No idea how accurate this truly is as I have no training in accounting whatsoever (other than a 3rd grade class on balancing my checkbook). Anyways, I wanted to know what my overall annualised yield was for my RateSetter account where I've made some deposits on certain dates and know my current balance. I would welcome improvements or correction…

determine_yield.rb

# Simple script to determine what your rough overall annual yield is for something that has gained value with multiple deposits and withdrawals.
#
# Expects an input file with each line in one of the following formats:
#
# 1. amount added:
# yyyy-mm-dd £###.##
#
# 2. amount subtracted:
# yyyy-mm-dd (£###.##)
#
# 3. current value (there should only be one line like this):
# current value as of yyyy-mm-dd: £###.##
#
# Lines that are blank or start with # will be ignored.  Lines that are none of the above will be ignored in the calculations and printed out to STDOUT.

# TODO: we really want to compare this to a savings account interest rate.
# 1. Determine the total yield
# 2. Determine an imaginary monthly interest payment, wherein each penny day contributes equally to that payment.  This should, with compound calculation, arrive at the total yield included in the "current value".
# 3. Calculate the annual interest rate on the basis of that imaginary monthly payment.
# Note: if done correctly, this should allow for withrawals wherein the balance is > 0, but the 'paid-in principal' is < 0.


require 'date'

filename = ARGV.length > 0 ? ARGV[0] : "sums.txt"

class Transaction
  attr_reader :date, :amount_in_pennies, :type
  
  def initialize(date_str, amount_str, type)
    @date = Date.parse(date_str)
    @amount_in_pennies = amount_str.include?('.') ? amount_str.tr('.', '').to_i : "#{amount_str}00".to_i  # 12.23 => 1223, 12 => 1200
    @type = type
  end
  
  def apply(balance_in_pennies)
    if type == :withdrawal
      balance_in_pennies - @amount_in_pennies
    else
      balance_in_pennies + @amount_in_pennies
    end
  end
  
  def date_diff(other_transaction)
    @date - other_transaction.date
  end
end

transactions = []
current_value_in_pennies = nil
current_value_as_of_date = nil

File.readlines(filename).each do |line|
  if line =~ /^(\d{4}-\d{2}-\d{2}) [$\u00A3\u20AC\u00A5](\d+(\.\d\d)?)\s*$/
    transactions << Transaction.new($1, $2, :deposit)
  elsif line =~ /^(\d{4}-\d{2}-\d{2}) \([$\u00A3\u20AC\u00A5](\d+(\.\d\d)?)\)\s*$/  # note the parens
    transactions << Transaction.new($1, $2, :withdrawal)
  elsif line =~ /^current value as of (\d{4}-\d{2}-\d{2}):\s*[$\u00A3\u20AC\u00A5](\d+(\.\d\d)?)\s*$/
    raise "You cannot define current value more than once in input file!  It's defined at least twice: #{current_value_in_pennies} and #{$1}" unless current_value_in_pennies.nil?
    current_value_in_pennies = $2.include?('.') ? $2.tr('.', '').to_i : "#{$2}00".to_i
    current_value_as_of_date = Date.parse($1)
  elsif line =~ /^\s*(#.*)?$/
    # ignore this line, it's empty or a comment
  else
    puts "Ignoring this line as I cannot parse it: #{line}"
  end
end

sorted_transactions = transactions.sort_by &:date
puts "#{sorted_transactions.length} sorted transactions"

class PrincipalBalance
  def initialize(amount_in_pennies, days_duration)
    @amount_in_pennies, @days_duration = amount_in_pennies, days_duration
  end
  
  def penny_days
    @amount_in_pennies * @days_duration
  end
end

principal_balances = []
last_balance_in_pennies = 0
last_transaction = nil

sorted_transactions.each do |trans|
  if last_transaction.nil?
    last_transaction = trans
    last_balance_in_pennies = trans.apply(0)
  else
    days = trans.date_diff(last_transaction)
    principal_balances << PrincipalBalance.new(last_balance_in_pennies, days)
        
    new_balance = trans.apply(last_balance_in_pennies)

    last_transaction = trans
    last_balance_in_pennies = new_balance
  end  
end

# final balance period
principal_balances << PrincipalBalance.new(last_balance_in_pennies, current_value_as_of_date - last_transaction.date)


profit = (current_value_in_pennies - last_balance_in_pennies).to_f
total_penny_days = principal_balances.inject(0) {|sum, nxt| sum + nxt.penny_days}
total_penny_years = total_penny_days.to_f / 365

annualized_yield = (profit) / total_penny_years

puts "Annualized yield was: #{annualized_yield * 100}% APY"

sums.txt

# Example sums file to test-run this scriptlet:

2012-05-27 £1000
2012-11-12 £1000
current value as of 2013-01-10: £2030.83