  end

  def calculate(apr, monthly_payment, balance, payments, schedule_map) when balance < monthly_payment do
    IO.puts "We are there! Last payment of: #{balance}"
    IO.puts "We've made #{payments + 1} payments"
    monthly_interest_payment = ((apr / 12) * balance)
    current_payment_period = payments + 1
    final_payment = balance + monthly_interest_payment
    total_paid = (payments * monthly_payment) + final_payment

    current_period = [
      current_balance: 0,
      total_paid: total_paid,
      principal_payment: balance,
      interest_payment: monthly_interest_payment,
      current_payment_period: current_payment_period,
      monthly_payment: final_payment
    ]
    Map.put(schedule_map, current_payment_period, current_period)
  end

  def calculate(apr, monthly_payment, balance, payments, schedule_map) do
    %{interest: interest_payment, principal: principal_payment} = current_payments(apr, balance, monthly_payment)

    if principal_payment < 0 do
      raise "You can't pay this loan. Your minimum payment must be higher than #{interest_payment}"
    end

    if principal_payment == 0 do
      raise "You are paying just below the minimum payment"
    end

    current_payment_period = payments + 1
    next_month_balance = balance - principal_payment
    total_paid = current_payment_period * monthly_payment

    current_period = [
      apr: apr,
      current_balance: next_month_balance,
      total_paid: total_paid,
      principal_payment: principal_payment,
      interest_payment: interest_payment,
      current_payment_period: current_payment_period,
      monthly_payment: monthly_payment
    ]
    new_payment_schedule = Map.put(schedule_map, current_payment_period, current_period)

    calculate(apr, monthly_payment, next_month_balance, current_payment_period, new_payment_schedule)
  end

  def current_payments(apr, balance, monthly_payment) do
    monthly_interest_payment = ((apr / 12) * balance)
    monthly_principal_payment = monthly_payment - monthly_interest_payment

    %{
      interest: monthly_interest_payment,
      principal: monthly_principal_payment
    }
  end
end

An example use case, with a $10,000 loan, monthly payments of $1,000 and 5% APR:

iex(1)> LoanSchedule.perform(0.05, 1000, 10_000)

We are there! Last payment of: 234.96800783042113
We've made 11 payments
[payment_schedule: %{1 => [apr: 0.05, current_balance: 9041.666666666666,
    total_paid: 1000, principal_payment: 958.3333333333334,
    interest_payment: 41.666666666666664, current_payment_period: 1,
    monthly_payment: 1000],
   2 => [apr: 0.05, current_balance: 8079.340277777777, total_paid: 2000,
    principal_payment: 962.3263888888889, interest_payment: 37.67361111111111,
    current_payment_period: 2, monthly_payment: 1000],
   3 => [apr: 0.05, current_balance: 7113.004195601851, total_paid: 3000,
    principal_payment: 966.336082175926, interest_payment: 33.66391782407407,
    current_payment_period: 3, monthly_payment: 1000],
   4 => [apr: 0.05, current_balance: 6142.641713083525, total_paid: 4000,
    principal_payment: 970.3624825183256, interest_payment: 29.63751748167438,
    current_payment_period: 4, monthly_payment: 1000],
   5 => [apr: 0.05, current_balance: 5168.236053554707, total_paid: 5000,
    principal_payment: 974.4056595288187, interest_payment: 25.594340471181354,
    current_payment_period: 5, monthly_payment: 1000],
   6 => [apr: 0.05, current_balance: 4189.770370444518, total_paid: 6000,
    principal_payment: 978.4656831101887, interest_payment: 21.53431688981128,
    current_payment_period: 6, monthly_payment: 1000],
   7 => [apr: 0.05, current_balance: 3207.2277469880373, total_paid: 7000,
    principal_payment: 982.5426234564811, interest_payment: 17.457376543518826,
    current_payment_period: 7, monthly_payment: 1000],
   8 => [apr: 0.05, current_balance: 2220.591195933821, total_paid: 8000,
    principal_payment: 986.6365510542165, interest_payment: 13.363448945783489,
    current_payment_period: 8, monthly_payment: 1000],
   9 => [apr: 0.05, current_balance: 1229.8436592502119, total_paid: 9000,
    principal_payment: 990.747536683609, interest_payment: 9.252463316390921,
    current_payment_period: 9, monthly_payment: 1000],
   10 => [apr: 0.05, current_balance: 234.96800783042113, total_paid: 10000,
    principal_payment: 994.8756514197908, interest_payment: 5.124348580209216,
    current_payment_period: 10, monthly_payment: 1000],
   11 => [current_balance: 0, total_paid: 10235.947041196381,
    principal_payment: 234.96800783042113, interest_payment: 0.979033365960088,
    current_payment_period: 11, monthly_payment: 235.9470411963812]},
 loan_details: %{apr: 0.05, initial_balance: 10000, monthly_payment: 1000,
   total_paid: 10235.947041196381, total_payments: 11}]

Here, the loan_details provide an actual total_paid value of how much that loan actually costs over time.

The payment_schedule map describes each payment record that is made each month, as well as how much each payment contributed to interest vs principal on the loan.