Added computation of amortization schedule and some unit tests.

lib/amortization.rb

@@ -1,17 +1,32 @@
+require 'cashflows'
+
 class Amortization
 	attr_accessor :balance
 	attr_accessor :rate
 	attr_accessor :payment
+	attr_accessor :payments
 	attr_accessor :periods
 	attr_accessor :principal
 
 	def compute(balance, rate)
-		@payment = balance * (rate + (rate / ((1 + rate) ** rate.duration - 1)))
+		@payment = Amortization.payment balance, rate.monthly, rate.duration
+
+		rate.duration.times do
+			@payments << @payment
+			@balance += (@balance * rate.monthly.round(6)).round(2) + @payment
+		end
 	end
 
 	def initialize(principal, rate)
 		@principal = principal
 		@balance   = principal
 		@rate      = rate
+		@payments  = []
+
+		compute(@balance, @rate)
+	end
+
+	def Amortization.payment(balance, rate, periods)
+		-(balance * (rate + (rate / ((1 + rate) ** periods - 1)))).round(2)
 	end
 end

lib/rates.rb

@@ -59,4 +59,8 @@ def initialize(opts={})
 		elsif opts.has_key? :nominal: @nominal = decimal(opts[:nominal])
 		end
 	end
+
+	def monthly
+		self.effective / 12
+	end
 end

test/tc_amortization.rb

@@ -0,0 +1,29 @@
+require 'rubygems'
+require 'amortization'
+require 'flt/d'
+require 'rates'
+require 'test/unit'
+
+class TestBasicAmortization < Test::Unit::TestCase
+	def setup
+		@rate = Rate.new :effective => 0.0375
+		@rate.duration = 360
+		@amortization = Amortization.new(D(200000), @rate)
+	end
+
+	def test_balance
+		assert_equal D(0), @amortization.balance
+	end
+
+	def test_payment
+		assert_equal D('-926.23'), @amortization.payment
+	end
+
+	def test_payments
+		assert_equal [ D('-926.23') ] * @rate.duration, @amortization.payments
+	end
+
+	def test_principal
+		assert_equal D(200000), @amortization.principal
+	end
+end