Amount deviation

amount-deviation-spec.rb

require 'bigdecimal'

class BigDecimal
  def inspect; to_s('F'); end
end

class Account

  attr_reader :intial_amount, :balance_changes

  def initialize(amount)
    @initial_amount = amount
    @balance_changes = [BalanceChange.new(amount, BigDecimal('0'))]
  end

  def current_balance
    @balance_changes.last
  end

  def total_win_percentage
    ((current_balance.amount - @initial_amount) / @initial_amount).mult(BigDecimal('100'), 6)
  end

  def change_by!(amount_diff)
    new_amount = current_balance.amount + amount_diff
    new_percentage = ((new_amount - current_balance.amount) / current_balance.amount).mult(BigDecimal('100'), 6)

    @balance_changes << BalanceChange.new(new_amount, new_percentage)
  end

end

BalanceChange = Struct.new(:amount, :diff_to_last_percentage)

module Metrics
  module Common

    ONE = BigDecimal('1') unless const_defined?(:ONE)
    ONE_HUNDRED = BigDecimal('100') unless const_defined?(:ONE_HUNDRED)

    DECIMAL_ROUNDING_SCALE = 6 unless const_defined?(:DECIMAL_ROUNDING_SCALE)

    def self.sum_percent_deltas(pds)
      result = ONE
      pds.each { |pd|
        # There is no need to call #round explicitly.
        # BigDecimal#mult supports # inherent rounding to a specified scale.
        #
        # See http://ruby-doc.org/stdlib-1.8.7/libdoc/bigdecimal/rdoc/BigDecimal.html#method-i-mult
        result = result.mult(ONE + pd / ONE_HUNDRED, DECIMAL_ROUNDING_SCALE)
      }
      (result - 1) * ONE_HUNDRED
    end

  end
end

NR_OF_REPETITIONS = [10, 100, 1_000, 10_000]

INITIAL_AMOUNT = BigDecimal('10_000')
DELTA = BigDecimal('10')

describe Account do

  describe "#initialize with #{INITIAL_AMOUNT}" do

    subject { Account.new(INITIAL_AMOUNT) }

    it 'should have one balance change when initialized' do
      subject.balance_changes.should have(1).item
    end

    it "should have a current balance of #{INITIAL_AMOUNT}" do
      subject.current_balance.amount.should == INITIAL_AMOUNT
    end
    
  end

  describe "#change_by" do

    subject { Account.new(INITIAL_AMOUNT) }

    context 'when the change is positive' do

      before { subject.change_by!(BigDecimal('100')) }

      it 'should have a new amount' do
        subject.current_balance.amount.should == BigDecimal('10100')
      end

      it 'should record the correct diff percentage' do
        subject.current_balance.diff_to_last_percentage.should == BigDecimal('1')
      end

    end

    context 'when the change is negative' do

      before { subject.change_by!(BigDecimal('-100')) }

      it 'should have a new amount' do
        subject.current_balance.amount.should == BigDecimal('9900')
      end

      it 'should record the correct diff percentage' do
        subject.current_balance.diff_to_last_percentage.should == BigDecimal('-1')
      end

    end

  end

  NR_OF_REPETITIONS.each do |nr|
    describe "account after #{nr} changes" do
      subject { Account.new(INITIAL_AMOUNT) }

      before {
        nr.times do
          delta = BigDecimal((rand * 100 - 50).to_s) # a random number between -50 and 50
          subject.change_by!(delta)
        end
      }

      it "the cumulative balance changes should be equal to the absolute change" do
        cumulative_change = Metrics::Common::sum_percent_deltas(subject.balance_changes.map(&:diff_to_last_percentage))
        cumulative_change.should be_within(0.01).of(subject.total_win_percentage)
      end
    end
  end

end

gistfile1.txt

# run with

rspec -cf p amount-deviation-spec.rb