Repetition (Low Priority)

[BenAvtabi]

My current plan for this special case - keep a list of all previous moves (or last x previous moves for repetition) in a relatively high-level function, and if a pawn double-moved last round, then it passes his location in a list that is usually empty, which is then passed down all the way to the pawn move function, that checks it for "en passant opportunities"

[BenAvtabi]

I pass PreviousMoves around, just make a check that the past 3 pairs of moves are the same.
I don't know how your heuristic stuff currently work, it seemed to be a cumulative score thing?
Anyway, you need to add an "override" functionality to that:
If X happens, it should override the score.

For example, when there's repetition, no matter what else happens, the score should be 0.
Same thing goes for stalemate (valid_moves returns an empty list).
I assume also mates need to override the score to something stupid (unless you trust "taking the king" to give enough score, and so "finding mates" is the same as looking a turn deeper and "taking the king".