Excel IRR function in Ruby

excel_irr.rb

module I10
  # Returns Internal Rate of Return (TIR Spanish) for a cash
  # flow in numbers, with the payments as values.
  # @param transactions : Array which contains initial project cost,
  #                       and the payments over the time.
  def I10.irr(transactions, max_iterations = 20)
    min = 0.0
    max = 1.0
    npv = 0
    iterations = 0
    accuracy = 0.000001
    while npv.abs < accuracy || iterations != max_iterations do
      closest = (min + max)/2
      npv = transactions.each_with_index.map{|v,i| (v/(1+closest)**i) }.inject(:+)
      (npv > 0 ? min = closest : max = closest)
      iterations += 1
    end
    closest
  end

end