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Update Algorithm explanation.txt

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Coding with Tom
2025-01-19 22:02:36 +00:00
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Algorithm explanation.txt
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@@ -39,14 +39,14 @@ To remove a bit we need to set that bit on a ulong and then invert the bits:
bitboard &= ~SQUARE_BBS[50]; //bitboard now = 0 again
SQUARE_BBS[50] =
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 0 0 0 0 0
0 0 0 0 0 0 0 0
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ 1 _ _ _ _ _
_ _ _ _ _ _ _ _
~SQUARE_BBS[50] =
1 1 1 1 1 1 1 1
@@ -55,7 +55,7 @@ To remove a bit we need to set that bit on a ulong and then invert the bits:
1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1
1 1 0 1 1 1 1 1
1 1 _ 1 1 1 1 1
1 1 1 1 1 1 1 1
If you bitwise AND (&) with the inverted bitboard it will keep all bits the same
@@ -64,38 +64,38 @@ Let's say we want to move a pawn:
unsigned long long black_pawns = 65280;
in bits =
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
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
We need to remove the bit we want and place it somewhere else. Let's d7 to d5:
black_pawns |= SQUARE_BBS[D5]; //add the bit
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 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
_ _ _ _ _ _ _ _
_ _ _ _ 1 _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
This puts a bit on d5. Now we remove d7 like so:
black_pawns &= ~SQUARE_BBS[D7]; //remove the bit
0 0 0 0 0 0 0 0
1 1 1 1 0 1 1 1
0 0 0 0 0 0 0 0
0 0 0 0 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
_ _ _ _ _ _ _ _
1 1 1 1 _ 1 1 1
_ _ _ _ _ _ _ _
_ _ _ _ 1 _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
That's how we move pieces.
@@ -162,7 +162,7 @@ a lot slower and I found an explanation online of how to work this out.
First we get the king position, let's say it's white to play:
const int whiteKingPosition = BitscanForward(bitboard_array[BK]);
const int whiteKingPosition = BitscanForward(bitboard_array[WK]);
From the king square we need to use the piece moves to see if there is a check or pin:
@@ -174,10 +174,8 @@ From the king square we need to use the piece moves to see if there is a check o
if (tempBitboard != 0) //if it's not zero then there is a pawn
{
int pawnSquare = (DEBRUIJN64[MAGIC * (tempBitboard ^ (tempBitboard - 1)) >> 58]); //This inlines BitscanForward
checkBitboard = SQUARE_BBS[pawnSquare]; //We then set the checkbitboard with that square
if (checkBitboard == 0) { //probably an unnecessary check
checkBitboard = SQUARE_BBS[pawnSquare]; //We then set the checkbitboard with that square
}
whiteKingCheckCount++;
}
@@ -210,24 +208,24 @@ It also includes the last square.
Example:
INBETWEEN_BITBOARD[E1][E8] =
0 0 0 0 1 0 0 0
0 0 0 0 1 0 0 0
0 0 0 0 1 0 0 0
0 0 0 0 1 0 0 0
0 0 0 0 1 0 0 0
0 0 0 0 1 0 0 0
0 0 0 0 1 0 0 0
0 0 0 0 0 0 0 0
_ _ _ _ 1 _ _ _
_ _ _ _ 1 _ _ _
_ _ _ _ 1 _ _ _
_ _ _ _ 1 _ _ _
_ _ _ _ 1 _ _ _
_ _ _ _ 1 _ _ _
_ _ _ _ 1 _ _ _
_ _ _ _ _ _ _ _
INBETWEEN_BITBOARD[B7][G2] =
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 1 0 0 0 0 0
0 0 0 1 0 0 0 0
0 0 0 0 1 0 0 0
0 0 0 0 0 1 0 0
0 0 0 0 0 0 1 0
0 0 0 0 0 0 0 0
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ 1 _ _ _ _ _
_ _ _ 1 _ _ _ _
_ _ _ _ 1 _ _ _
_ _ _ _ _ 1 _ _
_ _ _ _ _ _ 1 _
_ _ _ _ _ _ _ _
We use these for pins and check for rooks, queens and bishops.
@@ -324,83 +322,83 @@ Here both rooks are pinning each other.
The white rook bitboard looks like this:
tempBitboard = bitboard_array[WR];
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 0 0 0
0 0 0 0 0 0 0 0
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ 1 _ _ _
_ _ _ _ _ _ _ _
The rook moves will look like this:
rook_attacks = GetRookAttackFast(rook_square, COMBINED_OCCUPANCIES);
0 0 0 0 0 0 0 0
0 0 0 0 1 0 0 0
0 0 0 0 1 0 0 0
0 0 0 0 1 0 0 0
0 0 0 0 1 0 0 0
0 0 0 0 1 0 0 0
1 1 1 1 0 1 1 1
0 0 0 0 1 0 0 0
_ _ _ _ _ _ _ _
_ _ _ _ 1 _ _ _
_ _ _ _ 1 _ _ _
_ _ _ _ 1 _ _ _
_ _ _ _ 1 _ _ _
_ _ _ _ 1 _ _ _
1 1 1 1 _ 1 1 1
_ _ _ _ 1 _ _ _
We AND this with empty squares to get non capture moves:
EMPTY_OCCUPANCIES:
1 1 1 1 0 1 1 1
1 1 1 1 0 1 1 1
1 1 1 1 _ 1 1 1
1 1 1 1 _ 1 1 1
1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1
1 1 1 1 0 1 1 1
1 1 1 1 0 1 1 1
1 1 1 1 _ 1 1 1
1 1 1 1 _ 1 1 1
unsigned long long non_capture_moves = EMPTY_OCCUPANCIES & rook_attacks;
non_capture_moves:
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 1 0 0 0
0 0 0 0 1 0 0 0
0 0 0 0 1 0 0 0
0 0 0 0 1 0 0 0
1 1 1 1 0 1 1 1
0 0 0 0 0 0 0 0
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ 1 _ _ _
_ _ _ _ 1 _ _ _
_ _ _ _ 1 _ _ _
_ _ _ _ 1 _ _ _
1 1 1 1 _ 1 1 1
_ _ _ _ _ _ _ _
We get the capture moves by ANDing the attacks with black occupancies:
BLACK_OCCUPANCIES:
0 0 0 0 1 0 0 0
0 0 0 0 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
_ _ _ _ 1 _ _ _
_ _ _ _ 1 _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
unsigned long long capture_moves = BLACK_OCCUPANCIES & rook_attacks;
capture_moves:
0 0 0 0 0 0 0 0
0 0 0 0 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
_ _ _ _ _ _ _ _
_ _ _ _ 1 _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
The pin bitboard will look like this:
0 0 0 0 0 0 0 0
0 0 0 0 1 0 0 0
0 0 0 0 1 0 0 0
0 0 0 0 1 0 0 0
0 0 0 0 1 0 0 0
0 0 0 0 1 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
_ _ _ _ _ _ _ _
_ _ _ _ 1 _ _ _
_ _ _ _ 1 _ _ _
_ _ _ _ 1 _ _ _
_ _ _ _ 1 _ _ _
_ _ _ _ 1 _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
We then AND that with captures and non captures. So if there is a pin, only these squares are valid.
Similar with check:
@@ -416,37 +414,37 @@ Similar with check:
The check bitboard here:
0 0 0 0 0 0 0 0
0 0 0 0 1 0 0 0
0 0 0 0 1 0 0 0
0 0 0 0 1 0 0 0
0 0 0 0 1 0 0 0
0 0 0 0 1 0 0 0
0 0 0 0 1 0 0 0
0 0 0 0 0 0 0 0
_ _ _ _ _ _ _ _
_ _ _ _ 1 _ _ _
_ _ _ _ 1 _ _ _
_ _ _ _ 1 _ _ _
_ _ _ _ 1 _ _ _
_ _ _ _ 1 _ _ _
_ _ _ _ 1 _ _ _
_ _ _ _ _ _ _ _
The white rook attacks:
1 0 0 0 0 0 0 0
1 0 0 0 0 0 0 0
1 0 0 0 0 0 0 0
1 0 0 0 0 0 0 0
1 0 0 0 0 0 0 0
1 0 0 0 0 0 0 0
0 1 1 1 1 1 1 1
1 0 0 0 0 0 0 0
1 _ _ _ _ _ _ _
1 _ _ _ _ _ _ _
1 _ _ _ _ _ _ _
1 _ _ _ _ _ _ _
1 _ _ _ _ _ _ _
1 _ _ _ _ _ _ _
_ 1 1 1 1 1 1 1
1 _ _ _ _ _ _ _
AND then together:
unsigned long long valid_rook_moves = checkBitboard & rook_attacks;
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 0 0 0
0 0 0 0 0 0 0 0
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ 1 _ _ _
_ _ _ _ _ _ _ _
Only these moves are valid.
@@ -519,14 +517,14 @@ To remove the least significant bit we use this method:
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:
@@ -538,43 +536,43 @@ 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
_ 1 1 1 1 1 1 1
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
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 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
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
=
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
_ _ _ _ _ _ _ _
_ 1 1 1 1 1 1 1
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _