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ladybird/Libraries/LibChess/Chess.cpp
Brendan Coles d4c34d50c9 LibChess: Forbid King moving into check by a pawn on the home rank 
A player can no longer move the King piece into any position on
their home rank if the move would place the King in check.

A player can also no longer ignore a check position when in check
by a pawn on their home rank. The player must now resolve the check
during their move.
2020-10-30 16:51:41 +01:00

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/*
* Copyright (c) 2020, the SerenityOS developers.
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are met:
*
* 1. Redistributions of source code must retain the above copyright notice, this
* list of conditions and the following disclaimer.
*
* 2. Redistributions in binary form must reproduce the above copyright notice,
* this list of conditions and the following disclaimer in the documentation
* and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
* DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
* SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
* CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
* OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
#include <AK/Assertions.h>
#include <AK/LogStream.h>
#include <AK/String.h>
#include <AK/StringBuilder.h>
#include <AK/Vector.h>
#include <LibChess/Chess.h>
#include <stdlib.h>
namespace Chess {
String char_for_piece(Chess::Type type)
{
switch (type) {
case Type::Knight:
return "N";
case Type::Bishop:
return "B";
case Type::Rook:
return "R";
case Type::Queen:
return "Q";
case Type::King:
return "K";
case Type::Pawn:
default:
return "";
}
}
Chess::Type piece_for_char_promotion(const StringView& str)
{
String string = String(str).to_lowercase();
if (string == "")
return Type::None;
if (string == "n")
return Type::Knight;
if (string == "b")
return Type::Bishop;
if (string == "r")
return Type::Rook;
if (string == "q")
return Type::Queen;
if (string == "k")
return Type::King;
return Type::None;
}
Colour opposing_colour(Colour colour)
{
return (colour == Colour::White) ? Colour::Black : Colour::White;
}
Square::Square(const StringView& name)
{
ASSERT(name.length() == 2);
char filec = name[0];
char rankc = name[1];
if (filec >= 'a' && filec <= 'h') {
file = filec - 'a';
} else if (filec >= 'A' && filec <= 'H') {
file = filec - 'A';
} else {
ASSERT_NOT_REACHED();
}
if (rankc >= '1' && rankc <= '8') {
rank = rankc - '1';
} else {
ASSERT_NOT_REACHED();
}
}
String Square::to_algebraic() const
{
StringBuilder builder;
builder.append(file + 'a');
builder.append(rank + '1');
return builder.build();
}
Move::Move(const StringView& algebraic)
: from(algebraic.substring_view(0, 2))
, to(algebraic.substring_view(2, 2))
, promote_to(piece_for_char_promotion((algebraic.length() >= 5) ? algebraic.substring_view(4, 1) : ""))
{
}
String Move::to_long_algebraic() const
{
StringBuilder builder;
builder.append(from.to_algebraic());
builder.append(to.to_algebraic());
builder.append(char_for_piece(promote_to).to_lowercase());
return builder.build();
}
Board::Board()
{
// Fill empty spaces.
for (unsigned rank = 2; rank < 6; ++rank) {
for (unsigned file = 0; file < 8; ++file) {
set_piece({ rank, file }, EmptyPiece);
}
}
// Fill white pawns.
for (unsigned file = 0; file < 8; ++file) {
set_piece({ 1, file }, { Colour::White, Type::Pawn });
}
// Fill black pawns.
for (unsigned file = 0; file < 8; ++file) {
set_piece({ 6, file }, { Colour::Black, Type::Pawn });
}
// Fill while pieces.
set_piece(Square("a1"), { Colour::White, Type::Rook });
set_piece(Square("b1"), { Colour::White, Type::Knight });
set_piece(Square("c1"), { Colour::White, Type::Bishop });
set_piece(Square("d1"), { Colour::White, Type::Queen });
set_piece(Square("e1"), { Colour::White, Type::King });
set_piece(Square("f1"), { Colour::White, Type::Bishop });
set_piece(Square("g1"), { Colour::White, Type::Knight });
set_piece(Square("h1"), { Colour::White, Type::Rook });
// Fill black pieces.
set_piece(Square("a8"), { Colour::Black, Type::Rook });
set_piece(Square("b8"), { Colour::Black, Type::Knight });
set_piece(Square("c8"), { Colour::Black, Type::Bishop });
set_piece(Square("d8"), { Colour::Black, Type::Queen });
set_piece(Square("e8"), { Colour::Black, Type::King });
set_piece(Square("f8"), { Colour::Black, Type::Bishop });
set_piece(Square("g8"), { Colour::Black, Type::Knight });
set_piece(Square("h8"), { Colour::Black, Type::Rook });
}
Piece Board::get_piece(const Square& square) const
{
ASSERT(square.rank < 8);
ASSERT(square.file < 8);
return m_board[square.rank][square.file];
}
Piece Board::set_piece(const Square& square, const Piece& piece)
{
ASSERT(square.rank < 8);
ASSERT(square.file < 8);
return m_board[square.rank][square.file] = piece;
}
bool Board::is_legal_promotion(const Move& move, Colour colour) const
{
auto piece = get_piece(move.from);
if (move.promote_to == Type::Pawn || move.promote_to == Type::King) {
// attempted promotion to invalid piece
return false;
}
if (piece.type != Type::Pawn && move.promote_to != Type::None) {
// attempted promotion from invalid piece
return false;
}
unsigned promotion_rank = (colour == Colour::White) ? 7 : 0;
if (move.to.rank != promotion_rank && move.promote_to != Type::None) {
// attempted promotion from invalid rank
return false;
}
if (piece.type == Type::Pawn && move.to.rank == promotion_rank && move.promote_to == Type::None) {
// attempted move to promotion rank without promoting
return false;
}
return true;
}
bool Board::is_legal(const Move& move, Colour colour) const
{
if (colour == Colour::None)
colour = turn();
if (!is_legal_no_check(move, colour))
return false;
if (!is_legal_promotion(move, colour))
return false;
Board clone = *this;
clone.apply_illegal_move(move, colour);
if (clone.in_check(colour))
return false;
// Don't allow castling through check or out of check.
Vector<Square> check_squares;
if (colour == Colour::White && move.from == Square("e1") && get_piece(Square("e1")) == Piece(Colour::White, Type::King)) {
if (move.to == Square("a1") || move.to == Square("c1")) {
check_squares = { Square("e1"), Square("d1"), Square("c1") };
} else if (move.to == Square("h1") || move.to == Square("g1")) {
check_squares = { Square("e1"), Square("f1"), Square("g1") };
}
} else if (colour == Colour::Black && move.from == Square("e8") && get_piece(Square("e8")) == Piece(Colour::Black, Type::King)) {
if (move.to == Square("a8") || move.to == Square("c8")) {
check_squares = { Square("e8"), Square("d8"), Square("c8") };
} else if (move.to == Square("h8") || move.to == Square("g8")) {
check_squares = { Square("e8"), Square("f8"), Square("g8") };
}
}
for (auto& square : check_squares) {
Board clone = *this;
clone.set_piece(move.from, EmptyPiece);
clone.set_piece(square, { colour, Type::King });
if (clone.in_check(colour))
return false;
}
return true;
}
bool Board::is_legal_no_check(const Move& move, Colour colour) const
{
auto piece = get_piece(move.from);
if (piece.colour != colour)
// attempted move of opponent's piece
return false;
if (move.to.rank > 7 || move.to.file > 7)
// attempted move outside of board
return false;
if (piece.type == Type::Pawn) {
int dir = (colour == Colour::White) ? +1 : -1;
unsigned start_rank = (colour == Colour::White) ? 1 : 6;
if (move.from.rank == start_rank && move.to.rank == move.from.rank + (2 * dir) && move.to.file == move.from.file
&& get_piece(move.to).type == Type::None && get_piece({ move.from.rank + dir, move.from.file }).type == Type::None) {
// 2 square pawn move from initial position.
return true;
}
if (move.to.rank != move.from.rank + dir)
// attempted backwards or sideways move
return false;
if (move.to.file == move.from.file && get_piece(move.to).type == Type::None) {
// Regular pawn move.
return true;
}
if (move.to.file == move.from.file + 1 || move.to.file == move.from.file - 1) {
unsigned other_start_rank = (colour == Colour::White) ? 6 : 1;
unsigned en_passant_rank = (colour == Colour::White) ? 4 : 3;
Move en_passant_last_move = { { other_start_rank, move.to.file }, { en_passant_rank, move.to.file } };
if (get_piece(move.to).colour == opposing_colour(colour)) {
// Pawn capture.
return true;
}
if (m_last_move.has_value() && move.from.rank == en_passant_rank && m_last_move.value() == en_passant_last_move
&& get_piece(en_passant_last_move.to) == Piece(opposing_colour(colour), Type::Pawn)) {
// En passant.
return true;
}
}
return false;
} else if (piece.type == Type::Knight) {
int rank_delta = abs(move.to.rank - move.from.rank);
int file_delta = abs(move.to.file - move.from.file);
if (get_piece(move.to).colour != colour && max(rank_delta, file_delta) == 2 && min(rank_delta, file_delta) == 1) {
return true;
}
} else if (piece.type == Type::Bishop) {
int rank_delta = move.to.rank - move.from.rank;
int file_delta = move.to.file - move.from.file;
if (abs(rank_delta) == abs(file_delta)) {
int dr = rank_delta / abs(rank_delta);
int df = file_delta / abs(file_delta);
for (Square sq = move.from; sq != move.to; sq.rank += dr, sq.file += df) {
if (get_piece(sq).type != Type::None && sq != move.from) {
return false;
}
}
if (get_piece(move.to).colour != colour) {
return true;
}
}
} else if (piece.type == Type::Rook) {
int rank_delta = move.to.rank - move.from.rank;
int file_delta = move.to.file - move.from.file;
if (rank_delta == 0 || file_delta == 0) {
int dr = (rank_delta) ? rank_delta / abs(rank_delta) : 0;
int df = (file_delta) ? file_delta / abs(file_delta) : 0;
for (Square sq = move.from; sq != move.to; sq.rank += dr, sq.file += df) {
if (get_piece(sq).type != Type::None && sq != move.from) {
return false;
}
}
if (get_piece(move.to).colour != colour) {
return true;
}
}
} else if (piece.type == Type::Queen) {
int rank_delta = move.to.rank - move.from.rank;
int file_delta = move.to.file - move.from.file;
if (abs(rank_delta) == abs(file_delta) || rank_delta == 0 || file_delta == 0) {
int dr = (rank_delta) ? rank_delta / abs(rank_delta) : 0;
int df = (file_delta) ? file_delta / abs(file_delta) : 0;
for (Square sq = move.from; sq != move.to; sq.rank += dr, sq.file += df) {
if (get_piece(sq).type != Type::None && sq != move.from) {
return false;
}
}
if (get_piece(move.to).colour != colour) {
return true;
}
}
} else if (piece.type == Type::King) {
int rank_delta = move.to.rank - move.from.rank;
int file_delta = move.to.file - move.from.file;
if (abs(rank_delta) <= 1 && abs(file_delta) <= 1) {
if (get_piece(move.to).colour != colour) {
return true;
}
}
if (colour == Colour::White) {
if ((move.to == Square("a1") || move.to == Square("c1")) && m_white_can_castle_queenside && get_piece(Square("b1")).type == Type::None && get_piece(Square("c1")).type == Type::None && get_piece(Square("d1")).type == Type::None) {
return true;
} else if ((move.to == Square("h1") || move.to == Square("g1")) && m_white_can_castle_kingside && get_piece(Square("f1")).type == Type::None && get_piece(Square("g1")).type == Type::None) {
return true;
}
} else {
if ((move.to == Square("a8") || move.to == Square("c8")) && m_black_can_castle_queenside && get_piece(Square("b8")).type == Type::None && get_piece(Square("c8")).type == Type::None && get_piece(Square("d8")).type == Type::None) {
return true;
} else if ((move.to == Square("h8") || move.to == Square("g8")) && m_black_can_castle_kingside && get_piece(Square("f8")).type == Type::None && get_piece(Square("g8")).type == Type::None) {
return true;
}
}
}
return false;
}
bool Board::in_check(Colour colour) const
{
Square king_square = { 50, 50 };
Square::for_each([&](const Square& square) {
if (get_piece(square) == Piece(colour, Type::King)) {
king_square = square;
return IterationDecision::Break;
}
return IterationDecision::Continue;
});
bool check = false;
Square::for_each([&](const Square& square) {
if (is_legal_no_check({ square, king_square }, opposing_colour(colour))) {
check = true;
return IterationDecision::Break;
}
return IterationDecision::Continue;
});
return check;
}
bool Board::apply_move(const Move& move, Colour colour)
{
if (colour == Colour::None)
colour = turn();
if (!is_legal(move, colour))
return false;
return apply_illegal_move(move, colour);
}
bool Board::apply_illegal_move(const Move& move, Colour colour)
{
Board clone = *this;
clone.m_previous_states = {};
clone.m_moves = {};
auto state_count = 0;
if (m_previous_states.contains(clone))
state_count = m_previous_states.get(clone).value();
m_previous_states.set(clone, state_count + 1);
m_moves.append(move);
m_turn = opposing_colour(colour);
m_last_move = move;
m_moves_since_capture++;
if (move.from == Square("a1") || move.to == Square("a1") || move.from == Square("e1"))
m_white_can_castle_queenside = false;
if (move.from == Square("h1") || move.to == Square("h1") || move.from == Square("e1"))
m_white_can_castle_kingside = false;
if (move.from == Square("a8") || move.to == Square("a8") || move.from == Square("e8"))
m_black_can_castle_queenside = false;
if (move.from == Square("h8") || move.to == Square("h8") || move.from == Square("e8"))
m_black_can_castle_kingside = false;
if (colour == Colour::White && move.from == Square("e1") && get_piece(Square("e1")) == Piece(Colour::White, Type::King)) {
if (move.to == Square("a1") || move.to == Square("c1")) {
set_piece(Square("e1"), EmptyPiece);
set_piece(Square("a1"), EmptyPiece);
set_piece(Square("c1"), { Colour::White, Type::King });
set_piece(Square("d1"), { Colour::White, Type::Rook });
return true;
} else if (move.to == Square("h1") || move.to == Square("g1")) {
set_piece(Square("e1"), EmptyPiece);
set_piece(Square("h1"), EmptyPiece);
set_piece(Square("g1"), { Colour::White, Type::King });
set_piece(Square("f1"), { Colour::White, Type::Rook });
return true;
}
} else if (colour == Colour::Black && move.from == Square("e8") && get_piece(Square("e8")) == Piece(Colour::Black, Type::King)) {
if (move.to == Square("a8") || move.to == Square("c8")) {
set_piece(Square("e8"), EmptyPiece);
set_piece(Square("a8"), EmptyPiece);
set_piece(Square("c8"), { Colour::Black, Type::King });
set_piece(Square("d8"), { Colour::Black, Type::Rook });
return true;
} else if (move.to == Square("h8") || move.to == Square("g8")) {
set_piece(Square("e8"), EmptyPiece);
set_piece(Square("h8"), EmptyPiece);
set_piece(Square("g8"), { Colour::Black, Type::King });
set_piece(Square("f8"), { Colour::Black, Type::Rook });
return true;
}
}
if (get_piece(move.from).type == Type::Pawn && ((colour == Colour::Black && move.to.rank == 0) || (colour == Colour::White && move.to.rank == 7))) {
// Pawn Promotion
set_piece(move.to, { colour, move.promote_to });
set_piece(move.from, EmptyPiece);
return true;
}
if (get_piece(move.from).type == Type::Pawn && move.from.file != move.to.file && get_piece(move.to).type == Type::None) {
// En passant.
if (colour == Colour::White) {
set_piece({ move.to.rank - 1, move.to.file }, EmptyPiece);
} else {
set_piece({ move.to.rank + 1, move.to.file }, EmptyPiece);
}
m_moves_since_capture = 0;
}
if (get_piece(move.to).colour != Colour::None)
m_moves_since_capture = 0;
set_piece(move.to, get_piece(move.from));
set_piece(move.from, EmptyPiece);
return true;
}
Move Board::random_move(Colour colour) const
{
if (colour == Colour::None)
colour = turn();
Move move = { { 50, 50 }, { 50, 50 } };
int probability = 1;
generate_moves([&](Move m) {
if (rand() % probability == 0)
move = m;
++probability;
return IterationDecision::Continue;
});
return move;
}
Board::Result Board::game_result() const
{
bool sufficient_material = false;
bool no_more_pieces_allowed = false;
Optional<Square> bishop;
Square::for_each([&](Square sq) {
if (get_piece(sq).type == Type::Queen || get_piece(sq).type == Type::Rook || get_piece(sq).type == Type::Pawn) {
sufficient_material = true;
return IterationDecision::Break;
}
if (get_piece(sq).type != Type::None && get_piece(sq).type != Type::King && no_more_pieces_allowed) {
sufficient_material = true;
return IterationDecision::Break;
}
if (get_piece(sq).type == Type::Knight)
no_more_pieces_allowed = true;
if (get_piece(sq).type == Type::Bishop) {
if (bishop.has_value()) {
if (get_piece(sq).colour == get_piece(bishop.value()).colour) {
sufficient_material = true;
return IterationDecision::Break;
} else if (sq.is_light() != bishop.value().is_light()) {
sufficient_material = true;
return IterationDecision::Break;
}
no_more_pieces_allowed = true;
} else {
bishop = sq;
}
}
return IterationDecision::Continue;
});
if (!sufficient_material)
return Result::InsufficientMaterial;
bool are_legal_moves = false;
generate_moves([&](Move m) {
(void)m;
are_legal_moves = true;
return IterationDecision::Break;
});
if (are_legal_moves) {
if (m_moves_since_capture >= 75 * 2)
return Result::SeventyFiveMoveRule;
if (m_moves_since_capture == 50 * 2)
return Result::FiftyMoveRule;
auto repeats = m_previous_states.get(*this);
if (repeats.has_value()) {
if (repeats.value() == 3)
return Result::ThreeFoldRepetition;
if (repeats.value() >= 5)
return Result::FiveFoldRepetition;
}
return Result::NotFinished;
}
if (in_check(turn()))
return Result::CheckMate;
return Result::StaleMate;
}
Colour Board::game_winner() const
{
if (game_result() == Result::CheckMate)
return opposing_colour(turn());
return Colour::None;
}
int Board::game_score() const
{
switch (game_winner()) {
case Colour::White:
return +1;
case Colour::Black:
return -1;
case Colour::None:
return 0;
}
return 0;
}
bool Board::game_finished() const
{
return game_result() != Result::NotFinished;
}
int Board::material_imbalance() const
{
int imbalance = 0;
Square::for_each([&](Square square) {
int value = 0;
switch (get_piece(square).type) {
case Type::Pawn:
value = 1;
break;
case Type::Knight:
case Type::Bishop:
value = 3;
break;
case Type::Rook:
value = 5;
break;
case Type::Queen:
value = 9;
break;
default:
break;
}
if (get_piece(square).colour == Colour::White) {
imbalance += value;
} else {
imbalance -= value;
}
return IterationDecision::Continue;
});
return imbalance;
}
bool Board::is_promotion_move(const Move& move, Colour colour) const
{
if (colour == Colour::None)
colour = turn();
unsigned promotion_rank = (colour == Colour::White) ? 7 : 0;
if (move.to.rank != promotion_rank)
return false;
if (get_piece(move.from).type != Type::Pawn)
return false;
Move queen_move = move;
queen_move.promote_to = Type::Queen;
if (!is_legal(queen_move, colour))
return false;
return true;
}
bool Board::operator==(const Board& other) const
{
bool equal_squares = true;
Square::for_each([&](Square sq) {
if (get_piece(sq) != other.get_piece(sq)) {
equal_squares = false;
return IterationDecision::Break;
}
return IterationDecision::Continue;
});
if (!equal_squares)
return false;
if (m_white_can_castle_queenside != other.m_white_can_castle_queenside)
return false;
if (m_white_can_castle_kingside != other.m_white_can_castle_kingside)
return false;
if (m_black_can_castle_queenside != other.m_black_can_castle_queenside)
return false;
if (m_black_can_castle_kingside != other.m_black_can_castle_kingside)
return false;
return turn() == other.turn();
}
}
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