Calculate monthly payments to be done in N months

payment_calculator.rb

#!/usr/bin/env ruby

# This prints a table showing how much a monthly payment
#   would need to be to pay off a loan.  It takes a max and
#   min loan amount, and how much to jump between them.  I 
#   use it to dream about paying off student loans.  It allows
#   me to see what my 1 year pay-off payments are as the loan
#   gets paid back.

class PaymentCalculator
  def initialize args
    monthly_interest = (args[:interest] > 1 ? args[:interest] / 100.0 : args[:interest]) / 12.0
    payment_exponent = -1.0 * args[:number_of_payments]

    @payment_formula = proc{ |principal| principal * (monthly_interest / (1 - (1 + monthly_interest) ** payment_exponent)) }
    @payment_enumerator = args[:max_principal].step(args[:min_principal], args[:step])
  end

  def show_payments
    @payment_enumerator.each do |principal|
      puts "\t%.2f\t%.2f" % [principal, @payment_formula.call(principal)]
    end
  end
end

if __FILE__==$0
  params = {
    max_principal: 100_000,  # most to check
    min_principal: 50_000,   # least to check
    interest:  4.0,          # as 5.0 or .005, annual
    number_of_payments: 60,  # pay of in N months
    step: -1000              # its setup to step from max to min by step
  }

  PaymentCalculator.new(params).show_payments
  
# $ ./loan_repayment.rb 
#        100000.00       1841.65
#        99000.00        1823.24
#        98000.00        1804.82
#        97000.00        1786.40
#        96000.00        1767.99
#        95000.00        1749.57
#        94000.00        1731.15
#        93000.00        1712.74
#        92000.00        1694.32
#        91000.00        1675.90
#        90000.00        1657.49

end