QBBEngine

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[ A KTM Quad 990 ATV [1] QBBEngine,

a didactic UCI compliant open source chess engine by Fabio Gobbato, written in C and published as single source file along with Pedone, which apparently uses a similar representation [2]. The QBBEngine demonstrates the use of a compact quad-bitboard structure to represent the board, and further applies alpha-beta search and an evaluation based on static values and piece-square tables [3]. The program performs a color agnostic move generation by flipping the board each time in make move.

C Structure


/*
Board structure definition

PM,P0,P1,P2 are the 4 bitboards that contain the whole board
PM is the bitboard with the side to move pieces
P0,P1 and P2: with these bitboards you can obtain every type of pieces and every pieces combinations.
*/
typedef struct
{
   TBB PM;
   TBB P0;
   TBB P1;
   TBB P2;
   uint8_t CastleFlags; /* ..sl..SL  short long opponent SHORT LONG side to move */
   uint8_t EnPassant; /* enpassant column, =8 if not set */
   uint8_t Count50; /* 50 move rule counter */
   uint8_t Rep; /* 0 if it's not a repetition, 1 if it is */
   uint8_t STM; /* side to move */
} TBoard;

Piece Coding

The board-definition with vertical nibbles as piece or empty square codes, i.e. the initial position:


Square
6
6
6
6
5
5
5
5
5
~
0
…
			0

3
2
1
0
9
8
7
6
5
~
8
7
6
5
4
3
2
1
0

Piece
r
n
b
k
q
b
n
r
p
~
P
R
N
B
K
Q
B
N
R

PM
0
0
0
0
0
0
0
0
0
~
1
1
1
1
1
1
1
1
1
Side to Move

P0
0
0
1
0
1
1
0
0
1
~
1
0
0
1
0
1
1
0
0
	P
.
B
.
Q

P1
0
1
1
1
0
1
1
0
0
~
0
0
1
1
1
0
1
1
0
		N
B
.
	K

P2
1
0
0
1
1
0
0
1
0
~
0
1
0
0
1
1
0
0
1
		.
.
R
Q
K


P2     RQK        P1    NB  K       P0   P B Q         PM side to move
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 1 1 1