No checkmate detection

[JohnathonNow]

Right now check is checked but not chessmate.

[JohnathonNow]

Checkmate is of course being:

1. In Check
2. Having no legal moves

1 is easy to detect - we simply iterate over the opponent's pieces and see if they can move to the king - if so, we are in check. This is already done.
2 seems harder - we cannot iterate over every move, as there are always an infinite number of moves we could make. What we need to do is look for moves that stop check. There are three ways to stop check:

1. Move the king out of check
2. Capture the attacking piece
3. Block the attacking piece

1 is trivial - there are 8 moves to consider the legality of, and for each legal move, check if the king would still be in check. 2 is also fairly easy - for each opponent piece that can attack our king, see if any of our pieces can capture them, and then see if we are in check. Pawns are only a slight complication - ones that have moved are as regular pieces, and for the ones that have not yet moved we need only check if the attacking piece is on the rank one beyond the pawn walls, and if so, we only need to check two pawns.

3 is difficult. We only need to consider bishops, rooks, and queens, as pawns and knights cannot be blocked. WLOG we can consider only rooks, which move along a vertical or horizontal line. For each rook that is attacking our king, we can look at each of our rooks and find the intercept between the lines the rooks can move on. We can then consider that move, and if it takes us out of check, we are not in checkmate. If it does not, we consider our other pieces. For knights, we see if any of the squares they can move into is on the line (really we can just check if these 8 moves block check), and for moved pawns we can also check if they block check.

For unmoved pawns, however, there are potentially many pawns to consider. For rooks we first check if the rook is on the rank one or two beyond the pawns. We can then check each file between the rook and king until we find just one unmoved pawn, if it cannot block then no pawn can. Bishops (and therefore queens) are simpler - we simply find the two pawns that could move to the line between the bishop (and likewise queen) and see if they can move there to stop check.