calcs

calcs.rb

  # p = Principle (rrp, amount financed, vehicle cost)
  # i = interest rate (pass in as rate/100/12, so 8.56% = 8.56/100/12 = 0.00713, i = 0.00713
  # t = term
  # r = residual
  # pw = 1 or 0, payment when, 1 = advance, 0 = arrears
  def self.monthly_payment(p,i,t,r,pw)
    #
    # Monthly rate is computed as follows:
    #
    #        p*i*(1+i)**t - i*r
    #   M = --------------------
    #       (1+i)*((1+i)**t - 1)
    #
    # or:
    #
    #   M = A*p + B*r
    #
    # where:
    #
    #   denom = (1+i)*((1+i)**t - 1)
    #   A = i*(1+i)**t / denom
    #   B =         -i / denom
    #
    if pw == 1 # 0 for end of period and 1 for beginning of period
      denom = (1+i)*((1+i)**t - 1)  
    else
      denom = ((1+i)**t - 1)
    end
  
    a = i*(1+i)**t / denom
    b = -i / denom
    BigDecimal.new("#{a*p + b*r}")
  end