Stockfish/src/tools/sfen_packer.cpp

#include "sfen_packer.h"

#include "packed_sfen.h"

#include "misc.h"
#include "position.h"

#include <sstream>
#include <fstream>
#include <cstring> // std::memset()

using namespace std;

namespace Tools {

    // Class that handles bitstream
    // useful when doing aspect encoding
    struct BitStream
    {
        // Set the memory to store the data in advance.
        // Assume that memory is cleared to 0.
        void set_data(std::uint8_t* data_) { data = data_; reset(); }

        // Get the pointer passed in set_data().
        uint8_t* get_data() const { return data; }

        // Get the cursor.
        int get_cursor() const { return bit_cursor; }

        // reset the cursor
        void reset() { bit_cursor = 0; }

        // Write 1bit to the stream.
        // If b is non-zero, write out 1. If 0, write 0.
        void write_one_bit(int b)
        {
            if (b)
                data[bit_cursor / 8] |= 1 << (bit_cursor & 7);

            ++bit_cursor;
        }

        // Get 1 bit from the stream.
        int read_one_bit()
        {
            int b = (data[bit_cursor / 8] >> (bit_cursor & 7)) & 1;
            ++bit_cursor;

            return b;
        }

        // write n bits of data
        // Data shall be written out from the lower order of d.
        void write_n_bit(int d, int n)
        {
            for (int i = 0; i <n; ++i)
                write_one_bit(d & (1 << i));
        }

        // read n bits of data
        // Reverse conversion of write_n_bit().
        int read_n_bit(int n)
        {
            int result = 0;
            for (int i = 0; i < n; ++i)
                result |= read_one_bit() ? (1 << i) : 0;

            return result;
        }

    private:
        // Next bit position to read/write.
        int bit_cursor;

        // data entity
        std::uint8_t* data;
    };

    // Class for compressing/decompressing sfen
    // sfen can be packed to 256bit (32bytes) by Huffman coding.
    // This is proven by mini. The above is Huffman coding.
    //
    // Internal format = 1-bit turn + 7-bit king position *2 + piece on board (Huffman coding) + hand piece (Huffman coding)
    // Side to move (White = 0, Black = 1) (1bit)
    // White King Position (6 bits)
    // Black King Position (6 bits)
    // Huffman Encoding of the board
    // Castling availability (1 bit x 4)
    // En passant square (1 or 1 + 6 bits)
    // Rule 50 (6 bits)
    // Game play (8 bits)
    //
    // TODO(someone): Rename SFEN to FEN.
    //
    struct SfenPacker
    {
        void pack(const Position& pos);

        // sfen packed by pack() (256bit = 32bytes)
        // Or sfen to decode with unpack()
        uint8_t *data; // uint8_t[32];

        BitStream stream;

        // Output the board pieces to stream.
        void write_board_piece_to_stream(Piece pc);

        // Read one board piece from stream
        Piece read_board_piece_from_stream();
    };


    // Huffman coding
    // * is simplified from mini encoding to make conversion easier.
    //
    // Huffman Encoding
    //
    // Empty  xxxxxxx0
    // Pawn   xxxxx001 + 1 bit (Color)
    // Knight xxxxx011 + 1 bit (Color)
    // Bishop xxxxx101 + 1 bit (Color)
    // Rook   xxxxx111 + 1 bit (Color)
    // Queen   xxxx1001 + 1 bit (Color)
    //
    // Worst case:
    // - 32 empty squares    32 bits
    // - 30 pieces           150 bits
    // - 2 kings             12 bits
    // - castling rights     4 bits
    // - ep square           7 bits
    // - rule50              7 bits
    // - game ply            16 bits
    // - TOTAL               228 bits < 256 bits

    struct HuffmanedPiece
    {
        int code; // how it will be coded
        int bits; // How many bits do you have
    };

    constexpr HuffmanedPiece huffman_table[] =
    {
        {0b0000,1}, // NO_PIECE
        {0b0001,4}, // PAWN
        {0b0011,4}, // KNIGHT
        {0b0101,4}, // BISHOP
        {0b0111,4}, // ROOK
        {0b1001,4}, // QUEEN
    };

    // Pack sfen and store in data[32].
    void SfenPacker::pack(const Position& pos)
    {
        memset(data, 0, 32 /* 256bit */);
        stream.set_data(data);

        // turn
        // Side to move.
        stream.write_one_bit((int)(pos.side_to_move()));

        // 7-bit positions for leading and trailing balls
        // White king and black king, 6 bits for each.
        for(auto c: Colors)
            stream.write_n_bit(pos.king_square(c), 6);

        // Write the pieces on the board other than the kings.
        for (Rank r = RANK_8; r >= RANK_1; --r)
        {
            for (File f = FILE_A; f <= FILE_H; ++f)
            {
                Piece pc = pos.piece_on(make_square(f, r));
                if (type_of(pc) == KING)
                    continue;
                write_board_piece_to_stream(pc);
            }
        }

        // TODO(someone): Support chess960.
        stream.write_one_bit(pos.can_castle(WHITE_OO));
        stream.write_one_bit(pos.can_castle(WHITE_OOO));
        stream.write_one_bit(pos.can_castle(BLACK_OO));
        stream.write_one_bit(pos.can_castle(BLACK_OOO));

        if (pos.ep_square() == SQ_NONE) {
            stream.write_one_bit(0);
        }
        else {
            stream.write_one_bit(1);
            stream.write_n_bit(static_cast<int>(pos.ep_square()), 6);
        }

        stream.write_n_bit(pos.state()->rule50, 6);

        const int fm = 1 + (pos.game_ply()-(pos.side_to_move() == BLACK)) / 2;
        stream.write_n_bit(fm, 8);

        // Write high bits of half move. This is a fix for the
        // limited range of half move counter.
        // This is backwards compatibile.
        stream.write_n_bit(fm >> 8, 8);

        // Write the highest bit of rule50 at the end. This is a backwards
        // compatibile fix for rule50 having only 6 bits stored.
        // This bit is just ignored by the old parsers.
        stream.write_n_bit(pos.state()->rule50 >> 6, 1);

        assert(stream.get_cursor() <= 256);
    }

    // Output the board pieces to stream.
    void SfenPacker::write_board_piece_to_stream(Piece pc)
    {
        // piece type
        PieceType pr = type_of(pc);
        auto c = huffman_table[pr];
        stream.write_n_bit(c.code, c.bits);

        if (pc == NO_PIECE)
            return;

        // first and second flag
        stream.write_one_bit(color_of(pc));
    }

    // Read one board piece from stream
    Piece SfenPacker::read_board_piece_from_stream()
    {
        PieceType pr = NO_PIECE_TYPE;
        int code = 0, bits = 0;
        while (true)
        {
            code |= stream.read_one_bit() << bits;
            ++bits;

            assert(bits <= 6);

            for (pr = NO_PIECE_TYPE; pr <KING; ++pr)
                if (huffman_table[pr].code == code
                    && huffman_table[pr].bits == bits)
                    goto Found;
        }
    Found:;
        if (pr == NO_PIECE_TYPE)
            return NO_PIECE;

        // first and second flag
        Color c = (Color)stream.read_one_bit();

        return make_piece(c, pr);
    }

    int set_from_packed_sfen(Position& pos, const PackedSfen& sfen, StateInfo* si, Thread* th)
    {
        SfenPacker packer;
        auto& stream = packer.stream;

        // TODO: separate streams for writing and reading. Here we actually have to
        // const_cast which is not safe in the long run.
        stream.set_data(const_cast<uint8_t*>(reinterpret_cast<const uint8_t*>(&sfen)));

        pos.clear();
        std::memset(si, 0, sizeof(StateInfo));
        std::fill_n(&pos.pieceList[0][0], sizeof(pos.pieceList) / sizeof(Square), SQ_NONE);
        pos.st = si;

        // Active color
        pos.sideToMove = (Color)stream.read_one_bit();

        pos.pieceList[W_KING][0] = SQUARE_NB;
        pos.pieceList[B_KING][0] = SQUARE_NB;

        // First the position of the ball
        for (auto c : Colors)
            pos.board[stream.read_n_bit(6)] = make_piece(c, KING);

        // Piece placement
        for (Rank r = RANK_8; r >= RANK_1; --r)
        {
            for (File f = FILE_A; f <= FILE_H; ++f)
            {
                auto sq = make_square(f, r);

                // it seems there are already balls
                Piece pc;
                if (type_of(pos.board[sq]) != KING)
                {
                    assert(pos.board[sq] == NO_PIECE);
                    pc = packer.read_board_piece_from_stream();
                }
                else
                {
                    pc = pos.board[sq];
                    // put_piece() will catch ASSERT unless you remove it all.
                    pos.board[sq] = NO_PIECE;
                }

                // There may be no pieces, so skip in that case.
                if (pc == NO_PIECE)
                    continue;

                pos.put_piece(Piece(pc), sq);

                if (stream.get_cursor()> 256)
                    return 1;
            }
        }

        // Castling availability.
        // TODO(someone): Support chess960.
        pos.st->castlingRights = 0;
        if (stream.read_one_bit()) {
            Square rsq;
            for (rsq = relative_square(WHITE, SQ_H1); pos.piece_on(rsq) != W_ROOK; --rsq) {}
            pos.set_castling_right(WHITE, rsq);
        }
        if (stream.read_one_bit()) {
            Square rsq;
            for (rsq = relative_square(WHITE, SQ_A1); pos.piece_on(rsq) != W_ROOK; ++rsq) {}
            pos.set_castling_right(WHITE, rsq);
        }
        if (stream.read_one_bit()) {
            Square rsq;
            for (rsq = relative_square(BLACK, SQ_H1); pos.piece_on(rsq) != B_ROOK; --rsq) {}
            pos.set_castling_right(BLACK, rsq);
        }
        if (stream.read_one_bit()) {
            Square rsq;
            for (rsq = relative_square(BLACK, SQ_A1); pos.piece_on(rsq) != B_ROOK; ++rsq) {}
            pos.set_castling_right(BLACK, rsq);
        }

        // En passant square. Ignore if no pawn capture is possible
        if (stream.read_one_bit()) {
            Square ep_square = static_cast<Square>(stream.read_n_bit(6));
            pos.st->epSquare = ep_square;

            if (!(pos.attackers_to(pos.st->epSquare) & pos.pieces(pos.sideToMove, PAWN))
                || !(pos.pieces(~pos.sideToMove, PAWN) & (pos.st->epSquare + pawn_push(~pos.sideToMove))))
                pos.st->epSquare = SQ_NONE;
        }
        else {
            pos.st->epSquare = SQ_NONE;
        }

        // Halfmove clock
        pos.st->rule50 = stream.read_n_bit(6);

        // Fullmove number
        pos.gamePly = stream.read_n_bit(8);

        // Read the highest bit of rule50. This was added as a fix for rule50
        // counter having only 6 bits stored.
        // In older entries this will just be a zero bit.
        pos.gamePly |= stream.read_n_bit(8) << 8;

        // Read the highest bit of rule50. This was added as a fix for rule50
        // counter having only 6 bits stored.
        // In older entries this will just be a zero bit.
        pos.st->rule50 |= stream.read_n_bit(1) << 6;

        // Convert from fullmove starting from 1 to gamePly starting from 0,
        // handle also common incorrect FEN with fullmove = 0.
        pos.gamePly = std::max(2 * (pos.gamePly - 1), 0) + (pos.sideToMove == BLACK);

        assert(stream.get_cursor() <= 256);

        pos.chess960 = false;
        pos.thisThread = th;
        pos.set_state(pos.st);

        assert(pos.pos_is_ok());

        return 0;
    }

    PackedSfen sfen_pack(Position& pos)
    {
        PackedSfen sfen;

        SfenPacker sp;
        sp.data = (uint8_t*)&sfen;
        sp.pack(pos);

        return sfen;
    }
}