diff --git a/.rvmrc b/.rvmrc
index ee3b09c..ca9e207 100644
--- a/.rvmrc
+++ b/.rvmrc
@@ -1,2 +1,3 @@
 rvm_gemset_create_on_use_flag=1
-rvm gemset use monetico 
+rvm gemset use monetico
+rvm use 1.9.3@monetico --create
diff --git a/Gemfile b/Gemfile
index 4a8b71e..b6cc049 100644
--- a/Gemfile
+++ b/Gemfile
@@ -2,3 +2,6 @@ source 'http://rubygems.org'
 
 # Specify your gem's dependencies in monetico.gemspec
 gemspec
+
+group :development, :test do
+end
diff --git a/Rakefile b/Rakefile
index f57ae68..0d17dde 100644
--- a/Rakefile
+++ b/Rakefile
@@ -1,2 +1,38 @@
+# encoding: utf-8
+
 #!/usr/bin/env rake
 require "bundler/gem_tasks"
+
+require 'rubygems'
+require 'bundler'
+begin
+  Bundler.setup(:default, :development)
+rescue Bundler::BundlerError => e
+  $stderr.puts e.message
+  $stderr.puts "Run `bundle install` to install missing gems"
+  exit e.status_code
+end
+require 'rake'
+
+require 'rspec/core'
+require 'rspec/core/rake_task'
+RSpec::Core::RakeTask.new(:spec) do |spec|
+  spec.pattern = FileList['spec/**/*_spec.rb']
+end
+
+RSpec::Core::RakeTask.new(:rcov) do |spec|
+  spec.pattern = 'spec/**/*_spec.rb'
+  spec.rcov = true
+end
+
+task :default => :spec
+
+require 'rake/rdoctask'
+Rake::RDocTask.new do |rdoc|
+  version = File.exist?('VERSION') ? File.read('VERSION') : ""
+
+  rdoc.rdoc_dir = 'rdoc'
+  rdoc.title = "simple_metar_parser #{version}"
+  rdoc.rdoc_files.include('README*')
+  rdoc.rdoc_files.include('lib/**/*.rb')
+end
diff --git a/lib/excel.rb b/lib/excel.rb
new file mode 100644
index 0000000..1bf6758
--- /dev/null
+++ b/lib/excel.rb
@@ -0,0 +1,35 @@
+module Excel
+  def pmt(rate, nper, pv, fv=0, type=0)
+    ((-pv * pvif(rate, nper) - fv ) / ((1.0 + rate * type) * fvifa(rate, nper)))
+  end
+
+  def ipmt(rate, per, nper, pv, fv=0, type=0)
+    p = pmt(rate, nper, pv, fv, 0);
+    ip = -(pv * pow1p(rate, per - 1) * rate + p * pow1pm1(rate, per - 1))
+    (type == 0) ? ip : ip / (1 + rate)
+  end
+
+  def ppmt(rate, per, nper, pv, fv=0, type=0)
+    p = pmt(rate, nper, pv, fv, type)
+    ip = ipmt(rate, per, nper, pv, fv, type)
+    p - ip
+  end
+  
+  protected
+  
+  def pow1pm1(x, y)
+    (x <= -1) ? ((1 + x) ** y) - 1 : Math.exp(y * Math.log(1.0 + x)) - 1
+  end
+
+  def pow1p(x, y)
+    (x.abs > 0.5) ? ((1 + x) ** y) : Math.exp(y * Math.log(1.0 + x))
+  end
+
+  def pvif(rate, nper)
+    pow1p(rate, nper)
+  end
+
+  def fvifa(rate, nper)
+    (rate == 0) ? nper : pow1pm1(rate, nper) / rate
+  end
+end
diff --git a/lib/monetico.rb b/lib/monetico.rb
index 04cbbc5..24e19c5 100644
--- a/lib/monetico.rb
+++ b/lib/monetico.rb
@@ -1,23 +1,22 @@
 require "bigdecimal"
+require "excel"
+require "monetico/money_array"
+require "monetico/calculable"
 require "monetico/loan"
 require "monetico/version"
 
 class Float
-  def big; BigDecimal(self.to_s); end
+  include Monetico::Calculable
+end
 
-  def round_to(x)
-    (self * 10**x).ceil.to_f / 10**x
-  end
+class BigDecimal
+  include Monetico::Calculable
+end
 
-  def round_down(x)
-    if self >= 0 
-      (self * 10**x).floor.to_f / 10**x
-    else
-      -((-self * 10**x).floor.to_f / 10**x)
-    end
-  end
+class Fixnum
+  # usable for #big
+  include Monetico::Calculable
 end
 
 module Monetico
-  # Your code goes here...
 end
diff --git a/lib/monetico/calculable.rb b/lib/monetico/calculable.rb
new file mode 100644
index 0000000..2ae9eb2
--- /dev/null
+++ b/lib/monetico/calculable.rb
@@ -0,0 +1,25 @@
+module Monetico
+  module Calculable
+    DEF_PREC = 2
+
+    def big
+      BigDecimal(self.to_s);
+    end
+
+    alias_method :to_big, :big
+
+    def round_to(x = DEF_PREC)
+      (self * 10**x).ceil.to_f / 10**x
+    end
+
+    alias_method :round_up, :round_to
+
+    def round_down(x = DEF_PREC)
+      if self >= 0
+        (self * 10**x).floor.to_f / 10**x
+      else
+        -((-self * 10**x).floor.to_f / 10**x)
+      end
+    end
+  end
+end
\ No newline at end of file
diff --git a/lib/monetico/loan.rb b/lib/monetico/loan.rb
index 8b26154..2c3778f 100644
--- a/lib/monetico/loan.rb
+++ b/lib/monetico/loan.rb
@@ -1,57 +1,215 @@
 module Monetico
   class Loan
+    include Excel
+
     CADENCE = {
       monthly: 12,
       weekly: 52,
     }
 
-    def initialize(amount, interest_rate, no_installments, cadence=:monthly, kind=:desc)
+    # methods
+    # <ex. capital_real> - rounded
+    # <ex. capital_real_real> - float, used in internal calculations
+    # <ex. calculate_capital> - calculate for the first time, float
+
+    def initialize(amount, interest_rate, no_installments, cadence = :monthly, kind = :desc)
       @amount = amount.big
-      @interest_rate = interest_rate.big / CADENCE[cadence] 
+      @interest_rate = interest_rate.big / CADENCE[cadence]
       @no_installments = no_installments
       @kind = kind
     end
 
-    def capital 
+    private
+
+    # Capital - amount / number of installments
+    def capital
+      round capital_real
+    end
+
+    public :capital
+
+    def capital_real
+      @capital = calculate_capital if @capital.nil?
+      @capital
+    end
+
+    def calculate_capital
       @amount / @no_installments
     end
 
+    # Total amount of interests
     def total_interests
+      round total_interests_real
+    end
+
+    public :total_interests
+
+    def total_interests_real
+      @total_interests = calculate_total_interests if @total_interests.nil?
+      @total_interests
+    end
+
+    def calculate_total_interests
       if const?
         par = (1 + @interest_rate) ** @no_installments
-        payback_amount = @amount * @interest_rate * par / (par - 1) 
-
-        payback_amount * @no_installments - @amount 
+        payback_amount = @amount * @interest_rate * par / (par - 1)
+        payback_amount * @no_installments - @amount
       else
-        0.5.big * @interest_rate * @no_installments * (@amount + capital)
+        0.5.big * @interest_rate * @no_installments * (@amount + capital_real)
       end
     end
 
+    # Interests for period/installment
     def interests(idx)
-      (@amount - (idx - 1) * capital) * @interest_rate
+      round interests_real(idx)
+    end
+
+    public :interests
+
+    def interests_real(idx)
+      @interests = Array.new if @interests.nil?
+      @interests[idx] = calculate_interests(idx) if @interests[idx].nil?
+      @interests[idx]
+    end
+
+    # little refactoring
+    alias_method :interests_for_period, :interests
+
+    def calculate_interests(idx)
+      if const?
+        ipmt(@interest_rate, idx, @no_installments, @amount).abs
+      else
+        (@amount - (idx - 1) * capital_real) * @interest_rate
+      end
+    end
+
+    # Monthly payment
+    def monthly_payment
+      round monthly_payment_real
+    end
+
+    public :monthly_payment
+
+    def monthly_payment_real
+      @monthly_payment = calculate_monthly_payment if @monthly_payment_float.nil?
+      @monthly_payment
+    end
+
+    def calculate_monthly_payment
+      pmt(@interest_rate, @no_installments, @amount).abs
+    end
+
+    # Capital for period
+    def capital_for_period(idx)
+      round capital_for_period_real(idx)
+    end
+
+    public :capital_for_period
+
+    def capital_for_period_real(idx)
+      # only for desc
+      return capital_real if not const?
+
+      # only for const
+      @capitals = Array.new if @capitals.nil?
+      @capitals[idx] = calculate_capital_for_period(idx) if @capitals[idx].nil?
+      @capitals[idx]
+    end
+
+    def calculate_capital_for_period(idx)
+      ppmt(@interest_rate, idx, @no_installments, @amount).abs
+    end
+
+    # Amount for period
+    def amounts(idx)
+      capital_for_period(idx) + interests(idx)
+    end
+
+    public :amounts
+
+    def const?
+      @kind == :const
     end
 
-    def payback(range) 
-      from = range.begin
-      to = range.end
+    public 'const?'
+
+    # round money value
+    def round(v)
+      MoneyArray.money_round_value(v)
+    end
+
+    # Loan debug
+    def to_s
+      s = "Loan\n"
+      table = payback_all
+      table.each_with_index do |t, i|
+        s += "#{t[:no]}: #{t[:amount].to_f};\t#{t[:balance].to_f};\t#{t[:capital].to_f};\t#{t[:interests].to_f}\n"
+      end
+      return s
+    end
+
+    public :to_s
+
+    # Paybacks items
+    # all payback items for all
+    def calculate_payback
+      from = 1
+      to = @no_installments
+      range = (from..to)
+
+      current_amount = 0.0
 
       if const?
         par = (1 + @interest_rate) ** @no_installments
 
-        range.map do |n|
-          { no: n, interests: interests(n), amount: @amount * @interest_rate * par / (par - 1) }
+        res = range.map do |n|
+          current_amount += monthly_payment_real
+          { no: n, interests: interests(n), amount: monthly_payment_real, capital: capital_for_period(n), balance: @amount + total_interests_real - current_amount }
         end
       else
-        range.map do |n|
-          { no: n, interests: interests(n), amount: capital + interests(n) }
+        res = range.map do |n|
+          amount = capital_real + interests(n)
+          current_amount += amount
+          { no: n, interests: interests(n), amount: amount, capital: capital_real, balance: @amount + total_interests_real - current_amount }
         end
-      end 
+      end
+      @paybacks = res
+      @paybacks_round = round_paybacks(res)
     end
 
-    def const?
-      @kind == :const
+    def payback_real(range)
+      calculate_payback if @paybacks.nil?
+      @paybacks[range]
+    end
+
+    def payback(range)
+      calculate_payback if @paybacks.nil?
+      @paybacks_round[range]
     end
-    private :const?
+
+    def payback_all
+      calculate_payback if @paybacks.nil?
+      @paybacks_round
+    end
+
+    public :payback_all
+
+    public :payback
+
+
+    def round_paybacks(res)
+      [:interests, :amount, :capital, :balance].each do |k|
+        tmp = MoneyArray.factory(res.collect { |r| r[k] })
+        tmp = tmp.money_round
+
+        res.each_with_index do |r, i|
+          r[k] = tmp[i]
+        end
+
+      end
+      return res
+    end
+
 
   end
-end
\ No newline at end of file
+end
diff --git a/lib/monetico/money_array.rb b/lib/monetico/money_array.rb
new file mode 100644
index 0000000..878f504
--- /dev/null
+++ b/lib/monetico/money_array.rb
@@ -0,0 +1,55 @@
+module Monetico
+  class MoneyArray < Array
+    # round money values to proper precision, ...
+    def money_round
+      return self.class.money_round(self)
+    end
+
+    def array_sum
+      self.inject(nil) { |sum, x| sum ? sum+x : x }
+    end
+
+    def self.factory(_from)
+      obj = self.new
+      _from.each do |f|
+        obj << f
+      end
+      return obj
+    end
+
+    # round up currency value
+    def self.money_round_value(v)
+      return ((v * 100).round / 100.0).to_big
+      # TODO delete it
+      #return ((v * 100).floor / 100.0)
+      #return ((v * 100).ceil / 100.0)
+    end
+
+    # round series of values to achieve proper sum
+    def self.money_round(a)
+      sum = a.array_sum
+      rest = 0.0
+      result = MoneyArray.new
+
+      a.each do |b|
+        rounded = money_round_value(b)
+        real = b
+
+        # compensation
+        if rest >= 1.0
+          rounded -= 1.0
+          rest -= 1.0
+        elsif rest <= -1.0
+          rounded -= -1.0
+          rest -= -1.0
+        end
+
+        result << rounded.to_big
+        rest += real - rounded
+      end
+
+      return result
+
+    end
+  end
+end
\ No newline at end of file
diff --git a/monetico.gemspec b/monetico.gemspec
index 29c704c..86014d7 100644
--- a/monetico.gemspec
+++ b/monetico.gemspec
@@ -18,4 +18,11 @@ Gem::Specification.new do |gem|
   gem.name          = "monetico"
   gem.require_paths = ["lib"]
   gem.version       = Monetico::VERSION
+  
+  gem.add_development_dependency "rspec"
+  gem.add_development_dependency "rake"
+
+  gem.add_development_dependency "rdoc"
+  gem.add_development_dependency "shoulda"
+  gem.add_development_dependency "simplecov"
 end
diff --git a/spec/loan_spec.rb b/spec/loan_spec.rb
new file mode 100644
index 0000000..13d0adf
--- /dev/null
+++ b/spec/loan_spec.rb
@@ -0,0 +1,106 @@
+require "spec_helper"
+
+describe Monetico::Loan do
+  describe "const loan" do
+    before :all do
+      # tests were updated using
+      # http://www.money.pl/banki/kalkulatory/kredytowy/
+      @loan = Monetico::Loan.new(200000.0, 0.065, 360, :monthly, :const)
+    end
+
+    it "first monthly_payment" do
+      @loan.monthly_payment.to_f.should == 1264.14
+      @loan.monthly_payment.to_f.should == 1264.14
+    end
+
+    it "total_interests returns sum of interests" do
+      @loan.total_interests.to_f.should == 255088.98
+    end
+
+    it "monthly_capital(1) returns capital for first payment" do
+      @loan.capital_for_period(1).to_f.abs.should == 180.80
+    end
+
+    it "monthly_interests(1) returns capital for first payment" do
+      @loan.interests_for_period(1).to_f.abs.should == 1083.33
+    end
+
+    it "monthly_capital(117) returns capital for payment" do
+      @loan.capital_for_period(117).to_f.abs.should == 338.34
+    end
+
+    it "monthly_intersts(117) returns capital for payment" do
+      @loan.interests_for_period(117).to_f.abs.should == 925.80
+    end
+
+    it "monthly_capital(231) returns capital for payment" do
+      @loan.capital_for_period(231).to_f.abs.should == 626.33
+    end
+
+    it "monthly_intersts(231) returns capital for payment" do
+      @loan.interests_for_period(231).to_f.abs.should == 637.81
+    end
+
+    # TODO
+    it "payback_all returns payback table and last item balance should be 0.0" do
+      #table = @loan.payback(1..360) # 1 or 0 should be the first payback item?
+      table = @loan.payback_all
+
+      #table.size.should == 360
+
+      table[0][:capital].to_f.abs.should == 180.80
+      table[0][:interests].to_f.abs.should == 1083.33
+      #table[0][:amount].to_f.abs.should == 180.80
+      #table[0][:balance].to_f.abs.should == 180.80
+
+
+      #table.last[:capital].to_f.abs.should == 180.80
+      #table.last[:interests].to_f.abs.should == 1083.33
+      #table.last[:amount].to_f.abs.should == 180.80
+      table.last[:balance].to_f.should == 0.0
+    end
+
+  end
+
+  describe "desc loan" do
+    before :all do
+      @loan = Monetico::Loan.new(200000.0, 0.065, 360, :monthly, :desc)
+    end
+
+    it "capital returns amount/no_installments" do
+      @loan.capital.to_f.abs.should == 555.56
+    end
+
+    it "total_interests returns sum of interests" do
+      @loan.total_interests.to_f.should == 195541.67
+    end
+
+    it "interests(1) returns intersts for 1 payment" do
+      @loan.interests(1).to_f.should == 1083.33
+    end
+
+    it "amount(1) returns amount for 1 payment" do
+      @loan.amounts(1).to_f.should == 1638.89
+    end
+
+    it "interests(123) returns intersts for 1 payment" do
+      @loan.interests(123).to_f.should == 716.20
+    end
+
+    it "amount(123) returns amount for 1 payment" do
+      @loan.amounts(123).to_f.should == 1271.76
+    end
+
+    it "payback_all returns payback table and last item balance should be 0.0" do
+      table = @loan.payback_all
+      table[359][:balance].to_f.should == 0.0
+    end
+
+    it "can return debug string using to_s" do
+      @loan.to_s.should be_kind_of(String)
+    end
+
+  end
+
+end
+
diff --git a/spec/monetico_spec.rb b/spec/monetico_spec.rb
new file mode 100644
index 0000000..40554ea
--- /dev/null
+++ b/spec/monetico_spec.rb
@@ -0,0 +1,38 @@
+require 'spec_helper'
+require 'rspec'
+
+describe Monetico do
+  before :each  do
+  end
+
+  it "should round up/down BigDecimal" do
+    b = BigDecimal.new('1.0155')
+    b.round_down(2).should == 1.01
+    b.round_up(2).should == 1.02
+    b.round_to(2).should == 1.02
+    b.round_down.should == 1.01
+    b.round_up.should == 1.02
+
+    b.round_down(1).should == 1.0
+    b.round_up(1).should == 1.1
+
+    b.round_down(0).should == 1.0
+    b.round_up(0).should == 2.0
+  end
+
+  it "should round up/down Float" do
+    b = 1.0155
+    b.round_down(2).should == 1.01
+    b.round_up(2).should == 1.02
+    b.round_to(2).should == 1.02
+    b.round_down.should == 1.01
+    b.round_up.should == 1.02
+
+    b.round_down(1).should == 1.0
+    b.round_up(1).should == 1.1
+
+    b.round_down(0).should == 1.0
+    b.round_up(0).should == 2.0
+  end
+
+end
diff --git a/spec/money_round_spec.rb b/spec/money_round_spec.rb
new file mode 100644
index 0000000..902ebfa
--- /dev/null
+++ b/spec/money_round_spec.rb
@@ -0,0 +1,42 @@
+require "spec_helper"
+
+describe Monetico::MoneyArray do
+  describe "round simple array" do
+    before :all do
+    end
+
+    it "should round simple array" do
+      ma = Monetico::MoneyArray.new
+      ma << 10.001
+      ma << 9.999
+
+      ma.money_round.each do |m|
+        # precision check
+        rest = (m * 100.00) % 1
+        #puts "#{m} - rest #{rest}"
+        rest.should == 0.0
+      end
+
+    end
+
+    it "should round simple loan results" do
+      ma = Monetico::MoneyArray.new
+      # also testes using floats
+      ma << BigDecimal.new("327.8389946270847")
+      ma << BigDecimal.new("333.3029778708694")
+      ma << BigDecimal.new("338.85802750205056")
+
+      ma.money_round.each do |m|
+        # precision check
+        rest = (m * 100.00) % 1
+        #puts "#{m} - rest #{rest}"
+        rest.should == 0.0
+      end
+
+      # sum after rounding
+      ma.money_round.array_sum.should == 1000.0
+    end
+
+  end
+end
+
diff --git a/spec/spec_helper.rb b/spec/spec_helper.rb
new file mode 100644
index 0000000..2df4502
--- /dev/null
+++ b/spec/spec_helper.rb
@@ -0,0 +1,6 @@
+require "bundler"
+Bundler.setup
+
+require "rspec"
+
+Dir[File.expand_path(File.dirname(__FILE__) + "/../lib/**/*.rb")].each { |f| require f }