Creating Special Rules

Special rules are ‘special’ because they can only occur under certain conditions. For example, Castling which can only occur if the king and rook have not moved from their initial positions.

In our variant, the unique special rule is that a player cannot move the same piece twice in a row. These rules are applied when we fetch the moves a player can make to either filter the moves, or add furthr special moves. In this case, we are restricting the moves a player can make by removing the moves which move the same piece the player moved previously.

Step 1: Implement the SpecialRules interface

1. Create a file called “NoRepeatedMoveFromSamePieceRule.kt” in the tutorials package you have created.

2. Make this class implement the SpecialRules2D<AbstractChess> interface.

package tutorial

import gameTypes.chess.AbstractChess2D
import moves.Move2D
import players.Player
import rules.SpecialRules2D

class NoRepeatedMoveFromSamePieceRule() : SpecialRules2D<AbstractChess2D> {
    override fun getPossibleMoves(game: AbstractChess2D, player: Player, moves: MutableList<Move2D>) {
        TODO()
    }
}

Step 2: Implement the getPossibleMoves() method

This function should modify the given moves list by removing all the moves that contain the piece that the player moved previously. We can get the move the player previously made like so:

val prevMove = game.moveLog[game.moveLog.size - 2]

This means we can easily remove the moves where the same piece is moved like so:

override fun getPossibleMoves(game: AbstractChess2D, player: Player, moves: MutableList<Move2D>) {
    if (game.moveLog.size <= 1) {
        return
    }
    val prevMove = game.moveLog[game.moveLog.size - 2]
    moves.removeAll { it.displayPieceMoved === prevMove.displayPieceMoved }
}

Step 3: Overall

The NoRepeatedMoveFromSamePieceRule class should now look like this.

package tutorial

import gameTypes.chess.AbstractChess2D
import moves.Move2D
import players.Player
import rules.SpecialRules2D

class NoRepeatedMoveFromSamePieceRule() : SpecialRules2D<AbstractChess2D> {
    override fun getPossibleMoves(game: AbstractChess2D, player: Player, moves: MutableList<Move2D>) {
        if (game.moveLog.size < 2) {
            return
        }

        val prevMove = game.moveLog[game.moveLog.size - 2]
        moves.removeAll { it.displayPieceMoved === prevMove.displayPieceMoved }
    }
}