Mizar chess engine

Xiangqi, is a two-player Chinese board game in the same family as Western chess, chaturanga, shogi and janggi. The present-day form of Xiangqi originated in China and is therefore commonly called Chinese chess in English. Xiangqi has a long history. Though its precise origins have not yet been confirmed, the earliest literary reference comes from the 9th century.

Xiangqi is one of the most popular board games in the world. Distinctive features of Xiangqi include the unique movement of the pao (“cannon”) piece, a rule prohibiting the generals (similar to chess kings) from facing each other directly, and the river and palace board features, which restrict the movement of some pieces.

Rules

Board setup

Xiangqi is played on a board that is 9 lines wide by 10 lines long. In a manner similar to the game Go, the pieces are played on the intersections, which are known as points. The vertical lines are known as files, while the horizontal lines are known as ranks. With a few awkward substitutions, it is possible to play this game using a standard chess set.

Centered at the first through third ranks of the board is a square zone also mirrored in the opponent’s territory. The three point by three point zone is demarcated by two diagonal lines connecting opposite corners and intersecting at the center point. This area is known as the palace or fortress.

Dividing the two opposing sides (between the fifth and sixth ranks) is the river. The river is often marked with the phrases “Chu River”, and “Han border”, a reference to the Chu-Han War. Although the river provides a visual division between the two sides, only a few pieces are affected by its presence: soldiers are promoted after crossing, and elephants cannot cross the river.

The starting points of the soldiers and cannons are typically marked with small crosses, but not all boards have these marks.

The Pieces

The pieces are small discs of wood, plastic, or some other material. Pieces are identified by Chinese ideograms in the team colours, typically black (sometimes another dark colour) and red. The names of some of the pieces differ on the two sides. The character on the red elephant, for example, actually means minister or augur. However, discussions of the game in English invariably assign the same names to the pieces on both sides. There is also some variation in the form of the characters, especially in older sets.

General. The general starts the game at the midpoint of the back edge (within the castle). One point in any non-diagonal direction. Cannot move outside the castle. In addition, the general has the theoretical power of moving like a rook along a file from his own castle to the enemy castle, to capture the opposing general. Therefore it is illegal to make any move that leaves your own general on an open file opposite the opposing general, because to do so would be to move into check.

Mandarin (also known as guard, advisor or minister, and less commonly as assistant or warrior). The mandarins start to the sides of the general. One point in any diagonal direction. Cannot move outside the castle.

Elephant. They are located next to the mandarins. Two points in any diagonal direction. It must move two points, and cannot leap another piece of either colour. Cannot cross the river. An elephant can thus reach only seven points on the board.

Horse. They begin the game next to the elephants. One point in any non-diagonal direction, followed by one point in a diagonal direction, so that it ends two points away from where it started. This is similar to the knight’s move in Western chess, except that the move is blocked by any piece occupying the point at the “elbow” of the move. Hence it is important to remember that the non-diagonal part of the move comes first.

Chariot. The chariots begin the game on the points at the corners of the board. Any number of points in any non-diagonal direction. Cannot leap. This is just like the rook’s move in Western chess.

Cannon. The cannons start on the row behind the soldiers, two points in front of the horses. When not capturing, moves just like the chariot. When capturing, must leap a single piece of either colour before proceeding to the point occupied by the target piece. This intervening piece is called a screen.

Soldier. Soldiers are placed on alternating points, one row back from the edge of the river. One point straight forward. After it reaches the opposite river bank, can move one point forward or directly sideways. Never moves diagonally or backward. No further promotion is gained when a soldier reaches the farthest rank of the board

Player take alternate turns. In each turn, a player must make a single move with a single piece. If a piece ends its move on a point occupied by an enemy piece, that piece is captured and permanently removed from play. The object of the game is to capture the enemy general. The game is won as soon as one player can make no move that prevents capture of his general. This is checkmate. Stalemate, where one player has no legal move but is not in check, is a win for the last player to move. It is illegal to make any move that exposes your general to immediate capture. This is called moving into check. It is illegal to avoid defeat or attempt to force a draw by repeating the same series of moves over and over. In particular, perpetual check is not allowed, and the onus is on the attacker to vary his move.

Strategy

Xiangqi is a fast game for several reasons. First, the barrier of pawns is reduced dramatically. Second, the cannons jump to capture, making them a long-range threat early in the game. In addition, since the general is confined to only moving within the palace, it can be checkmated more easily unless it is protected by other pieces. Because of the size of the board and the relative low number of long-range pieces, it may take time to move one’s army of pieces from place to place on the board, and there is a tendency for the battle to focus on a particular area of the board. Common strategies used in Western chess such as forking with horse and pinning with chariot (sometimes the cannon and general can also pin) are also applicable in xiangqi. Usually, the soldiers do not support each other unless the player has no better move. This is because from the initial position, it takes a minimum of 5 moves of a soldier to allow twin soldiers to protect each other. Defensively, a common configuration is to leave the general at his or her starting position, deploy one advisor and one elephant on the two points directly in front of the general, and to leave the other advisor and the other elephant in their starting positions, to the side of the general. In this setup, the paired-up advisors and elephants support each other, and the general is immune from attacks by cannons. However, with the loss of a single advisor or elephant, the general becomes vulnerable to cannons, and this setup may need to be abandoned. The defender may move advisors or elephants away from the general, or even sacrifice them intentionally, to ward off attack by a cannon. The two chariots are not normally lined up together as they are the most powerful piece and in doing so, a player risks the chances of losing at least one chariot to an inferior piece of the enemy. Depending on the situation, it may be advantageous to position a chariot at one of the corners of the enemy’s side of the board, where it is very difficult to dislodge, and threatens the enemy general. It is common to use the cannons independently to control particular ranks and files. Using a cannon to control the middle file is often considered vital strategy, because it helps to lock certain pieces such as the advisors and elephants in certain positions to prevent a check. The two files adjacent to the middle rank are also considered important and knights and chariots can be used to push for mate here. In addition, the cannons can also be used one in front of one another in the centre line, therefore checkmating the general/marshall in almost all scenarios, as the front cannon ensures that nothing can block, and the rear ensuring that if the front cannon is taken, the general/marshall is still check, and therefore resulting in checkmate. Unfortunately, there are ways to block this tactic, but there is less chance that the opponent can block this manoeuvre in time.

Opening

Since the left and right flank of the starting setup are symmetrical and therefore equivalent, it is customary to always make the first move from the right flank. Starting on the left flank is considered to be needlessly confusing.

The most common opening is to move the cannon to the central column, the most common reply is to advance the horse on the same flank. This is usually followed by the most common second move, in which the first player moves a chariot forward one space, usually the right one (moving the left one loses the horse, but you can reply by trapping the cannon with your chariots). The most common reply is to move the right advisor diagonally. This is to prevent a series of events that leads to the first player quickly checkmating the second.

Less common first moves include:

• moving an elephant to the central column
• advancing the soldier on the third or seventh file
• moving a horse forward
• moving either cannon behind the 2nd pawn from the left or right

General advice for the opening includes rapid development of at least one chariot, because it is the most powerful piece and the only long-range piece besides the cannon. It may not be a bad move to develop one horse to the edge of the board, for example, to avoid being blocked by one of one’s own pawns that cannot advance. Usually, at least one horse should be moved to the middle.

Beginners often succumb to an early checkmate with two cannons. This checkmate may be executed in four moves from the beginning of the game. However, it is easily countered by the horse reply. A double cannon technique involves 2 cannons of the same side lining up with the enemy general with no other pieces in between. This results in a check as the rear cannon uses the front cannon as cannon platform. The opponent cannot get away by placing a piece in front of the general to block the rear cannon because the front cannon will use that newly-moved piece as cannon platform to capture the general. The solution is either to move the general up before the check or to nullify the 2nd cannon either by taking it out or placing a piece between the two cannons.

Computer Xiangqi

Compared to other games such as Checkers, Othello, and Go, the complexity of Chinese chess can be said to be similar to that of Western chess, although it seems to lead to longer tactical exchanges. On average slightly more capturing moves are possible, the board is slightly larger, and a draw by repetition of forcing moves is not allowed. Despite that, the tactical exchange and long-term planning ideas from chess carry across. Thus most computer methods developed for chess apply equally well in Chinese Chess.

Chinese-chess programs are like Western-chess programs in that they can be divided into two types: knowledge-based and brute-force programs. A knowledge-based program emphasises the evaluation of positions. A brute-force program uses the ability to compute quickly and to explore the game tree deeply, so as to discover better moves. Even in Western-chess, the better approach has not been identified yet. Chinese chess has more different pieces and a larger board than chess, so knowledge-based programs are considered to have more potential.

A game of Chinese chess can usually be divided into three stages – the opening, the middle game, and the endgame. In the opening, Chinese-chess programs currently use an opening database to support computation. An opening database is enlarged by Chinese-chess game records from tournaments on the Internet and by the results of Chinese-chess tournaments. According to the results of the competition between human and computer, the best program with an adequate opening database is currently above 7-dan and the program that implements adequate searches in the middle game can exceed 6-dan. These ratings may be improved as hardware advances. The endgame is the weakest part of the present Chinese-chess programs. A massive memory is necessary to combine endgame database and the search system. The development of hardware may overcome this difficulty.

[from Wikipedia.org; "An Introduction to Chinese Chess" by Peter Donnelly; "A gamut of games" by Jonathan Schaeffer; "Searching for solutions in games and artificial intelligence" L. V. Allis; "Computer Chinese Chess" Yen, Chen, Yang Hsu]