lib/exonio/financial.rb in exonio-0.5.3 vs lib/exonio/financial.rb in exonio-0.5.4

@@ -48,21 +48,23 @@
 
     # 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
-    # @param pmt [Float] The number of payments to be made
-    # @param pv [Float] The present value
+    # @param pmt [Float] The payment amount made each period
+    # @param pv [Float] The present value of the payments
     # @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.nper(0.07 / 12, -150, 8000) # ==> 64.07334877066185
     #
     def nper(rate, pmt, pv, fv = 0, end_or_beginning = 0)
+      return (-pv - fv) / pmt if rate.zero?
+
       z = pmt * (1 + rate * end_or_beginning) / rate
       temp = Math.log((-fv + z) / (pv + z))
 
       temp / Math.log(1 + rate)
     end