Start BUY X GET Y promotion Special Case

Consider Example:

Buy 2 Product get 1 Product Free. i.e. X and Y are same product.

----------------------------------------------------------
Product In Cart  |  X  |  Y  | Paid Product | Free Product
----------------------------------------------------------
      2          |  2  |  1  |     2        |     0
      3          |  2  |  1  |     2        |     1
      4          |  2  |  1  |     3        |     1
      5          |  2  |  1  |     4        |     1
      6          |  2  |  1  |     4        |     2

i.e. This will apply discount on current items in order.

This is special case so explained here. Rest will work as expected.