Another chess related one: Given a chessboard with some pieces placed, how many valid moves are there? I suggest having only one colour to make it simpler (no need to consider capturing).

Example input:

_ _ _ _ _ _ _ _
_ _ N _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ R _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _

Example output:

18

(knight has 6 moves, rook has 12).

Difficulty = 4/5 (?). Can be made easier by restricting the problem only to one specific kind of chess piece (only queens, for example).

Please reply if you like this idea!

I like this idea :)