I don't even understand how Applet generated presets evaluate legal moves.
Conceptually this is not very difficult. In principle legality of each psudo-legal move M can be tested by performing the move, and then trying all pseudo-legal moves of the opponent to see if any of those captures the King. To speed this up it only tries the opponent moves that were already attacking the King before move M was made, and all moves that were affected by M because they encounter a square the occupancy of which was changed by M in their path. Most moves only affect two squares, the origin and the destination, and only a small fraction of all moves would have found one of these two squares in their path.
Moves that were not attacking the King before M, and did not encounter a square mutated by M, will not attack the King the King after M.
Conceptually this is not very difficult. In principle legality of each psudo-legal move M can be tested by performing the move, and then trying all pseudo-legal moves of the opponent to see if any of those captures the King. To speed this up it only tries the opponent moves that were already attacking the King before move M was made, and all moves that were affected by M because they encounter a square the occupancy of which was changed by M in their path. Most moves only affect two squares, the origin and the destination, and only a small fraction of all moves would have found one of these two squares in their path.
Moves that were not attacking the King before M, and did not encounter a square mutated by M, will not attack the King the King after M.