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We can now move through each move in sequence, but what happens if we wish to step back one or more moves? All of these moves can be handled by simply reversing the parameters that we provide to the makeMove function, but in the following cases we need to do a little more:
The changes required for a standard move and to update the move counters are shown in red in this listing; changes for the other functions are shown in this listing comprising the reworked en Passant, Castling and Pawn Promotion functions.
This leaves the undoing of a capture of one piece by another, which can happen as a normal move, as part of Pawn Promotion or en Passant. While the actual move can be handled by the standard move functionality, we need to put the piece that was taken back on the board (although in the latter case we have handled it within the en Passant function as it is a simple change and the piece captured will always be a pawn of the opposite color).
To achieve this, we need to step back through all the previous moves in the game until we find a piece that was on the correct square. Matters are further complicated if the square happens to be one that is used in Castling, as the move is recorded as 0-0 or 0-0-0, so we'll need to unpack those as well. We'll use the result - or returned variable - of this function in the moveHandler function. Since this is the most complex of the 'undo' functions, we'll use the following Pseudo Code, which is a little complex, so take your time when working through it:
Get the correct set of moves - either White's or Blacks
Set matchFound to false
Starting with the current move go back until matchFound is true
If either a match is found or no more moves then matchFound
= true
Next move
If the square we are looking at is a square which a Castled piece may end on
then
Set matchFound to false
Starting with the current move step back until matchFound
is true
If the move number is equal
to where the match has already been found then matchFound=true
If there has been a Castling
move which has landed on that square then matchFound=true
Next move
End if
If we have found a move then
Get the move
If it is not a Castling one get the piece that was there
Else work out which Castling piece it is
Else
No moves have been made, so get the piece that started
on that square
End if
Put the piece back on the correct square
This complex function is shown in this listing. However, we need to call this function if we're undoing a capture as a normal move, but also if we have a Pawn promotion that involved a capture; so we need to amend that function as well, as shown in this listing (with changes in red as usual).
Now that we have the functionality to handle all the moves within our game universe, we need to link that to the user's interface. If you think back to the previous section, we created an information box which had two buttons, one to move forward and one to move backwards. Now, all we need to do is to create the functionality behind these buttons and then link that to the movement functionality managed by the moveHandler function. These functions will also handle the bounds checking to ensure that we have a move to process. The code for this function can be found in this listing.
The only other way of making a move is by clicking on a given Move Number in the Move List; this allows us to jump to any given position in the game. To accomplish this, we need to compare the move against the current move number and then loop in the correct direction until they are equal. Here is the resulting Pseudo Code:
Get the move number and color of the move we are moving to
Work out if the new move number is high that the current move number for the
color
If it is higher, then call the moveForward function until we get there
If it is lower, then call the moveBack function until we get there
If it is the next or previous move for either color, then call either moveForward
or moveBack once
If it is the current move then inform the user
The other component of the information box that we have not yet created is the current position information. This shows the current move number and move for each player. Since we've created the move number and move areas as standard DHTML div's, we can use the innerHTML functionality to populate them with the correct information. The best time to do this is at the end of the moveHandler function. The function itself will be relatively simple; just updating the current values into the information box as shown in this listing.
You may have spotted in that function a variable called str_comment which is placed in one of the div's in the information box. This is simply a text string that can be populated from any of our functions to display a message about the game to the user, for example, Black Castles Queenside or White Resigns.
Here is a demonstration of these changes which displays the functionality implemented thus far.
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Created: March 27, 2003
Revised: June 10, 2003
URL: http://webreference.com/programming/javascript/javachess/chap2
