Thread: Legal moves in a chess game

  1. #1
    Registered User Finchie_88's Avatar
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    Aug 2004

    Legal moves in a chess game

    Hello everyone. I'm in the process of writing a chess game in the console. I have reached a problem when it comes to testing whether or not the entered move is legal. I was hope that someone could give me a few pointers (no pun intended) as to the easiest way to do it.

  2. #2
    Dump Truck Internet valis's Avatar
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    Jul 2005
    One of the simplest methods would be to create a two dimensional array where each element represent a board space. Have something like 0 represent an empty space and each piece it's own identifier. When a piece is told to move you know its position and its type which is all you need. For example a pawn can only move straight if it's not capturing, check if pawnPosition - rowLength - 1 is occupied and whether pawnPosition - rowLength + 1 is occupied (by an opponent's piece), if so the corresponding diagonal is a valid move, otherwise only pawnPosition - rowLength is valid.

    If you wanted to optimize space you could also take a bitboard approach.

  3. #3
    (?<!re)tired Mario F.'s Avatar
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    May 2006
    With the strict positional rules, do as Valis suggested. Each type of piece has different movement rules. These rules can be easily coded if you look at the board as having a cartesian coordinate system like the one provided by a two dimensional array.

    Remember that 3 pieces have special moves:
    Pawn en-passant, Pawn first move
    Rook big castle, Rook small castle
    King big castle, King small castle

    Very few things will stop a piece from moving in chess. "Another piece of the same side occupying the destination space" is the only one for regular moves. So this is actually quiet easy to code. The two pawn special moves and the the two castle have well known rules, but easy to code too.
    Originally Posted by brewbuck:
    Reimplementing a large system in another language to get a 25% performance boost is nonsense. It would be cheaper to just get a computer which is 25% faster.

  4. #4
    The larch
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    May 2006
    You should also keep in mind that the king should not be in check after the move.

  5. #5
    Frequently Quite Prolix dwks's Avatar
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    Apr 2005
    There's an article on that's somewhat relevant:

    Seek and ye shall find. quaere et invenies.

    "Simplicity does not precede complexity, but follows it." -- Alan Perlis
    "Testing can only prove the presence of bugs, not their absence." -- Edsger Dijkstra
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  6. #6
    The superhaterodyne twomers's Avatar
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    Dec 2005
    I made one before (AGES ago), and I programmed in all the allowable moves ....

    I had a switch statement, and depending on whether the piece was a p, k, q, n, b, r it implemented a different function. I had four variables - the start x, start y, end x and end y, which were indices to the starting and ending positions of the piece. For the bishop, I first tested to see if there was anything in the diagonal and that the x deviation ==ed the y one ... just think about the moves, write them down, and see what you come up with. It's not all that hard, and gives you lots of experience with messing around with arrays!

    EDIT - you can also recycle some of the functions. I think for the Queen's movement, I did something to do with an amalgamation of the Bishop's and the Rook's.

  7. #7
    Registered User Finchie_88's Avatar
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    Aug 2004
    Thank you for the replies. If I have anymore problems, I will post again. I was using the 2D array idea to start with (and I have a couple of classes to store the coords of the pieces). Thanks again, much appreciated.

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