The generalized pull-down problem is nothing more that drawing the best discrete approximation of the ideal line from (1, 1) to (desired frame rate, source frame rate), as shown in the following diagram for the (30 fps, 24 fps).

Moreover, it is a very special case of a line. Can you think of addition optimizations to our Bresenham line drawing code that could be applyed for the special case of pull-down?