Bitboards in Rust

Introduction

Hello there! I haven’t posted in a while because I was working on my chess engine. Specifically, representing the pieces as bitboards. Now you may ask what a bitboard is. You can use bitboards to represent a chess board in a piece-centric manner. What is so special about bitboards then? Doesn’t a piece list do the same thing? Well, representing a bitboard only requires a single unsigned 64 bit integer!

As taken from the chessprogramming wiki:

To represent the board we typically need one bitboard for each piece-type and color - likely encapsulated inside a class or structure, or as an array of bitboards as part of a position object. A one-bit inside a bitboard implies the existence of a piece of this piece-type on a certain square - one to one associated by the bit-position.

For example, a bitboard containing all of white’s pawns in the starting position would look like this:

. . . . . . . .
. . . . . . . .
. . . . . . . .
. . . . . . . .
. . . . . . . .
. . . . . . . .
1 1 1 1 1 1 1 1
. . . . . . . .

As you can observe, the squares containing the pawns have a value of 1 and others have a value of 0. A bitboard representing a particular piece will have 1 in the squares the piece occupies and 0 in all the other squares.

Before proceding further, I’d like to shed light on the fact that I’m fairly new to the chess engine development scene, so all suggestions are welcome!

Implementation

I decided to go with representing all the bitboards in a single position as a struct.

#[derive(Debug, Clone)]
struct BitPos {
	wp: BitBoard, // white pawns
	wn: BitBoard, // white knights
	wb: BitBoard, // white bishops
	wr: BitBoard, // white rooks
	wq: BitBoard, // white queen
	wk: BitBoard, // white king
	bp: BitBoard, // black pawns
	bn: BitBoard, // black knights
	bb: BitBoard, // black bishops
	br: BitBoard, // black rooks
	bq: BitBoard, // black queen
	bk: BitBoard, // black king
}

BitBoard is a struct containing a u64.

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
struct BitBoard(pub u64);

Then I wrote a simple function to generate a bitboard from my piece list array (see fen-string-parsing-in-rust)

fn gen(pieces: &mut [Option<Piece>; 64], compare: char) -> Self {
    let mut bin_str = String::new();
    // reverse pieces array
    pieces.reverse();

    for piece in pieces {
        if let Some(piece) = piece {
            if !(piece.symbol == compare) {
                bin_str.push('0');
            } else {
                bin_str.push('1');
            }
        } else {
            bin_str.push('0');
        }
    }

    // convert binary string to decimal (u64)
    Self(u64::from_str_radix(&bin_str, 2).unwrap())
}

And another function to print out the bitboard:

fn print(&self) {
    println!("Value: {}", &self.0);
    println!();
    let mut x = 8;
    for rank in 0..8 {
        print!("\x1b[34m{}\x1b[0m  ", x);
        x -= 1;
        for file in 0..8 {
            let square = rank * 8 + file;
            if self.0 & (1 << square) >= 1 {
                print!("\x1b[1m1\x1b[0m ");
            } else {
                print!("\x1b[38;5;8m0\x1b[0m ");
            }
        }
        println!();
    }
    println!();
    println!("\x1b[34m   a b c d e f g h\x1b[0m");
    println!();
}

And some other functions for convenience:

// generate bitboard from square
fn from_sq(square: Square) -> Self {
    Self(1 << square as u8)
}
// init empty bitboard
fn empty() -> Self {
    Self(0)
}
// set bit at given square (0 -> 1)
fn set_bit(&mut self, square: Square) {
    self.0 |= BitBoard::from_sq(square).0;
}
// toggle bit at given square
fn toggle_bit(&mut self, square: Square) {
    self.0 ^= BitBoard::from_sq(square).0;
}

What fascinated me is the fact that how easy it was to perform actions on bitboards.

I also made an enum to represent the squares (just for convenience and better code readability).

// to represent squares
#[rustfmt::skip]
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
enum Square {
    A8, B8, C8, D8, E8, F8, G8, H8,
    A7, B7, C7, D7, E7, F7, G7, H7,
    A6, B6, C6, D6, E6, F6, G6, H6,
    A5, B5, C5, D5, E5, F5, G5, H5,
    A4, B4, C4, D4, E4, F4, G4, H4,
    A3, B3, C3, D3, E3, F3, G3, H3,
    A2, B2, C2, D2, E2, F2, G2, H2,
    A1, B1, C1, D1, E1, F1, G1, H1,
}

Piece attack generation

I found this very interesting. I still haven’t finished working on move generation for sliding pieces at the time of writing this.

Anyways, for generating pawn attacks, you can shift the bitboard by 7 and 9 in specific directions to achieve the desired result. That is, capturing diagonally.

For example:

const NOT_A_FILE: BitBoard = BitBoard(18374403900871474942);
const NOT_H_FILE: BitBoard = BitBoard(9187201950435737471);

pub fn east(board: BitBoard, colour: Colour) -> BitBoard {
    match colour {
        Colour::White => BitBoard((board.0 >> 9) & NOT_H_FILE.0),
        Colour::Black => BitBoard((board.0 << 9) & NOT_A_FILE.0),
        Colour::Undefined => exit(1),
    }
}

pub fn west(board: BitBoard, colour: Colour) -> BitBoard {
    match colour {
        Colour::White => BitBoard((board.0 >> 7) & NOT_A_FILE.0),
        Colour::Black => BitBoard((board.0 << 7) & NOT_H_FILE.0),
        Colour::Undefined => exit(1),
    }
}

I also a made a function for convenience (lookup pawn attacks for a particular square):

pub fn lookup(square: Square, colour: Colour) -> BitBoard {
    let board = BitBoard::from_sq(square);

    match colour {
        Colour::White => BitBoard(east(board, Colour::White).0 | west(board, Colour::White).0),
        Colour::Black => BitBoard(east(board, Colour::Black).0 | west(board, Colour::Black).0),
        Colour::Undefined => exit(1),
    }
}

And another one for all pawns:

pub fn all(board: BitBoard, colour: Colour) -> BitBoard {
    match colour {
        Colour::White => BitBoard(east(board, Colour::White).0 | west(board, Colour::White).0),
        Colour::Black => BitBoard(east(board, Colour::Black).0 | west(board, Colour::Black).0),
        Colour::Undefined => exit(1),
    }
}

Attacks can be generated in the same manner for knights and kings:

Knights

        noNoWe    noNoEa
            +15  +17
             |     |
noWeWe  +6 __|     |__+10  noEaEa
              \   /
               >0<
           __ /   \ __
soWeWe -10   |     |   -6  soEaEa
             |     |
            -17  -15
        soSoWe    soSoEa

const NOT_HG_FILE: BitBoard = BitBoard(4557430888798830399);
const NOT_AB_FILE: BitBoard = BitBoard(18229723555195321596);

pub fn no_no_east(board: BitBoard) -> BitBoard {
    BitBoard((board.0 << 17) & NOT_A_FILE.0)
}

pub fn no_no_west(board: BitBoard) -> BitBoard {
    BitBoard((board.0 << 15) & NOT_H_FILE.0)
}

pub fn so_so_east(board: BitBoard) -> BitBoard {
    BitBoard((board.0 >> 15) & NOT_A_FILE.0)
}

pub fn so_so_west(board: BitBoard) -> BitBoard {
    BitBoard((board.0 >> 17) & NOT_H_FILE.0)
}

pub fn no_ea_east(board: BitBoard) -> BitBoard {
    BitBoard((board.0 << 10) & NOT_AB_FILE.0)
}

pub fn so_ea_east(board: BitBoard) -> BitBoard {
    BitBoard((board.0 >> 6) & NOT_AB_FILE.0)
}

pub fn no_we_west(board: BitBoard) -> BitBoard {
    BitBoard((board.0 << 6) & NOT_HG_FILE.0)
}

pub fn so_we_west(board: BitBoard) -> BitBoard {
    BitBoard((board.0 >> 10) & NOT_HG_FILE.0)
}

// and then just get the union of all the other boards
pub fn knight(sq: Square) -> BitBoard {
    let board = BitBoard::from_sq(sq);

    let no_no_east = knight::no_no_east(board);
    let no_no_west = knight::no_no_west(board);
    let so_so_east = knight::so_so_east(board);
    let so_so_west = knight::so_so_west(board);
    let no_ea_east = knight::no_ea_east(board);
    let so_ea_east = knight::so_ea_east(board);
    let no_we_west = knight::no_we_west(board);
    let so_we_west = knight::so_we_west(board);

    BitBoard(
        no_no_east.0
            | no_no_west.0
            | so_so_east.0
            | so_so_west.0
            | no_ea_east.0
            | so_ea_east.0
            | no_we_west.0
            | so_we_west.0,
    )
}

Result (knight on e4):

8  . . . . . . . . 
7  . . . . . . . . 
6  . . . 1 . 1 . . 
5  . . 1 . . . 1 . 
4  . . . . . . . . 
3  . . 1 . . . 1 . 
2  . . . 1 . 1 . . 
1  . . . . . . . . 

   a b c d e f g h

King


// north
pub fn no(board: BitBoard) -> BitBoard {
    BitBoard(board.0 << 8)
}

// south
pub fn so(board: BitBoard) -> BitBoard {
    BitBoard(board.0 >> 8)
}

// east
pub fn ea(board: BitBoard) -> BitBoard {
    BitBoard((board.0 << 1) & NOT_A_FILE.0)
}

// west
pub fn we(board: BitBoard) -> BitBoard {
    BitBoard((board.0 >> 1) & NOT_H_FILE.0)
}

// north east
pub fn no_ea(board: BitBoard) -> BitBoard {
    BitBoard((board.0 << 9) & NOT_A_FILE.0)
}

// north west
pub fn no_we(board: BitBoard) -> BitBoard {
    BitBoard((board.0 << 7) & NOT_H_FILE.0)
}

// south east
pub fn so_ea(board: BitBoard) -> BitBoard {
    BitBoard((board.0 >> 7) & NOT_A_FILE.0)
}

// south west
pub fn so_we(board: BitBoard) -> BitBoard {
    BitBoard((board.0 >> 9) & NOT_H_FILE.0)
}

pub fn king(sq: Square) -> BitBoard {
    let board = BitBoard::from_sq(sq);

    let no = king::no(board);
    let so = king::so(board);
    let ea = king::ea(board);
    let we = king::we(board);
    let no_ea = king::no_ea(board);
    let no_we = king::no_we(board);
    let so_ea = king::so_ea(board);
    let so_we = king::so_we(board);

    BitBoard(no.0 | so.0 | ea.0 | we.0 | no_ea.0 | no_we.0 | so_ea.0 | so_we.0)
}

Result (king on e4):

8  . . . . . . . . 
7  . . . . . . . . 
6  . . . . . . . . 
5  . . . 1 1 1 . . 
4  . . . 1 . 1 . . 
3  . . . 1 1 1 . . 
2  . . . . . . . . 
1  . . . . . . . . 

   a b c d e f g h

That’s it for this blog, see you soon!

Introduction#

Implementation#

Piece attack generation#

Knights#

King#

References#

Introduction

Implementation

Piece attack generation

Knights

King

References