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Piece Square Table

Today I want to discuss the Piece Square Table

Originally the piece square tables were declared in a separate class that was used by the Evaluation function.  However I found that it is more efficient to save the extra method calls and perform the piece square table lookups directly in the evaluation function. 

Hence I have modified this post to simply describe the piece square table and the logic behind the numbers assigned to each position.

As I have already stated the piece square tables are used chess board Evaluation class to score points based on the current position of the chess piece.  The main idea behind this code is that certain positions for chess pieces are better than others.  Fox example it is better for knights to stay away from the edge of the board.  Pawns should control the center of the board and advance forward. 

I have decided not to create a piece square table for every single chess piece.  I have omitted Queens and Rooks.  I could not find good enough positional tactical advantages for Rooks and Queens to warrant the performance cost of a table lookup for each of their positions.

Here are the piece square tables used by my chess engine:

Pawns are encouraged to stay in the center and advance forward:


private static readonly short[] PawnTable = new short[]
{
     0,  0,  0,  0,  0,  0,  0,  0,
    50, 50, 50, 50, 50, 50, 50, 50,
    10, 10, 20, 30, 30, 20, 10, 10,
     5,  5, 10, 27, 27, 10,  5,  5,
     0,  0,  0, 25, 25,  0,  0,  0,
     5, -5,-10,  0,  0,-10, -5,  5,
     5, 10, 10,-25,-25, 10, 10,  5,
     0,  0,  0,  0,  0,  0,  0,  0
};

Knights are encouraged to control the center and stay away from edges to increase mobility:

private static readonly short[] KnightTable = new short[]
{
    -50,-40,-30,-30,-30,-30,-40,-50,
    -40,-20,  0,  0,  0,  0,-20,-40,
    -30,  0, 10, 15, 15, 10,  0,-30,
    -30,  5, 15, 20, 20, 15,  5,-30,
    -30,  0, 15, 20, 20, 15,  0,-30,
    -30,  5, 10, 15, 15, 10,  5,-30,
    -40,-20,  0,  5,  5,  0,-20,-40,
    -50,-40,-20,-30,-30,-20,-40,-50,
};

Bishops are also encouraged to control the center and stay away from edges and corners:


private static readonly short[] BishopTable = new short[]
{
    -20,-10,-10,-10,-10,-10,-10,-20,
    -10,  0,  0,  0,  0,  0,  0,-10,
    -10,  0,  5, 10, 10,  5,  0,-10,
    -10,  5,  5, 10, 10,  5,  5,-10,
    -10,  0, 10, 10, 10, 10,  0,-10,
    -10, 10, 10, 10, 10, 10, 10,-10,
    -10,  5,  0,  0,  0,  0,  5,-10,
    -20,-10,-40,-10,-10,-40,-10,-20,
};

Kings have 2 piece square tables, one for the end game and one for the middle game.  During the middle game kings are encouraged to stay in the corners, while in the end game kings are encouraged to move towards the center.

private static readonly short[] KingTable = new short[]
{
  -30, -40, -40, -50, -50, -40, -40, -30,
  -30, -40, -40, -50, -50, -40, -40, -30,
  -30, -40, -40, -50, -50, -40, -40, -30,
  -30, -40, -40, -50, -50, -40, -40, -30,
  -20, -30, -30, -40, -40, -30, -30, -20,
  -10, -20, -20, -20, -20, -20, -20, -10,
   20,  20,   0,   0,   0,   0,  20,  20,
   20,  30,  10,   0,   0,  10,  30,  20
};

private static readonly short[] KingTableEndGame = new short[]
{
    -50,-40,-30,-20,-20,-30,-40,-50,
    -30,-20,-10,  0,  0,-10,-20,-30,
    -30,-10, 20, 30, 30, 20,-10,-30,
    -30,-10, 30, 40, 40, 30,-10,-30,
    -30,-10, 30, 40, 40, 30,-10,-30,
    -30,-10, 20, 30, 30, 20,-10,-30,
    -30,-30,  0,  0,  0,  0,-30,-30,
    -50,-30,-30,-30,-30,-30,-30,-50
};

The above tables are used during the evaluation method to lookup the positional values to help calculate the chess board score.

Here is an example of how the above tables would be used to lookup a value for a white pawn position:

score += PawnTable[position];

And here is the code to perform the same lookup for a black pawn:

byte index = (byte)(((byte)(position + 56)) - (byte)((byte)(position / 8) * 16));

score += PawnTable[index];

If you want to download a C# Solution project that contains all of the above code plus a graphical user interface that will allow you to make valid moves on the board have a look my C# Chess Game Starter Kit.

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