While I agree with iandanforth that you're missing an important aspect of this, I'd like to commend you on what you have. My first attempt at writing a spaced-r

While I agree with iandanforth that you're missing an important aspect of this, I'd like to commend you on what you have. My first attempt at writing a spaced-repetition algorithm turned out with the same results as yours, however your algorithm is a much neater way of looking at things. If you take iandanforth's comment to heart try to incorporate it into what you already have, instead of rewriting.