lib/exonio/financial.rb in exonio-0.2.0 vs lib/exonio/financial.rb in exonio-0.3.0

@@ -20,9 +20,32 @@
       fact = (1 + rate* end_or_beginning) * (temp - 1) / rate
 
       -(pv * temp + pmt * fact)
     end
 
+    # Calculates the payment on interest for an investment based on
+    # constant-amount periodic payments and a constant interest rate.
+    #
+    # @param rate [Float] The interest rate as decimal (not per cent) per period
+    # @param per [Integer] The amortization period, in terms of number of periods
+    # @param nper [Integer] The number of payments to be made
+    # @param pv [Float] The present value
+    # @param fv [Float] The future value remaining after the final payment has been made
+    # @param end_or_begining [Integer] Whether payments are due at the end (0) or
+    #   beggining (1) of each period
+    #
+    # @return [Float]
+    #
+    # @example
+    #   Exonio.ipmt(0.075 / 12, 8, 12 * 2, 5000) # ==> -22.612926783996798
+    #
+    def ipmt(rate, per, nper, pv, fv = 0, end_or_beginning = 0)
+      pmt = self.pmt(rate, nper, pv, fv, end_or_beginning)
+      fv = self.fv(rate, (per - 1), pmt, pv, end_or_beginning) * rate
+      temp = end_or_beginning == 1 ? fv / (1 + rate) : fv
+
+      (per == 1 && end_or_beginning == 1) ? 0.0 : temp
+    end
 
     # Calculates the number of payment periods for an investment based on
     # constant-amount periodic payments and a constant interest rate.
     #
     # @param rate [Float] The interest rate as decimal (not per cent) per period