module Xirr

  # Methods that will be included in Cashflow to calculate XIRR
  class Bisection
    include Base

    # Calculates yearly Internal Rate of Return
    # @return [BigDecimal]
    # @param midpoint [Float]
    # An initial guess rate will override the {Cashflow#irr_guess}
    def xirr(midpoint = nil)

      # Initial values
      left = [BigDecimal.new(-0.99, Xirr::PRECISION), cf.irr_guess].min
      right = [BigDecimal.new(9.99, Xirr::PRECISION), cf.irr_guess + 1].max
      midpoint ||= cf.irr_guess
      runs = 0

      # Loops until difference is within error margin
      while ((right - left).abs > Xirr::EPS && runs < Xirr.config.iteration_limit.to_i) do

        runs += 1
        npv_positive?(midpoint) == npv_positive?(left) ? left = midpoint : right = midpoint
        midpoint = format_irr(left, right)

      end

      if runs >= Xirr.config.iteration_limit.to_i
        raise ArgumentError, "Did not converge after #{runs} tries."
      end

      return midpoint.round Xirr::PRECISION

    end

    private

    # @param midpoint [Float]
    # @return [Bolean]
    # Returns true if result is to the right ot the range
    def npv_positive?(midpoint)
      xnpv(midpoint) > 0
    end

    # @param left [Float]
    # @param right [Float]
    # @return [Float] IRR of the Cashflow
    def format_irr(left, right)
      irr = (right+left) / 2
    end

  end

end