mambax7 / chess

class/ChessGame.php

<?php

namespace XoopsModules\Chess;

// ------------------------------------------------------------------------- //
//  This program is free software; you can redistribute it and/or modify     //
//  it under the terms of the GNU General Public License as published by     //
//  the Free Software Foundation; either version 2 of the License, or        //
//  (at your option) any later version.                                      //
//                                                                           //
//  You may not change or alter any portion of this comment or credits       //
//  of supporting developers from this source code or any supporting         //
//  source code which is considered copyrighted (c) material of the          //
//  original comment or credit authors.                                      //
//                                                                           //
//  This program is distributed in the hope that it will be useful,          //
//  but WITHOUT ANY WARRANTY; without even the implied warranty of           //
//  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the            //
//  GNU General Public License for more details.                             //
//                                                                           //
//  You should have received a copy of the GNU General Public License        //
//  along with this program; if not, write to the Free Software              //
//  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307 USA //
//  ------------------------------------------------------------------------ //
//  Author: Dave Lerner <http://Dave-L.com>                                  //
//  ------------------------------------------------------------------------ //
//  Adapted from Online Chess Club (OCC) version 1.0.10, which was written   //
//  by Michael Speck <http://lgames.sf.net> and published under the GNU      //
//  General Public License.                                                  //
//  ------------------------------------------------------------------------ //

/**
 * class ChessGame
 *
 * @package    chess
 * @subpackage game
 */

/**
 * The purpose of this class is to handle chess moves.
 *
 * An instantiation of this class comprises the data essential for handling chess
 * moves in a specific game, and provides the requisite methods.
 *
 * - Input:      Game state and proposed move.
 * - Processing: Check the legality of the move, and update the game state if the move is legal.
 * - Output:     Indication of the move's legality, and the (possibly) updated game state.
 *
 * In addition to the above, there are utility methods for converting between Standard Algebraic
 * Notation (SAN) and a notation similar to Long Algebraic Notation.
 *
 * @package    chess
 * @subpackage game
 */
class ChessGame
{
    /**
     * Indicates whether object is valid.
     *
     * If empty string (''), indicates this is a valid object; otherwise contains an error message.
     * Should be checked after creating an instance of this class.
     *
     * @var string  $error
     */

    public $error;
    /**
     * gamestate
     *
     * The game state is represented as an array with the following elements:
     *
     *  - 'fen_piece_placement'
     *  - 'fen_active_color'
     *  - 'fen_castling_availability'
     *  - 'fen_en_passant_target_square'
     *  - 'fen_halfmove_clock'
     *  - 'fen_fullmove_number'
     *  - 'pgn_result'
     *  - 'pgn_fen'
     *  - 'pgn_movetext'
     *
     * The elements prefixed with 'fen_' are standard Forsyth-Edwards Notation (FEN) elements,
     * and the elements prefixed with 'pgn_' are standard Portable Game Notation (PGN) elements.
     *
     * Each element is a string.
     *
     * @var array $gamestate
     */

    public $gamestate;
    /**
     * board
     *
     * A 64-element array, constructed from fen_piece_placement, is used for handling moves.
     * Its indices are related to the standard tile coordinates as follows:
     *
     * <pre>
     * 8 | 56 57 58 59 60 61 62 63
     * 7 | 48 49 50 51 52 53 54 55
     * 6 | 40 41 42 43 44 45 46 47
     * 5 | 32 33 34 35 36 37 38 39
     * 4 | 24 25 26 27 28 29 30 31
     * 3 | 16 17 18 19 20 21 22 23
     * 2 |  8  9 10 11 12 13 14 15
     * 1 |  0  1  2  3  4  5  6  7
     *    ------------------------
     *      a  b  c  d  e  f  g  h
     * </pre>
     *
     * For example, $board[17] is tile b3 and $board[55] is tile h7.
     *
     * @var array $board
     */

    public $board;
    /**
     * for auto-completion of moves
     * @var string $ac_move
     */

    public $ac_move;
    /**
     * array of white's pieces
     * @var array $w_figures
     */

    public $w_figures;
    /**
     * array of black's pieces
     * @var array $b_figures
     */

    public $b_figures;
    /**
     * updated by handleMove, not used now but might be used in future
     * @var string $last_move
     */

    public $last_move;
    /**
     * updated by handleMove, not used now but might be used in future
     * @var string $captured_piece
     */

    public $captured_piece;
    // --------------

    // PUBLIC METHODS

    // --------------

    /**
     * constructor
     *
     * If a failure occurs, $this->error is set to a string containing an error message;
     * otherwise $this->error is set to an empty string.
     *
     * Example:
     * <pre>
     *    $chessgame = new ChessGame($fen);
     *    if (!empty($chessgame->error)) {
     *      echo "'$fen' invalid: $chessgame->error\n";
     *    }
     * </pre>
     *
     * @param mixed $param If $param is an array, an existing game is loaded using $param as the nine-element gamestate array described above.
     *                     If $param is a non-empty string, a new game is created using $param as a FEN setup position.
     *                     Otherwise, a new game is created using the standard starting position.
     */
    public function __construct($param = null)
    {
        // for now

        $this->browsing_mode = 0;

The property browsing_mode does not exist. Although not strictly required by PHP, it is generally a best practice to declare properties explicitly.

        if (\is_array($param)) {
            $this->gamestate = $param;

            $this->error = '';
        } elseif (\is_string($param) && !empty($param)) {
            $this->error = $this->init_gamestate($param);
        } else {
            $this->init_gamestate();

            $this->error = '';
        }
    }

    /**
     * Handle a move.
     *
     * @param string $move
     * @return array A two-element array:
     *  - $move_performed: true if the move was performed and the game state has been updated, false otherwise
     *  - $move_result_text: text message
     */
    public function move($move)
    {
        empty($this->error) or \trigger_error(_MD_CHESS_ERROR, \E_USER_ERROR);

        return $this->handleMove($move);
    }

    /**
     * get game state
     *
     * @return array
     */
    public function gamestate()
    {
        empty($this->error) or \trigger_error(_MD_CHESS_ERROR, \E_USER_ERROR);

        return $this->gamestate;
    }

    // ----------------------------------------------------------------

    // PRIVATE METHODS - intended for use only by methods of this class

    // ----------------------------------------------------------------

    /**
     * Initialize gamestate for a new game.
     *
     * If a non-empty string $fen is provided, the game is initialized using $fen as a FEN setup position.
     * Otherwise the game is initialized using the standard starting position.
     *
     * @param string $fen
     * @return string  empty string on success, or error message on failure
     *
     * @access private
     */
    public function init_gamestate($fen = null)
    {
        $this->gamestate = [];

        if (!empty($fen)) {
            $setup = true;
        } else {
            $setup = false;

            $fen = 'rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1';
        }

        // check that data is not unreasonably short or long

        if (\mb_strlen($fen) < 23 || \mb_strlen($fen) > 100) {
            return _MD_CHESS_FENBAD_LENGTH; // invalid length
        }

        $fen_data = \explode(' ', $fen);

        if (6 != \count($fen_data)) {
            return _MD_CHESS_FENBAD_FIELD_COUNT; // wrong number of fields
        }

        $this->gamestate['fen_piece_placement'] = $fen_data[0];

        $this->gamestate['fen_active_color'] = $fen_data[1];

        $this->gamestate['fen_castling_availability'] = $fen_data[2];

        $this->gamestate['fen_en_passant_target_square'] = $fen_data[3];

        $this->gamestate['fen_halfmove_clock'] = $fen_data[4];

        $this->gamestate['fen_fullmove_number'] = $fen_data[5];

        $this->gamestate['pgn_fen'] = $setup ? $fen : null;

        $this->gamestate['pgn_result'] = '*';

        $this->gamestate['pgn_movetext'] = '*';

        if (!$this->fen_piece_placement_to_board()) {
            return _MD_CHESS_FENBAD_PP_INVALID; // piece_placement invalid
        } elseif ('w' != $this->gamestate['fen_active_color'] && 'b' != $this->gamestate['fen_active_color']) {
            return _MD_CHESS_FENBAD_AC_INVALID; // active_color invalid
        } // Since fen_piece_placement_to_board() checked $fen for the correct number of fields above, $castling_availability is non-empty.

        elseif ('-' != $this->gamestate['fen_castling_availability'] && !\preg_match('/^K?Q?k?q?$/', $this->gamestate['fen_castling_availability'])) {
            return _MD_CHESS_FENBAD_CA_INVALID; // castling_availability invalid
        } elseif ('-' != $this->gamestate['fen_en_passant_target_square'] && !\preg_match('/^[a-h][36]$/', $this->gamestate['fen_en_passant_target_square'])) {
            return _MD_CHESS_FENBAD_EP_INVALID; // en_passant_target_square invalid
        } elseif (!\preg_match('/^\d{0,4}$/', $this->gamestate['fen_halfmove_clock'])) {
            return _MD_CHESS_FENBAD_HC_INVALID; // halfmove_clock invalid
        } elseif (!\preg_match('/^\d{0,4}$/', $this->gamestate['fen_fullmove_number']) || $this->gamestate['fen_fullmove_number'] < 1) {
            return _MD_CHESS_FENBAD_FN_INVALID; // fullmove_number invalid
        } elseif ($this->insufficient_mating_material()) {
            return _MD_CHESS_FENBAD_MATERIAL; // insufficient mating material
        } elseif (('w' == $this->gamestate['fen_active_color'] && $this->kingIsUnderAttack('b', 'w'))
                  || ('b' == $this->gamestate['fen_active_color'] && $this->kingIsUnderAttack('w', 'b'))) {
            return _MD_CHESS_FENBAD_IN_CHECK; // player to move cannot have opponent in check
        } elseif ((\mb_strstr($this->gamestate['fen_castling_availability'], 'K') && ('wK' != $this->board[4] || 'wR' != $this->board[7]))
                  || (\mb_strstr($this->gamestate['fen_castling_availability'], 'Q') && ('wK' != $this->board[4] || 'wR' != $this->board[0]))
                  || (\mb_strstr($this->gamestate['fen_castling_availability'], 'k') && ('bK' != $this->board[60] || 'bR' != $this->board[63]))
                  || (\mb_strstr($this->gamestate['fen_castling_availability'], 'q') && ('bK' != $this->board[60] || 'bR' != $this->board[56]))) {
            return _MD_CHESS_FENBAD_CA_INCONSISTENT; // castling availability inconsistent with piece placement
        } elseif (('-' != $this->gamestate['fen_en_passant_target_square'] && 3 == $this->gamestate['fen_en_passant_target_square'][1] && 'b' != $this->gamestate['fen_active_color'])
                  || ('-' != $this->gamestate['fen_en_passant_target_square'] && 6 == $this->gamestate['fen_en_passant_target_square'][1] && 'w' != $this->gamestate['fen_active_color'])) {
            return _MD_CHESS_FENBAD_EP_COLOR; // en passant target square wrong color
        } elseif ('-' != $this->gamestate['fen_en_passant_target_square'] && 3 == $this->gamestate['fen_en_passant_target_square'][1]
                  && 'wP' != $this->board[$this->boardCoordToIndex($this->gamestate['fen_en_passant_target_square'][0] . '4')]) {
            return _MD_CHESS_FENBAD_EP_NO_PAWN; // en passant target square for nonexistent pawn
        } elseif ('-' != $this->gamestate['fen_en_passant_target_square'] && 6 == $this->gamestate['fen_en_passant_target_square'][1]
                  && 'bP' != $this->board[$this->boardCoordToIndex($this->gamestate['fen_en_passant_target_square'][0] . '5')]) {
            return _MD_CHESS_FENBAD_EP_NO_PAWN; // en passant target square for nonexistent pawn
        }

        #echo "In " . __CLASS__ . '::' . __FUNCTION__ . "\n";#*#DEBUG#

        #var_dump('gamestate', $this->gamestate);#*#DEBUG#

        return ''; // successful
    }

    /**
     * Check whether a path is blocked.
     *
     * check a series of tiles given a start, an end tile
     * which is not included to the check and a position
     * change for each iteration. return true if not blocked.
     * all values are given for 1dim board.
     *
     * @param int $start
     * @param int $end
     * @param int $change
     * @return bool
     *
     * @access private
     */
    public function pathIsNotBlocked($start, $end, $change)
    {
        for ($pos = $start; $pos != $end; $pos += $change) {
            #echo "path: $pos: '$this->board[$pos]' "; #*#DEBUG#

            if (!$this->is_empty_tile($pos)) {
                return false;
            }
        }

        return true;
    }

    /**
     * Get path.
     *
     * get the empty tiles between start and end as an 1dim array.
     * whether the path is clear is not checked.
     *
     * @param int $start
     * @param int $end
     * @param int $change
     * @return array
     *
     * @access private
     */
    public function getPath($start, $end, $change)
    {
        $path = [];

        for ($pos = $start; $pos != $end; $pos += $change) {
            $path[] = $pos;
        }

        return $path;
    }

    /**
     * get path change
     *
     * get the change value that must be added to create
     * the 1dim path for figure moving from fig_pos to
     * dest_pos. it is assumed that the movement is valid!
     * no additional checks as in tileIsReachable are
     * performed. rook, queen and bishop are the only
     * units that can have empty tiles in between.
     *
     * @param string $fig
     * @param int    $fig_pos
     * @param int    $dest_pos
     * @return int
     *
     * @access private
     */
    public function getPathChange($fig, $fig_pos, $dest_pos)
    {
        $change = 0;

        $fy = \floor($fig_pos / 8);

        $fx = $fig_pos % 8;

        $dx = $dest_pos % 8;

        $dy = \floor($dest_pos / 8);

        switch ($fig) {
            /* bishop */ case 'B':
            $change = $dy < $fy ? -8 : 8;
            $change += $dx < $fx ? -1 : 1;
            break;
            /* rook */ case 'R':
            if ($fx == $dx) {
                $change = $dy < $fy ? -8 : 8;
            } else {
                $change = $dx < $fx ? -1 : 1;
            }
            break;
            /* queen */ case 'Q':
            if (\abs($fx - $dx) == \abs($fy - $dy)) {
                $change = $dy < $fy ? -8 : 8;

                $change += $dx < $fx ? -1 : 1;
            } elseif ($fx == $dx) {
                $change = $dy < $fy ? -8 : 8;
            } else {
                $change = $dx < $fx ? -1 : 1;
            }
            break;
        }

        return $change;
    }

    /**
     * Check whether a tile is reachable.
     *
     * check whether dest_pos is in reach for unit of fig_type
     * at tile fig_pos. it is not checked whether the tile
     * itself is occupied but only the tiles in between.
     * this function does not check pawns.
     *
     * @param string $fig
     * @param int    $fig_pos
     * @param int    $dest_pos
     * @return bool
     *
     * @access private
     */
    public function tileIsReachable($fig, $fig_pos, $dest_pos)
    {
        if ($fig_pos == $dest_pos) {
            return;
        }

        $result = 0;

        $fy = \floor($fig_pos / 8);

        $fx = $fig_pos % 8;

        $dy = \floor($dest_pos / 8);

        $dx = $dest_pos % 8;

        /* DEBUG:  echo "$fx,$fy --> $dx,$dy: "; */

        switch ($fig) {
            /* knight */ case 'N':
            if (1 == \abs($fx - $dx) && 2 == \abs($fy - $dy)) {
                $result = 1;
            }
            if (1 == \abs($fy - $dy) && 2 == \abs($fx - $dx)) {
                $result = 1;
            }
            break;
            /* bishop */ case 'B':
            if (\abs($fx - $dx) != \abs($fy - $dy)) {
                break;
            }
            if ($dy < $fy) {
                $change = -8;
            } else {
                $change = 8;
            }
            if ($dx < $fx) {
                --$change;
            } else {
                ++$change;
            }
            if ($this->pathIsNotBlocked($fig_pos + $change, $dest_pos, $change)) {
                $result = 1;
            }
            break;
            /* rook */ case 'R':
            if ($fx != $dx && $fy != $dy) {
                break;
            }
            if ($fx == $dx) {
                if ($dy < $fy) {
                    $change = -8;
                } else {
                    $change = 8;
                }
            } else {
                if ($dx < $fx) {
                    $change = -1;
                } else {
                    $change = 1;
                }
            }
            if ($this->pathIsNotBlocked($fig_pos + $change, $dest_pos, $change)) {
                $result = 1;
            }
            break;
            /* queen */ case 'Q':
            if (\abs($fx - $dx) != \abs($fy - $dy) && $fx != $dx && $fy != $dy) {
                break;
            }
            if (\abs($fx - $dx) == \abs($fy - $dy)) {
                if ($dy < $fy) {
                    $change = -8;
                } else {
                    $change = 8;
                }

                if ($dx < $fx) {
                    --$change;
                } else {
                    ++$change;
                }
            } elseif ($fx == $dx) {
                if ($dy < $fy) {
                    $change = -8;
                } else {
                    $change = 8;
                }
            } else {
                if ($dx < $fx) {
                    $change = -1;
                } else {
                    $change = 1;
                }
            }
            if ($this->pathIsNotBlocked($fig_pos + $change, $dest_pos, $change)) {
                $result = 1;
            }
            break;
            /* king */ case 'K':
            if (\abs($fx - $dx) > 1 || \abs($fy - $dy) > 1) {
                break;
            }
            $kings     = 0;
            $adj_tiles = $this->getAdjTiles($dest_pos);
            foreach ($adj_tiles as $tile) {
                if ('K' == $this->board[$tile][1]) {
                    $kings++;
                }
            }
            if (2 == $kings) {
                break;
            }
            $result = 1;
            break;
        }

        /* DEBUG: echo " $result<br>"; */

        return $result;

The expression return $result returns the type integer which is incompatible with the documented return type boolean.

    }

    /**
     * Check whether a pawn can attack a tile.
     *
     * @param int $fig_pos  Position of pawn
     * @param int $dest_pos Tile to check
     * @return bool True if pawn can attack
     *
     * @access private
     */
    public function checkPawnAttack($fig_pos, $dest_pos)
    {
        if ('w' == $this->board[$fig_pos][0]) {
            if ($fig_pos % 8 > 0 && $dest_pos == $fig_pos + 7) {
                return true;
            }

            if ($fig_pos % 8 < 7 && $dest_pos == $fig_pos + 9) {
                return true;
            }
        } elseif ('b' == $this->board[$fig_pos][0]) {
            if ($fig_pos % 8 < 7 && $dest_pos == $fig_pos - 7) {
                return true;
            }

            if ($fig_pos % 8 > 0 && $dest_pos == $fig_pos - 9) {
                return true;
            }
        }

        return false;
    }

    /**
     * Check whether a pawn move is legal.
     *
     * check whether pawn at figpos may move to destpos.
     * first move may be two tiles instead of just one.
     * again the last tile is not checked but just the path
     * in between.
     *
     * @param int $fig_pos  Position of pawn.
     * @param int $dest_pos Destination tile.
     * @return bool True if move is legal
     *
     * @access private
     */
    public function checkPawnMove($fig_pos, $dest_pos)
    {
        $first_move = 0;

        if ('w' == $this->board[$fig_pos][0]) {
            if ($fig_pos >= 8 && $fig_pos <= 15) {
                $first_move = 1;
            }

            if ($dest_pos == $fig_pos + 8) {
                return true;
            }

            if ($first_move && ($dest_pos == $fig_pos + 16)) {
                if ($this->is_empty_tile($fig_pos + 8)) {
                    return true;
                }
            }
        } elseif ('b' == $this->board[$fig_pos][0]) {
            if ($fig_pos >= 48 && $fig_pos <= 55) {
                $first_move = 1;
            }

            if ($dest_pos == $fig_pos - 8) {
                return true;
            }

            if ($first_move && ($dest_pos == $fig_pos - 16)) {
                if ($this->is_empty_tile($fig_pos - 8)) {
                    return true;
                }
            }
        }

        return false;
    }

    /**
     * Check whether a tile is under attack by the specified player.
     *
     * @param string $opp      Attacking color ('w' or 'b')
     * @param int    $dest_pos Tile to check
     * @return bool
     *
     * @access private
     */
    public function tileIsUnderAttack($opp, $dest_pos)
    {
        #var_dump('tileIsUnderAttack, opp', $opp, 'dest_pos', $dest_pos, 'board', $board);#*#DEBUG#

        for ($i = 0; $i < 64; $i++) {
            if ($this->board[$i][0] == $opp) {
                if (('P' == $this->board[$i][1] && $this->checkPawnAttack($i, $dest_pos))
                    || ('P' != $this->board[$i][1]
                        && $this->tileIsReachable($this->board[$i][1], $i, $dest_pos))) {
                    /*DEBUG: echo "attack test: $i: ",$opp,"P<br>"; */

                    return true;
                }
            }
        }

        return false;
    }

    /**
     * Check whether a player's king can be attacked by his opponent.
     *
     * @param string $player Player's color ('w' or 'b')
     * @param string $opp    Opponent's color ('w' or 'b')
     * @return bool
     *
     * @access private
     */
    public function kingIsUnderAttack($player, $opp)
    {
        #var_dump('kingIsUnderAttack, player', $player, 'opp', $opp, 'board', $board);#*#DEBUG#

        for ($i = 0; $i < 64; $i++) {
            if ($this->board[$i][0] == $player && 'K' == $this->board[$i][1]) {
                $king_pos = $i;

                break;
            }
        }

        /*DEBUG: echo "$player king is at $king_pos<br>"; */

        return $this->tileIsUnderAttack($opp, $king_pos);

The variable $king_pos does not seem to be defined for all execution paths leading up to this point.

    }

    /**
     * Check whether player's king is checkmated by his opponent.
     *
     * @param string $player Player's color ('w' or 'b')
     * @param string $opp    Opponent's color ('w' or 'b')
     * @return bool
     *
     * @access private
     */
    public function isCheckMate($player, $opp)
    {
        for ($i = 0; $i < 64; $i++) {
            if ($this->board[$i][0] == $player && 'K' == $this->board[$i][1]) {
                $king_pos = $i;

                $king_x = $i % 8;

                $king_y = \floor($i / 8);

                break;
            }
        }

        /* test adjacent tiles while king is temporarly removed */

        $adj_tiles = $this->getAdjTiles($king_pos);

The variable $king_pos does not seem to be defined for all execution paths leading up to this point.

        $contents = $this->board[$king_pos];

        $this->clear_tile($king_pos);

        foreach ($adj_tiles as $dest_pos) {
            if ($this->board[$dest_pos][0] == $player) {
                continue;
            }

            if ($this->tileIsUnderAttack($opp, $dest_pos)) {
                continue;
            }

            $this->board[$king_pos] = $contents;

            return false;
        }

        $this->board[$king_pos] = $contents;

        /* DEBUG:  echo "King cannot escape by itself! "; */

        /* get all figures that attack the king */

        $attackers = [];

        $count = 0;

        for ($i = 0; $i < 64; $i++) {
            if ($this->board[$i][0] == $opp) {
                if (('P' == $this->board[$i][1] && $this->checkPawnAttack($i, $king_pos))
                    || ('P' != $this->board[$i][1]
                        && $this->tileIsReachable($this->board[$i][1], $i, $king_pos))) {
                    $attackers[$count++] = $i;
                }
            }
        }

        /* DEBUG:
        for( $i = 0; $i < $count; $i++ )
          echo "Attacker: $attackers[$i] ";
        echo "Attackercount: ",count($attackers), " "; */

        /* if more than one there is no chance to escape */

        if ($count > 1) {
            return true;
        }

        /* check whether attacker can be killed by own figure */

        $dest_pos = $attackers[0];

        for ($i = 0; $i < 64; $i++) {
            if ($this->board[$i][0] == $player) {
                if (('P' == $this->board[$i][1] && $this->checkPawnAttack($i, $dest_pos))
                    || ('P' != $this->board[$i][1] && 'K' != $this->board[$i][1]
                        && $this->tileIsReachable($this->board[$i][1], $i, $dest_pos))
                    || ('K' == $this->board[$i][1]
                        && $this->tileIsReachable($this->board[$i][1], $i, $dest_pos)
                        && !$this->tileIsUnderAttack($opp, $dest_pos))) {
                    /* DEBUG: echo "candidate: $i "; */

                    $can_kill_atk = 0;

                    $contents_def = $this->board[$i];

                    $contents_atk = $this->board[$dest_pos];

                    $this->board[$dest_pos] = $this->board[$i];

                    $this->clear_tile($i);

                    if (!$this->tileIsUnderAttack($opp, $king_pos)) {
                        $can_kill_atk = 1;
                    }

                    $this->board[$i] = $contents_def;

                    $this->board[$dest_pos] = $contents_atk;

                    if ($can_kill_atk) {
                        /* DEBUG: echo "$i can kill attacker"; */

                        return false;
                    }
                }
            }
        }

        /* check whether own unit can block the way */

        /* if attacking unit is a knight there
         * is no way to block the path */

        if ('N' == $this->board[$dest_pos][1]) {
            return true;
        }

        /* if enemy is adjacent to king there is no
         * way either */

        $dest_x = $dest_pos % 8;

        $dest_y = \floor($dest_pos / 8);

        if (\abs($dest_x - $king_x) <= 1 && \abs($dest_y - $king_y) <= 1) {

The variable $king_y does not seem to be defined for all execution paths leading up to this point.

The variable $king_x does not seem to be defined for all execution paths leading up to this point.

            return true;
        }

        /* get the list of tiles between king and attacking
         * unit that can be blocked to stop the attack */

        $change = $this->getPathChange($this->board[$dest_pos][1], $dest_pos, $king_pos);

        /* DEBUG:  echo "path change: $change "; */

        $path = $this->getPath($dest_pos + $change, $king_pos, $change);

        /* DEBUG: foreach( $path as $tile ) echo "tile: $tile "; */

        foreach ($path as $pos) {
            for ($i = 0; $i < 64; $i++) {
                if ($this->board[$i][0] == $player) {
                    if (('P' == $this->board[$i][1] && $this->checkPawnMove($i, $pos))
                        || ('P' != $this->board[$i][1] && 'K' != $this->board[$i][1]
                            && $this->tileIsReachable($this->board[$i][1], $i, $pos))) {
                        /* DEBUG: echo "$i can block "; */

                        return false;
                    }
                }
            }
        }

        return true;
    }

    /**
     * Check whether player is stalemated.
     *
     * @param string $player Color of player who has the move ('w' or 'b')
     * @param string $opp    Opponent's color ('w' or 'b')
     * @return bool
     *
     * @todo   recognize when move is not possible because of check
     *
     * @access private
     */
    public function isStaleMate($player, $opp)
    {
        for ($i = 0; $i < 64; $i++) {
            if ($this->board[$i][0] == $player) {
                switch ($this->board[$i][1]) {
                    case 'K':
                        $adj_tiles = $this->getAdjTiles($i);
                        foreach ($adj_tiles as $pos) {
                            if ($this->board[$pos][0] == $player) {
                                continue;
                            }

                            if ($this->tileIsUnderAttack($opp, $pos)) {
                                continue;
                            }

                            return false;
                        }
                        /* DEBUG:  echo "King cannot escape by itself! "; */
                        break;
                    case 'P':
                        if ('w' == $player) {
                            if ($this->is_empty_tile($i + 8)) {
                                return false;
                            }

                            if ($i % 8 > 0 && $this->board[$i + 7][0] != $player) {
                                return false;
                            }

                            if ($i % 8 < 7 && $this->board[$i + 9][0] != $player) {
                                return false;
                            }
                        } else {
                            if ($this->is_empty_tile($i - 8)) {
                                return false;
                            }

                            if ($i % 8 > 0 && $this->board[$i - 9][0] != $player) {
                                return false;
                            }

                            if ($i % 8 < 7 && $this->board[$i - 7][0] != $player) {
                                return false;
                            }
                        }
                        break;
                    case 'B':
                        if ($i - 9 >= 0 && $this->board[$i - 9][0] != $player) {
                            return false;
                        }
                        if ($i - 7 >= 0 && $this->board[$i - 7][0] != $player) {
                            return false;
                        }
                        if ($i + 9 <= 63 && $this->board[$i + 9][0] != $player) {
                            return false;
                        }
                        if ($i + 7 <= 63 && $this->board[$i + 7][0] != $player) {
                            return false;
                        }
                        break;
                    case 'R':
                        if ($i - 8 >= 0 && $this->board[$i - 8][0] != $player) {
                            return false;
                        }
                        if ($i - 1 >= 0 && $this->board[$i - 1][0] != $player) {
                            return false;
                        }
                        if ($i + 8 <= 63 && $this->board[$i + 8][0] != $player) {
                            return false;
                        }
                        if ($i + 1 <= 63 && $this->board[$i + 1][0] != $player) {
                            return false;
                        }
                        break;
                    case 'Q':
                        $adj_tiles = $this->getAdjTiles($i);
                        foreach ($adj_tiles as $pos) {
                            if ($this->board[$pos][0] != $player) {
                                return false;
                            }
                        }
                        break;
                    case 'N':
                        if ($i - 17 >= 0 && $this->board[$i - 17][0] != $player) {
                            return false;
                        }
                        if ($i - 15 >= 0 && $this->board[$i - 15][0] != $player) {
                            return false;
                        }
                        if ($i - 6 >= 0 && $this->board[$i - 6][0] != $player) {
                            return false;
                        }
                        if ($i + 10 <= 63 && $this->board[$i + 10][0] != $player) {
                            return false;
                        }
                        if ($i + 17 <= 63 && $this->board[$i + 17][0] != $player) {
                            return false;
                        }
                        if ($i + 15 <= 63 && $this->board[$i + 15][0] != $player) {
                            return false;
                        }
                        if ($i + 6 <= 63 && $this->board[$i + 6][0] != $player) {
                            return false;
                        }
                        if ($i - 10 >= 0 && $this->board[$i - 10][0] != $player) {
                            return false;
                        }
                        break;
                }
            }
        }

        return true;
    }

    /**
     * Generate informational text message with parameters.
     *
     * Example:
     * <pre>
     *   echo move_msg('cannot find {$param[1]} pawn in column {$param[2]}', 'b', 'e');
     *    - prints: "cannot find b pawn in column e"
     * </pre>
     *
     * @param string $text
     * @return string
     *
     * @access private
     */
    public function move_msg($text)
    {
        $param = \func_get_args();

The assignment to $param is dead and can be removed.

        #var_dump('move_msg, text', $text, 'param', $param);#*#DEBUG#

        return eval("return \"$text\";");

The use of eval() is discouraged.

    }

    /**
     * Translate Standard Algebraic Notation (SAN) into a full move description.
     *
     * The completed move is placed in $this->ac_move.
     *
     * @param string $player 'w' or 'b'
     * @param string $move
     *                       <pre>
     *                       [a-h][1-8|a-h][RNBQK]              pawn move/attack
     *                       [PRNBQK][a-h][1-8]                 figure move
     *                       [PRNBQK][:x][a-h][1-8]             figure attack
     *                       [PRNBQK][1-8|a-h][a-h][1-8]        ambigous figure move
     *                       [a-h][:x][a-h][1-8][[RNBQK]        ambigous pawn attack
     *                       [PRNBQK][1-8|a-h][:x][a-h][1-8]    ambigous figure attack
     *                       </pre>
     * @return string  Empty string if successful, otherwise error message
     *
     * @access private
     */
    public function completeMove($player, $move)
    {
        /*
         * [a-h][1-8|a-h][RNBQK]              pawn move/attack
         * [PRNBQK][a-h][1-8]                 figure move
         * [PRNBQK][:x][a-h][1-8]             figure attack
         * [PRNBQK][1-8|a-h][a-h][1-8]        ambigous figure move
         * [a-h][:x][a-h][1-8][[RNBQK]        ambigous pawn attack
         * [PRNBQK][1-8|a-h][:x][a-h][1-8]    ambigous figure attack
         */

        $error = _MD_CHESS_MOVE_UNKNOWN; // "format is totally unknown!"

        $this->ac_move = $move;

        if (\mb_strlen($move) >= 6) {
            /* full move: a pawn requires a ? in the end
             * to automatically choose a queen on last line */

            if (0 === \strpos($move, 'P')) {
                if ($move[\mb_strlen($move) - 1] < 'A' || $move[\mb_strlen($move) - 1] > 'Z') {
                    $this->ac_move = "$move?";
                }
            }

            return '';
        }

        /* allow last letter to be a capital one indicating
         * the chessmen a pawn is supposed to transform into,
         * when entering the last file. we split this character
         * to keep the autocompletion process the same. */

        $pawn_upg = '?';

        if ($move[\mb_strlen($move) - 1] >= 'A' && $move[\mb_strlen($move) - 1] <= 'Z') {
            $pawn_upg = $move[\mb_strlen($move) - 1];

            $move = \mb_substr($move, 0, -1);
        }

        // remove trailing '=', if present

        if ('=' == $move[mb_strlen($move) - 1]) {
            $move = \mb_substr($move, 0, -1);
        }

        if ('P' == $pawn_upg || 'K' == $pawn_upg) {
            return _MD_CHESS_MOVE_PAWN_MAY_BECOME;
        } // "A pawn may only become either a knight, a bishop, a rook or a queen!"

        if ($move[0] >= 'a' && $move[0] <= 'h') {
            /* pawn move. either it's 2 or for characters as
             * listed above */

            if (4 == mb_strlen($move)) {
                if ('x' != $move[1]) {
                    return _MD_CHESS_MOVE_USE_X;
                } // "use x to indicate an attack"

                $dest_x = $move[2];

                $dest_y = $move[3];

                $src_x = $move[0];

                if ('w' == $player) {
                    $src_y = $dest_y - 1;
                } else {
                    $src_y = $dest_y + 1;
                }

                $this->ac_move = \sprintf(
                    'P%s%dx%s%d%s',
                    $src_x,
                    $src_y,
                    $dest_x,
                    $dest_y,
                    $pawn_upg
                );

                return '';
            } elseif (2 == mb_strlen($move)) {
                $fig = \sprintf('%sP', $player);

                if ($move[1] >= '1' && $move[1] <= '8') {
                    /* pawn move */

                    $pos = $this->boardCoordToIndex($move);

                    if (64 == $pos) {
                        return $this->move_msg(_MD_CHESS_MOVE_COORD_INVALID, $move);
                    } // "coordinate $move is invalid"

                    if ('w' == $player) {
                        while ($pos >= 0 && $this->board[$pos] != $fig) {
                            $pos -= 8;
                        }

                        if ($pos < 0) {
                            $not_found = 1;
                        }
                    } else {
                        while ($pos <= 63 && $this->board[$pos] != $fig) {
                            $pos += 8;
                        }

                        if ($pos > 63) {
                            $not_found = 1;
                        }
                    }

                    $pos = $this->boardIndexToCoord($pos);

                    if ((isset($not_found) && $not_found) || '' == $pos) {
                        return $this->move_msg(_MD_CHESS_MOVE_CANNOT_FIND_PAWN, $player, $move[0]); // "cannot find $player pawn in column $move[0]"
                    }

                    $this->ac_move = \sprintf('P%s-%s%s', $pos, $move, $pawn_upg);

                    return '';
                }

                /* notation: [a-h][a-h] for pawn attack no longer allowed
                 * except for history browser */

                if (0 == $this->browsing_mode) {
                    return _MD_CHESS_MOVE_USE_NOTATION;
                } // "please use denotation [a-h]x[a-h][1-8] for pawn attacks (see help for more information)"

                /* pawn attack must be only one pawn in column! */

                $pawns = 0;

                $start = $this->boardCoordToIndex(\sprintf('%s1', $move[0]));

                if (64 == $start) {
                    return $this->move_msg(_MD_CHESS_MOVE_COORD_INVALID, $move[0]);
                } // "coordinate $move[0] is invalid"

                for ($i = 1; $i <= 8; $i++, $start += 8) {
                    if ($this->board[$start] == $fig) {
                        $pawns++;

                        $pawn_line = $i;
                    }
                }

                if (0 == $pawns) {

The condition 0 == $pawns is always true.

                    return $this->move_msg(_MD_CHESS_MOVE_NO_PAWN, $move[0]);
                } // "there is no pawn in column $move[0]"

                elseif ($pawns > 1) {
                    return $this->move_msg(_MD_CHESS_MOVE_TWO_PAWNS, $move[0]);
                } // "there is more than one pawn in column $move[0]"

                if ('w' == $player) {
                    $dest_line = $pawn_line + 1;

The variable $pawn_line does not seem to be defined for all execution paths leading up to this point.

                } else {
                    $dest_line = $pawn_line - 1;
                }

                $this->ac_move = \sprintf(
                    'P%s%dx%s%d',
                    $move[0],
                    $pawn_line,
                    $move[1],
                    $dest_line
                );

                return '';
            }
        } else {
            /* figure move */

            $dest_coord = \mb_substr($move, \mb_strlen($move) - 2, 2);

            $action = $move[\mb_strlen($move) - 3];

            if ('x' != $action) {
                $action = '-';
            }

            if ('w' == $player) {
                $figures = $this->w_figures;
            } else {
                $figures = $this->b_figures;
            }

            $fig_count = 0;

            foreach ($figures as $figure) {
                if ($figure[0] == $move[0]) {
                    $fig_count++;

                    if (1 == $fig_count) {
                        $pos1 = \mb_substr($figure, 1, 2);
                    } else {
                        $pos2 = \mb_substr($figure, 1, 2);
                    }
                }
            }

            if (0 == $fig_count) {
                return $this->move_msg(_MD_CHESS_MOVE_NO_FIGURE, $move[0], $this->getFullFigureName($move[0]));
            } // sprintf("there is no figure %s = %s", $move[0], $this->getFullFigureName($move[0]))

            elseif (1 == $fig_count) {
                $this->ac_move = \sprintf(
                    '%s%s%s%s',
                    $move[0],
                    $pos1,

The variable $pos1 does not seem to be defined for all execution paths leading up to this point.

                    $action,
                    $dest_coord
                );

                return '';
            }

            /* two figures which may cause ambiguity */

            $dest_pos = $this->boardCoordToIndex($dest_coord);

            if (64 == $dest_pos) {
                return $this->move_msg(_MD_CHESS_MOVE_COORD_INVALID, $dest_coord);
            } // "coordinate $dest_coord is invalid"

            $fig1_can_reach = $this->tileIsReachable(
                $move[0],
                $this->boardCoordToIndex($pos1),
                $dest_pos
            );

            $fig2_can_reach = $this->tileIsReachable(
                $move[0],
                $this->boardCoordToIndex($pos2),

The variable $pos2 does not seem to be defined for all execution paths leading up to this point.

                $dest_pos
            );

            if (!$fig1_can_reach && !$fig2_can_reach) {
                return $this->move_msg(_MD_CHESS_MOVE_NEITHER_CAN_REACH, $move[0], $this->getFullFigureName($move[0]), $dest_coord);
            } // sprintf("neither of the %s = %s can reach %s", $move[0], $this->getFullFigureName($move[0]), $dest_coord)

            elseif ($fig1_can_reach && $fig2_can_reach) {
                /* ambiguity - check whether a hint is given */

                if (('-' == $action && 4 == mb_strlen($move))
                    || ('x' == $action && 5 == mb_strlen($move))) {
                    $hint = $move[1];
                }

                if (empty($hint)) {
                    return $this->move_msg(_MD_CHESS_MOVE_BOTH_CAN_REACH, $move[0], $this->getFullFigureName($move[0]), $dest_coord);
                } // sprintf("both of the %s = %s can reach %s", $move[0], $this->getFullFigureName($move[0]), $dest_coord)

                $move_fig1 = 0;

                $move_fig2 = 0;

                if ($hint >= '1' && $hint <= '8') {

The variable $hint does not seem to be defined for all execution paths leading up to this point.

                    if ($pos1[1] == $hint && $pos2[1] != $hint) {
                        $move_fig1 = 1;
                    }

                    if ($pos2[1] == $hint && $pos1[1] != $hint) {
                        $move_fig2 = 1;
                    }
                } else {
                    if ($pos1[0] == $hint && $pos2[0] != $hint) {
                        $move_fig1 = 1;
                    }

                    if ($pos2[0] == $hint && $pos1[0] != $hint) {
                        $move_fig2 = 1;
                    }
                }

                if (!$move_fig1 && !$move_fig2) {
                    return _MD_CHESS_MOVE_AMBIGUOUS;
                } // "ambiguity is not properly resolved"

                if ($move_fig1) {
                    $this->ac_move = \sprintf(
                        '%s%s%s%s',
                        $move[0],
                        $pos1,
                        $action,
                        $dest_coord
                    );
                } else {
                    $this->ac_move = \sprintf(
                        '%s%s%s%s',
                        $move[0],
                        $pos2,
                        $action,
                        $dest_coord
                    );
                }

                return;
            }

            if ($fig1_can_reach) {
                $this->ac_move = \sprintf(
                    '%s%s%s%s',
                    $move[0],
                    $pos1,
                    $action,
                    $dest_coord
                );
            } else {
                $this->ac_move = \sprintf(
                    '%s%s%s%s',
                    $move[0],
                    $pos2,
                    $action,
                    $dest_coord
                );
            }

            return '';
        }

        return $error;
    }

    /**
     * A hacky function that uses autocomplete to short
     * a full move. if this fails there is no warning
     * but the move is kept unchanged.
     *
     * @param string $player 'w' or 'b'
     * @param string $move
     * @return string  new move
     *
     * @access private
     */
    public function convertFullToChessNotation($player, $move)
    {
        $new_move = $move;

        $old_ac_move = $this->ac_move; /* backup required as autocomplete
                              will overwrite it */

        /* valid pawn moves are always non-ambigious */

        if (0 === \strpos($move, 'P')) {
            /* skip P anycase. for attacks skip source digit
               and for moves skip source pos and - */

            if ('-' == $move[3]) {
                $new_move = \mb_substr($move, 4);
            } elseif ('x' == $move[3]) {
                $new_move = \sprintf('%s%s', $move[1], \mb_substr($move, 3));
            }
        } else {
            /* try to remove the source position and check whether it
             * is a non-ambigious move. if it is add one of the components
             * and check again */

            if ('-' == $move[3]) {
                $dest = \mb_substr($move, 4);
            } elseif ('x' == $move[3]) {
                $dest = \mb_substr($move, 3);
            }

            $new_move = \sprintf('%s%s', $move[0], $dest);

The variable $dest does not seem to be defined for all execution paths leading up to this point.

            if ('' != $this->completeMove($player, $new_move)) {
                /* add a component */

                $new_move = \sprintf('%s%s%s', $move[0], $move[1], $dest);

                if ('' != $this->completeMove($player, $new_move)) {
                    /* add other component */

                    $new_move = \sprintf('%s%s%s', $move[0], $move[2], $dest);

                    if ('' != $this->completeMove($player, $new_move)) {
                        $new_move = $move;
                    } /* give up */
                }
            }
        }

        $this->ac_move = $old_ac_move;

        return $new_move;
    }

    /**
     * Handle a move.
     *
     * check whether it is user's turn and the move is valid.
     * if the move is okay update the game file.
     *
     * @param string $move
     * @return array A two-element array:
     *  - $move_performed: true if the move was performed and the game state has been updated, false otherwise
     *  - $move_result_text: text message
     *
     * @access private
     */
    public function handleMove($move)
    {
        /* DEBUG: echo "HANDLE: $move, $comment<br>"; */

        $result = _MD_CHESS_MOVE_UNDEFINED;

The assignment to $result is dead and can be removed.

        $move_handled = 0;

        // Use $this->gamestate['fen_piece_placement'] to initialize $this->board, $this->w_figures and $this->b_figures.

        $this->fen_piece_placement_to_board() || \trigger_error('handleMove, piece_placement invalid', \E_USER_ERROR);

        // get color of current player

        $cur_player = $this->gamestate['fen_active_color']; /* b or w */

        if ('w' != $cur_player && 'b' != $cur_player) {
            return [false, "handleMove, internal error: player='$cur_player'"];
        }

        $cur_opp = 'w' == $cur_player ? 'b' : 'w';

        if ('*' != $this->gamestate['pgn_result']) {
            return [false, _MD_CHESS_MOVE_GAME_OVER];
        }

        // get castling availability flags

        $white_may_castle_short = \mb_strstr($this->gamestate['fen_castling_availability'], 'K') ? true : false;

        $white_may_castle_long = \mb_strstr($this->gamestate['fen_castling_availability'], 'Q') ? true : false;

        $black_may_castle_short = \mb_strstr($this->gamestate['fen_castling_availability'], 'k') ? true : false;

        $black_may_castle_long = \mb_strstr($this->gamestate['fen_castling_availability'], 'q') ? true : false;

        // Castling is supposed to use ohs, not zeros.  Allow zeros on input anyway.

        if ('0-0' == $move) {
            $move = 'O-O';
        } elseif ('0-0-0' == $move) {
            $move = 'O-O-O';
        }

        // allow two-step of king to indicate castling

        if ('w' == $cur_player && 'Ke1-g1' == $move) {
            $move = 'O-O';
        } elseif ('w' == $cur_player && 'Ke1-c1' == $move) {
            $move = 'O-O-O';
        } elseif ('b' == $cur_player && 'Ke8-g8' == $move) {
            $move = 'O-O';
        } elseif ('b' == $cur_player && 'Ke8-c8' == $move) {
            $move = 'O-O-O';
        }

        /* backup full move input for game history before
         * splitting figure type apart */

        $history_move = $move;

        /* clear last move - won't be saved yet if anything
           goes wrong */

        $this->last_move = 'x';

        $this->piece_captured = 'x';

The property piece_captured does not exist. Although not strictly required by PHP, it is generally a best practice to declare properties explicitly.

        /* HANDLE MOVES:
         * ---                               surrender
         * O-O                               short castling
         * O-O-O                             long castling
         * [PRNBQK][a-h][1-8][-:x][a-h][1-8] unshortened move
         */

        if ('O-O' == $move) {
            /* short castling */

            if ('b' == $cur_player) {
                if (!$black_may_castle_short) {
                    return [false, _MD_CHESS_MOVE_NO_CASTLE]; // You cannot castle short any longer!
                }

                if (!$this->is_empty_tile(61) || !$this->is_empty_tile(62)) {
                    return [false, _MD_CHESS_MOVE_NO_CASTLE]; // Cannot castle short because the way is blocked!
                }

                if ($this->kingIsUnderAttack('b', 'w')) {
                    return [false, _MD_CHESS_MOVE_NO_CASTLE]; // You cannot escape check by castling!
                }

                if ($this->tileIsUnderAttack('w', 62) || $this->tileIsUnderAttack('w', 61)) {
                    return [false, _MD_CHESS_MOVE_NO_CASTLE]; // A tile the king passes over or into would be under attack after short castling!
                }

                $this->clear_tile(60);

                $this->board[62] = 'bK';

                $this->board[61] = 'bR';

                $this->clear_tile(63);

                $black_may_castle_short = false;

                $black_may_castle_long = false;
            } else {
                if (!$white_may_castle_short) {
                    return [false, _MD_CHESS_MOVE_NO_CASTLE]; // You cannot castle short any longer!
                }

                if (!$this->is_empty_tile(5) || !$this->is_empty_tile(6)) {
                    return [false, _MD_CHESS_MOVE_NO_CASTLE]; // Cannot castle short because the way is blocked!
                }

                if ($this->kingIsUnderAttack('w', 'b')) {
                    return [false, _MD_CHESS_MOVE_NO_CASTLE]; // You cannot escape check by castling!
                }

                if ($this->tileIsUnderAttack('b', 5) || $this->tileIsUnderAttack('b', 6)) {
                    return [false, _MD_CHESS_MOVE_NO_CASTLE]; // A tile the king passes over or into would be under attack after short castling!
                }

                $this->clear_tile(4);

                $this->board[6] = 'wK';

                $this->board[5] = 'wR';

                $this->clear_tile(7);

                $white_may_castle_short = false;

                $white_may_castle_long = false;
            }

            $result = _MD_CHESS_MOVE_CASTLED_SHORT;

            $move_handled = 1;

            $this->last_move = 'O-O';
        } elseif ('O-O-O' == $move) {
            /* long castling */

            if ('b' == $cur_player) {
                if (!$black_may_castle_long) {
                    return [false, _MD_CHESS_MOVE_NO_CASTLE]; // You cannot castle long any longer!
                }

                if (!$this->is_empty_tile(57) || !$this->is_empty_tile(58) || !$this->is_empty_tile(59)) {
                    return [false, _MD_CHESS_MOVE_NO_CASTLE]; // Cannot castle long because the way is blocked!
                }

                if ($this->kingIsUnderAttack('b', 'w')) {
                    return [false, _MD_CHESS_MOVE_NO_CASTLE]; // You cannot escape check by castling!
                }

                if ($this->tileIsUnderAttack('w', 58) || $this->tileIsUnderAttack('w', 59)) {
                    return [false, _MD_CHESS_MOVE_NO_CASTLE]; // A tile the king passes over or into would be under attack after short castling!
                }

                $this->clear_tile(56);

                $this->board[58] = 'bK';

                $this->board[59] = 'bR';

                $this->clear_tile(60);

                $black_may_castle_short = false;

                $black_may_castle_long = false;
            } else {
                if (!$white_may_castle_long) {
                    return [false, _MD_CHESS_MOVE_NO_CASTLE]; // You cannot castle long any longer!
                }

                if (!$this->is_empty_tile(1) || !$this->is_empty_tile(2) || !$this->is_empty_tile(3)) {
                    return [false, _MD_CHESS_MOVE_NO_CASTLE]; // Cannot castle long because the way is blocked!
                }

                if ($this->kingIsUnderAttack('w', 'b')) {
                    return [false, _MD_CHESS_MOVE_NO_CASTLE]; // You cannot escape check by castling!
                }

                if ($this->tileIsUnderAttack('b', 2) || $this->tileIsUnderAttack('b', 3)) {
                    return [false, _MD_CHESS_MOVE_NO_CASTLE]; // A tile the king passes over or into would be under attack after short castling!
                }

                $this->clear_tile(0);

                $this->board[2] = 'wK';

                $this->board[3] = 'wR';

                $this->clear_tile(4);

                $white_may_castle_short = false;

                $white_may_castle_long = false;
            }

            $result = _MD_CHESS_MOVE_CASTLED_LONG;

            $move_handled = 1;

            $this->last_move = 'O-O-O';
        } else {
            /* [PRNBQK][a-h][1-8][-:x][a-h][1-8][RNBQK] full move */

            /* allow short move description by autocompleting to
             * full description */

            $ac_error = $this->completeMove($cur_player, \trim($move));

            if ('' != $ac_error) {
                return [false, $ac_error];
            } // "ERROR: autocomplete: $ac_error"

            $move = $this->ac_move;

            $this->last_move = \str_replace('?', '', $move);

            /* a final captial letter may only be N,B,R,Q for the
             * appropiate chessman */

            $c = $move[\mb_strlen($move) - 1];

            if ($c >= 'A' && $c <= 'Z') {
                if ('N' != $c && 'B' != $c && 'R' != $c && 'Q' != $c) {
                    return [false, _MD_CHESS_MOVE_INVALID_PIECE];
                }
            } // "ERROR: only N (knight), B (bishop), R (rook) and Q (queen) are valid chessman identifiers"

            /* if it is a full move, try to shorten the history move */

            if (\mb_strlen($history_move) >= 6) {
                $history_move = $this->convertFullToChessNotation($cur_player, $history_move);
            }

            /* DEBUG: echo "Move: $move ($history_move)<br>"; */

            /* validate figure and position */

            $fig_type = $move[0];

            $fig_name = $this->getFullFigureName($fig_type);

            if ('empty' == $fig_name) {
                return [false, $this->move_msg(_MD_CHESS_MOVE_UNKNOWN_FIGURE, $fig_type)];
            } // "ERROR: Figure $fig_type is unknown!"

            $fig_coord = \mb_substr($move, 1, 2);

            $fig_pos = $this->boardCoordToIndex($fig_coord);

            if (64 == $fig_pos) {
                return [false, $this->move_msg(_MD_CHESS_MOVE_COORD_INVALID, $fig_coord)];
            } // "ERROR: $fig_coord is invalid!"

            /* DEBUG  echo "fig_type: $fig_type, fig_pos: $fig_pos<br>"; */

            if ($this->is_empty_tile($fig_pos)) {
                return [false, $this->move_msg(_MD_CHESS_MOVE_TILE_EMPTY, $fig_coord)];
            } // "ERROR: Tile $fig_coord is empty."

            if ($this->board[$fig_pos][0] != $cur_player) {
                return [false, _MD_CHESS_MOVE_NOT_YOUR_PIECE];
            } // "ERROR: Figure does not belong to you!"

            if ($this->board[$fig_pos][1] != $fig_type) {
                return [false, _MD_CHESS_MOVE_NOEXIST_FIGURE];
            } // "ERROR: Figure does not exist!"

            /* get target index */

            $dest_coord = \mb_substr($move, 4, 2);

            $dest_pos = $this->boardCoordToIndex($dest_coord);

            if (64 == $dest_pos) {
                return [false, $this->move_msg(_MD_CHESS_MOVE_COORD_INVALID, $dest_coord)];
            } // "ERROR: $dest_coord is invalid!"

            if ($dest_pos == $fig_pos) {
                return [false, _MD_CHESS_MOVE_START_END_SAME];
            }

            /* DEBUG  echo "dest_pos: $dest_pos<br>"; */

            /* get action */

            $action = $move[3];

            if ('-' == $move[3]) {
                $action = 'M';
            } /* move */

            elseif ('x' == $move[3]) {
                $action = 'A';
            } /* attack */

            else {
                return [false, $this->move_msg(_MD_CHESS_MOVE_UNKNOWN_ACTION, $action)];
            } // "ERROR: $action is unknown! Please use \"-\" for a move and \"x\" for an attack."

            /* if attack an enemy unit must be present on tile
             * and if move then tile must be empty. in both cases
             * the king must not be checked after moving. */

            /* check whether the move is along a valid path and
             * whether all tiles in between are empty thus the path
             * is not blocked. the final destination tile is not
             * checked here. */

            if ('P' != $fig_type) {
                if (!$this->tileIsReachable($fig_type, $fig_pos, $dest_pos)) {
                    return [false, $this->move_msg(_MD_CHESS_MOVE_OUT_OF_RANGE, $dest_coord, $fig_name, $fig_coord)];
                } // "ERROR: Tile $dest_coord is out of moving range for $fig_name at $fig_coord!"
            } else {
                if ('M' == $action && !$this->checkPawnMove($fig_pos, $dest_pos)) {
                    return [false, $this->move_msg(_MD_CHESS_MOVE_OUT_OF_RANGE, $dest_coord, $fig_name, $fig_coord)];
                } // "ERROR: Tile $dest_coord is out of moving range for $fig_name at $fig_coord!"

                if ('A' == $action && !$this->checkPawnAttack($fig_pos, $dest_pos)) {
                    return [false, $this->move_msg(_MD_CHESS_MOVE_OUT_OF_RANGE, $dest_coord, $fig_name, $fig_coord)];
                } // "ERROR: Tile $dest_coord is out of attacking range for $fig_name at $fig_coord!"
            }

            $en_passant_capture_performed = 0; // 1 if en passant captured occurred, else 0

            /* check action */

            if ('M' == $action && !$this->is_empty_tile($dest_pos)) {
                return [false, $this->move_msg(_MD_CHESS_MOVE_OCCUPIED, $dest_coord)]; // "ERROR: Tile $dest_coord is occupied. You cannot move there."
            }

            if ('A' == $action && $this->is_empty_tile($dest_pos)) {
                /* en passant of pawn? */

                if ('P' == $fig_type) {
                    if ($this->boardIndexToCoord($dest_pos) == $this->gamestate['fen_en_passant_target_square']) {
                        $en_passant_capture_performed = 1;
                    }

                    if (0 == $en_passant_capture_performed) {
                        return [false, _MD_CHESS_MOVE_NO_EN_PASSANT]; // "ERROR: en-passant no longer possible!"
                    }
                } else {
                    return [false, $this->move_msg(_MD_CHESS_MOVE_ATTACK_EMPTY, $dest_coord)]; // "ERROR: Tile $dest_coord is empty. You cannot attack it."
                }
            }

            if ('A' == $action && $this->board[$dest_pos][0] == $cur_player) {
                return [false, $this->move_msg(_MD_CHESS_MOVE_ATTACK_SELF, $dest_coord)]; // "ERROR: You cannot attack own unit at $dest_coord."
            }

            /* backup affected tiles */

            $old_fig_tile = $this->board[$fig_pos];

            $old_dest_tile = $this->board[$dest_pos];

            /* perform move */

            $this->clear_tile($fig_pos);

            if (!$this->is_empty_tile($dest_pos)) {
                $this->piece_captured = \sprintf('%s%s', $this->board[$dest_pos], $dest_pos);
            }

            $this->board[$dest_pos] = "$cur_player$fig_type";

            if ($en_passant_capture_performed) {
                /* kill pawn */

                if ('w' == $cur_player) {
                    $this->clear_tile($dest_pos - 8);

                    $this->piece_captured = \sprintf('bP%s', $dest_pos - 8);
                } else {
                    $this->clear_tile($dest_pos + 8);

                    $this->piece_captured = \sprintf('wP%s', $dest_pos + 8);
                }
            }

            /* check check :) */

            if ($this->kingIsUnderAttack($cur_player, $cur_opp)) {
                $this->board[$fig_pos] = $old_fig_tile;

                $this->board[$dest_pos] = $old_dest_tile;

                if ($en_passant_capture_performed) {
                    // restore pawn that was captured above, since that move is invalid

                    if ('w' == $cur_player) {
                        $this->board[$dest_pos - 8] = 'bP';
                    } else {
                        $this->board[$dest_pos + 8] = 'wP';
                    }
                }

                return [false, _MD_CHESS_MOVE_IN_CHECK]; // "ERROR: Move is invalid because king would be under attack then."
            }

            // check whether this forbids any castling

            if ('K' == $fig_type) {
                if ('w' == $cur_player) {
                    $white_may_castle_short = false;

                    $white_may_castle_long = false;
                } else {
                    $black_may_castle_short = false;

                    $black_may_castle_long = false;
                }
            }

            if ('R' == $fig_type) {
                if ('w' == $cur_player) {
                    if (7 == $fig_pos) {
                        $white_may_castle_short = false;
                    } elseif (0 == $fig_pos) {
                        $white_may_castle_long = false;
                    }
                } else {
                    if (63 == $fig_pos) {
                        $black_may_castle_short = false;
                    } elseif (56 == $fig_pos) {
                        $black_may_castle_long = false;
                    }
                }
            }

            // if a pawn moved two tiles, this will allow 'en passant' on next move

            if ('P' == $fig_type && 16 == \abs($fig_pos - $dest_pos)) {
                $file_chars = 'abcdefgh';

                $this->gamestate['fen_en_passant_target_square'] = $file_chars[$fig_pos % 8] . ('w' == $cur_player ? '3' : '6');
            } else {
                // clear 'en passant'

                $this->gamestate['fen_en_passant_target_square'] = '-';
            }

            if ('M' == $action) {
                $result = $this->move_msg(_MD_CHESS_MOVE_MOVED, $fig_name, $fig_coord, $dest_coord);
            } else {
                $result = $this->move_msg(_MD_CHESS_MOVE_CAPTURED, $fig_name, $dest_coord, $fig_coord);
            }

            /* if pawn reached last line convert into a queen */

            if ('P' == $fig_type) {
                if (('w' == $cur_player && $dest_pos >= 56)
                    || ('b' == $cur_player && $dest_pos <= 7)) {
                    $pawn_upg = $move[\mb_strlen($move) - 1];

                    if ('?' == $pawn_upg) {
                        $pawn_upg = 'Q';

                        $history_move = \sprintf('%sQ', $history_move);
                    }

                    $this->board[$dest_pos] = "$cur_player$pawn_upg";

                    $result .= ' ... ' . $this->move_msg(_MD_CHESS_MOVE_PROMOTED, $this->getFullFigureName($pawn_upg));
                }
            }

            $move_handled = 1;
        }

        /* if a legal move was performed test whether you
         * check the opponent or even check-mate him. then
         * update castling and en-passant flags, select the
         * next player and add the move to the history. */

        if ($move_handled) {
            // Use $this->board to update $this->gamestate['fen_piece_placement'].

            $this->board_to_fen_piece_placement();

            // handle check, checkmate and stalemate

            $comment = '';

            if ($this->kingIsUnderAttack($cur_opp, $cur_player)) {
                // if this is check mate finish the game, otherwise if not just add a + to the move

                if ($this->isCheckMate($cur_opp, $cur_player)) {
                    $this->gamestate['pgn_result'] = 'w' == $cur_player ? '1-0' : '0-1';

                    $history_move .= '#';

                    $result .= ' ... ' . _MD_CHESS_MOVE_CHECKMATE;
                } else {
                    $history_move .= '+';
                }
            } elseif ($this->isStaleMate($cur_opp, $cur_player)) {
                $this->gamestate['pgn_result'] = '1/2-1/2';

                $result .= ' ... ' . _MD_CHESS_MOVE_STALEMATE;

                $comment = _MD_CHESS_DRAW_STALEMATE;
            } elseif ($this->insufficient_mating_material()) {
                $this->gamestate['pgn_result'] = '1/2-1/2';

                $result .= ' ... ' . _MD_CHESS_MOVE_MATERIAL;

                $comment = _MD_CHESS_DRAW_NO_MATE;
            }

            // store possible castling-availability modification

            $KQkq = ($white_may_castle_short ? 'K' : '') . ($white_may_castle_long ? 'Q' : '') . ($black_may_castle_short ? 'k' : '') . ($black_may_castle_long ? 'q' : '');

            $this->gamestate['fen_castling_availability'] = !empty($KQkq) ? $KQkq : '-';

            // strip old result-string from end of movetext

            // It's assumed that the movetext for a game in play is terminated by a '*', possibly preceded by whitespace.

            // (The whitespace will be present unless there are no moves in the game yet.)

            $this->gamestate['pgn_movetext'] = \preg_replace('/\s*\*$/', '', $this->gamestate['pgn_movetext']);

            // if white move, output move-number

            if ('w' == $this->gamestate['fen_active_color']) {
                if (!empty($this->gamestate['pgn_movetext'])) {
                    $this->gamestate['pgn_movetext'] .= ' ';
                }

                $this->gamestate['pgn_movetext'] .= $this->gamestate['fen_fullmove_number'] . '.';
                // if black move, no moves yet, and game is setup, output move number with special '...' terminator
            } elseif (empty($this->gamestate['pgn_movetext']) && !empty($this->gamestate['pgn_fen'])) {
                $this->gamestate['pgn_movetext'] .= $this->gamestate['fen_fullmove_number'] . '...';
            }

            // update movetext

            // comment is added only for a concluded game

            $this->gamestate['pgn_movetext'] .= ' ' . $history_move . ' ' . $this->gamestate['pgn_result'];

            if (!empty($comment)) {
                $this->gamestate['pgn_movetext'] .= ' {' . $comment . '}';
            }

            // if black move, increment move-number

            if ('b' == $this->gamestate['fen_active_color']) {
                ++$this->gamestate['fen_fullmove_number'];
            }

            // If pawn advance or capturing move, reset the halfmove clock. Otherwise increment it.

            if ('O-O' != $move && 'O-O-O' != $move && (0 === \strpos($move, 'P') || 'x' == $move[3])) {
                $this->gamestate['fen_halfmove_clock'] = 0;
            } else {
                ++$this->gamestate['fen_halfmove_clock'];
            }

            // set next player

            $this->gamestate['fen_active_color'] = 'w' == $this->gamestate['fen_active_color'] ? 'b' : 'w';
        }

        return [$move_handled, $result];
    }

    /**
     * Check whether a tile is empty.
     *
     * @param int $position
     * @return bool
     *
     * @access private
     */
    public function is_empty_tile($position)
    {
        return '00' == $this->board[$position];
    }

    /**
     * Clear a tile.
     *
     * @param int $position Position of tile
     *
     * @access private
     */
    public function clear_tile($position)
    {
        $this->board[$position] = '00';
    }

    /**
     * Convert FEN piece placement field to array.
     *
     * Use $this->gamestate['fen_piece_placement'] to initialize $this->board, $this->w_figures and $this->b_figures.
     *
     * @return bool True if piece placement is valid, otherwise false.
     *
     * @access private
     */
    public function fen_piece_placement_to_board()
    {
        if (empty($this->gamestate['fen_piece_placement']) || \mb_strlen($this->gamestate['fen_piece_placement']) > 71) {
            #trigger_error('piece placement empty or length invalid', E_USER_WARNING); #*#DEBUG#
            return false; // invalid length
        }

        $this->board = [];

        $piece_map = [
            'K' => 'wK',
            'Q' => 'wQ',
            'R' => 'wR',
            'B' => 'wB',
            'N' => 'wN',
            'P' => 'wP',
            'k' => 'bK',
            'q' => 'bQ',
            'r' => 'bR',
            'b' => 'bB',
            'n' => 'bN',
            'p' => 'bP',
        ];

        $tiles = \implode('', \array_reverse(\explode('/', $this->gamestate['fen_piece_placement'])));

        for ($i = 0, $iMax = mb_strlen($tiles); $i < $iMax; ++$i) {
            $tile = $tiles[$i];

            if (isset($piece_map[$tile])) {
                $this->board[] = $piece_map[$tile];
            } elseif (\is_numeric($tile) && $tile >= 1 && $tile <= 8) {
                for ($j = 0; $j < $tile; ++$j) {
                    $this->board[] = '00';
                }
            } else {
                #trigger_error("tile='$tile'", E_USER_WARNING); #*#DEBUG#
                return false; // unexpected character in piece_placement
            }
        }

        if (64 != \count($this->board)) {
            #trigger_error('count(board)=' . count($this->board), E_USER_WARNING); #*#DEBUG#
            return false; // piece_placement has incorrect number of tiles
        }

        $this->w_figures = [];

        $this->b_figures = [];

        for ($i = 0; $i < 64; ++$i) {
            $tile = $this->board[$i];

            $coordinates = $this->boardIndexToCoord($i);

            if ('w' == $tile[0]) {
                $this->w_figures[] = $tile[1] . $coordinates;
            } elseif ('b' == $tile[0]) {
                $this->b_figures[] = $tile[1] . $coordinates;
            }
        }

        return true;
    }

    /**
     * Convert array to FEN piece placement field.
     *
     * Use $this->board to initialize $this->gamestate['fen_piece_placement'].
     *
     * @access private
     */
    public function board_to_fen_piece_placement()
    {
        $rows = [];

        for ($rank = 7; $rank >= 0; --$rank) {
            $row = '';

            for ($file = 0; $file < 8; ++$file) {
                $index = 8 * $rank + $file;

                if (!$this->is_empty_tile($index)) {
                    $tile = $this->board[$index];

                    $piece = 'w' == $tile[0] ? $tile[1] : \mb_strtolower($tile[1]); // 'K','Q','R','B','N' or 'P' (uppercase for white, lowercase for black)
                } else {
                    $piece = 'x'; // temporarily mark each empty tile with 'x'
                }

                $row .= $piece; // append piece symbol to row-string
            }

            $rows[] = $row;
        }

        // Concatenate the eight row-strings with the separator '/'.

        // Then replace each string of x's with the string length.

        $this->gamestate['fen_piece_placement'] = \preg_replace_callback('/(x+)/', static function($matches) {return \strlen($matches[1]);}, \implode('/', $rows));
    }

    /**
     * Determine whether there is insufficient material to mate.
     *
     * @return bool  True if only the following pieces remain: K vs. K, K vs. K+B or K vs. K+N; otherwise false.
     *
     * @access private
     */
    public function insufficient_mating_material()
    {
        $pieces = \mb_strtoupper($this->gamestate['fen_piece_placement']);

        $counts = \count_chars($pieces, 1);

        $num_queens = (int)@$counts[\ord('Q')];

        $num_rooks = (int)@$counts[\ord('R')];

        $num_bishops = (int)@$counts[\ord('B')];

        $num_knights = (int)@$counts[\ord('N')];

        $num_pawns = (int)@$counts[\ord('P')];

        return 0 == $num_queens && 0 == $num_rooks && ($num_bishops + $num_knights) <= 1 && 0 == $num_pawns;
    }

    // --------------------------

    // INCIDENTAL PRIVATE METHODS

    // --------------------------

    // These functions don't really need to be class methods, since they don't access class properties.

    // They're placed within the class only for name-scoping.

    /**
     * Convert board coordinates [a-h][1-8] to board index [0..63]
     *
     * @param string $coord Example: 'b3'
     * @return int  Example: 17
     *
     * @access private
     */
    public function boardCoordToIndex($coord)
    {
        //echo $coord," --> ";

        switch ($coord[0]) {
            case 'a':
                $x = 0;
                break;
            case 'b':
                $x = 1;
                break;
            case 'c':
                $x = 2;
                break;
            case 'd':
                $x = 3;
                break;
            case 'e':
                $x = 4;
                break;
            case 'f':
                $x = 5;
                break;
            case 'g':
                $x = 6;
                break;
            case 'h':
                $x = 7;
                break;
            default:
                return 64; /* erronous coord */
        }

        $y = $coord[1] - 1;

        if ($y < 0 || $y > 7) {
            return 64;
        } /* erronous coord */

        return $y * 8 + $x;
        //echo "$index | ";
    }

    /**
     * Convert board index [0..63] to board coordinates [a-h][1-8].
     *
     * @param int $index Example: 17
     * @return string  Example: 'b3'
     *
     * @access private
     */
    public function boardIndexToCoord($index)
    {
        //echo $index," --> ";

        if ($index < 0 || $index > 63) {
            return '';
        }

        $y = \floor($index / 8) + 1;

        $x = \chr(($index % 8) + 97);

        return "$x$y";
    }

    /**
     * Get piece name from piece symbol.
     *
     * @param string $short Piece symbol
     * @return string        Piece name
     *
     * @access private
     */
    public function getFullFigureName($short)
    {
        static $names = [
            'K' => _MD_CHESS_MOVE_KING,
            'Q' => _MD_CHESS_MOVE_QUEEN,
            'R' => _MD_CHESS_MOVE_ROOK,
            'B' => _MD_CHESS_MOVE_BISHOP,
            'N' => _MD_CHESS_MOVE_KNIGHT,
            'P' => _MD_CHESS_MOVE_PAWN,
        ];

        return $names[$short] ?? _MD_CHESS_MOVE_EMPTY;
    }

    /**
     * Get tiles adjacent to specified tile.
     *
     * @param int $fig_pos
     * @return array
     *
     * @access private
     */
    public function getAdjTiles($fig_pos)
    {
        $adj_tiles = [];

        $i = 0;

        $x = $fig_pos % 8;

        $y = \floor($fig_pos / 8);

        if ($x > 0 && $y > 0) {
            $adj_tiles[$i++] = $fig_pos - 9;
        }

        if ($y > 0) {
            $adj_tiles[$i++] = $fig_pos - 8;
        }

        if ($x < 7 && $y > 0) {
            $adj_tiles[$i++] = $fig_pos - 7;
        }

        if ($x < 7) {
            $adj_tiles[$i++] = $fig_pos + 1;
        }

        if ($x < 7 && $y < 7) {
            $adj_tiles[$i++] = $fig_pos + 9;
        }

        if ($y < 7) {
            $adj_tiles[$i++] = $fig_pos + 8;
        }

        if ($x > 0 && $y < 7) {
            $adj_tiles[$i++] = $fig_pos + 7;
        }

        if ($x > 0) {
            $adj_tiles[$i++] = $fig_pos - 1;
        }

        /* DEBUG:  foreach( $adj_tiles as $tile )
          echo "adj: $tile "; */

        return $adj_tiles;
    }
} // class ChessGame

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