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🕸Fergus Duniho wrote on Wed, Jul 1, 2020 06:07 PM UTC:

Since it's most similar to the Aanca, let's first look at how the code for that piece works: 

def Aanca fn (checkride #0 #1 1 1 and empty #0) 
where #0 0 sign - rank #1 rank #0 
#1 
or fn (checkride #0 #1 1 1 and empty #0) 
where #0 sign - file #1 file #0 0 
#1
or checkleap #0 #1 1 0;

The code works backward, as usual. So it first checks whether the move is a one-space orthogonal move. If it is, it returns true immediately. If not, it continues. To calculate whether it makes a legal Aanca move, it needs three coordinates. #0 is the origin, and #1 is the destination, but it also needs the coordinate for the space on which it changes direction. Unfortunately, variables cannot be assigned in functions. The next best thing is to use a Lambda function that can take arguments. These appear in parentheses after the fn operator. Each one is the same, but each one gets fed different arguments. The second argument is always #1, which is the destination space. The first argument is the space where the move changes direction. Since each destination space falls into one of four quadrants, and there are two possible paths to the destination through a particular quadrant, this information is used to reduce the paths that need to be checked to two.

It first checks whether the move is going left or right, and based on that, it selects the space to the left or right of the origin space as the turning space. It then makes sure it is empty and that there is a legal Bishop move from it to the destination space. If that fails, it checks whether the move is going forward or backward, and based on that, it selects the space ahead or behind the origin space as the turning space. It makes sure this space is empty, and if it is, it makes sure there is a legal Bishop move from that space to the destination. If there is, it returns true, and if there is not, it returns false. I expect that the piece you want can be done with only slight modifications to this function.


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