xirr

[wkranec]

Finance should have an implementation of the XIRR function. Some details are available on the OpenOffice.org wiki:

http://wiki.services.openoffice.org/wiki/Documentation/How_Tos/Calc:_XIRR_function

Other implementations I've found on the web:

• http://stackoverflow.com/questions/5179866/xirr-calculation
• http://search.cpan.org/~erwan/Finance-Math-IRR-0.10/lib/Finance/Math/IRR.pm

Some thoughts on the implementation:

• The xirr method will be defined as an Array method (similar to Array#irr) inside Finance::Cashflow.
• Instead of values, the Array should contain Transaction objects, which will have date and amount attributes.
  • The date attribute hasn't been defined yet, but it should probably accept a String, and convert internally to a Ruby Date (or Datetime) object.
• Since the relevant time periods are known, the xirr method ought to return a Rate object (this is not the case in the example below).

Example

> t = []
> t << Transaction.new(-5000, :date => '3/1/2011')
> t << Transaction.new(100, :date => '4/1/2011')
> t << Transaction.new(100, :date => '5/15/2011')
> t << Transaction.new(100, :date => '6/29/2011')
> t << Transaction.new(5000, :date => '7/4/2011')
> t.xirr
#=> 0.19021145803047

[wkranec]

Added basic working support in 1.1.0. Tests and feedback are welcome!