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Update Algorithm explanation.txt

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Coding with Tom
2025-01-14 15:00:55 +00:00
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commit 2364223f45
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Algorithm explanation.txt
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@@ -513,6 +513,65 @@ To remove the least significant bit we use this method:
tempBitboard &= tempBitboard - 1;
Let's say we have the black pawns in the starting position:
tempBitboard = 65280;
0 0 0 0 0 0 0 0
1 1 1 1 1 1 1 1
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
Inside a loop we get the square and then need to remove that.
We could do this:
int square = BitscanForward(tempBitboard);
tempBitboard & ~SQUARE_BBS[square];
This can be a problem if "square" is not valid though. I do it this way:
tempBitboard - 1:
1 1 1 1 1 1 1 1
0 1 1 1 1 1 1 1
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
When you minus 1 bitboard, it removes the smallest bit and sets all bits below it.
We then just AND this with the bitboard to remove the smallest bit.
1 1 1 1 1 1 1 1
0 1 1 1 1 1 1 1
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
&
1 1 1 1 1 1 1 1
0 1 1 1 1 1 1 1
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
=
0 0 0 0 0 0 0 0
0 1 1 1 1 1 1 1
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0