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Chapter 42. Chess Game Notation

See the section called “Chessboard Locations” for some additional background.

Chess is played on an 8x8 board. One player has white pieces, one has black pieces. Each player's pieces include eight pawns, two rooks, two knights, two bishops, a king and a queen. The various pieces have different rules for movement. Players move alternately until one player's king is in a position from which it cannot escape but must be taken, or there is a draw. There are a number of rules that lead to a draw, all beyond the scope of this problem. White moves first.

A game is recorded as a log of the numbered moves of pieces, first white then black. The Portable Game Notation (PGN) standard includes additional descriptive information about the players and venue.

There are two notations for logging a chess game. The newer, algebraic notation and the older descriptive notation. We will write a program that will process a log in either notation and play out the game, showing the chess board after each of black's moves. It can also be extended to convert logs to completely standard PGN notation.

Algebraic Notation

We'll present the formal definition of algebraic notation including Algebraic Notation (LAN) and Short Algebraic Notation (SAN). We'll follow this with a summary and some examples. This section will end with some Algorithm R, used to resolve which of the available pieces could perform a legal move.

Definition

Algebraic notations uses letters a-h for the files (columns across the board) from white's left to right, and numbers for the ranks (rows of the board) from white (1) to black (8).

Piece symbols in the log are as follows:

Piece Symbol Move Summary
Pawn (omitted) 1 or 2 spaces forward
Rook R anywhere in the same rank or same file
Knight N 2 in one direction and 1 in the other “L-shaped
Bishop B diagonally, any distance
Queen Q horizontal, vertical or diagonal, any distance
King K 1 space in any direction

The game beings in the following starting position. For white the pieces are arranged as follows:

White Black Piece
a1 a8 rook
b1 b8 knight
c1 c8 bishop
d1 d8 queen
e1 e8 king
f1 f8 bishop
g1 g8 knight
h1 h8 rook
a2-h2 a7-h7 pawns

There are two forms of algebraic notation: short (or standard) algebraic notation (SAN), where only the destination is shown, and long algebraic notation (LAN) that shows the piece, the starting location and the destination.

The basic syntax for LAN is as follows:

[ P ] frmfr [ n ]

P is the piece name (omitted for pawns), f is file (a-h), r is rank (1-8), m is move (- or x) and n is any notes about the move (+, #, !, !!, ?, ??). The notes may include = and a piece letter when a pawn is promoted.

Short notation omits the starting file and rank unless they are essential for disambiguating a move. The basic syntax is as follows:

[ P ] [ m ] [ d ] fr [ n ]

The move, m , is only specified for a capture (x). The optional d is either a file (preferred) or a rank or both used to choose which piece moved when there are multiple possibilities.

In both notations, the castle moves are written O-O or O-O-O (capital letters, not numbers). Similarly, the end of a game is often followed with a 1-0 (white wins), 0-1 (black wins), 1/2-1/2 (a draw), or * for a game that is unknown or abandoned.

Each turn is preceeded by a turn number. Typically the number and a . preceeds the white move. Sometimes (because of commentary), the number and ... preceed the black move.

The PGN standard for notes is $ and a number, common numbers are as follows:

1 good move (traditional “!”)

2 poor move (traditional “?”)

3 very good move (traditional “!!”)

4 very poor move (traditional “??”)

Each piece has a legal move. This is a critical part of processing abbreviated notation where the log gives the name of the piece and where it wound up. The legal moves to determine which of two (or eight) pieces could have made the requested move. This requires a simple search of pieces to see which could have made the move.

A pawn moves forward only, in its same file. For white, the rank number must increase by 1. It can increase by 2 when it is in its starting position (rank 7 for white or 2 for black). For black, the rank number must decrease by 1 or 2. A pawn captures on the diagonal: it will move into an adjacent file and forward one rank, replacing the piece that was there. There is one origin for all but the opening pawn moves (one rank back on the file in which the pawn ended its move); one origin for an opening pawn move that lands in rank 4 or 5 (two ranks back on the file where the pawn ended its move); two possible origins for any pawn capture (one position on a file adjacent to the one in which the pawn ended its move).

A rook moves in ranks or files only, with no limit on distance. There are 16 possible origins for any rook move, including the entire rank or the entire file in which the rook ended its move.

A knight makes an “L-shaped” move. It moves two spaces in one direction, turns 90-degrees and moves one more space. From g1, a knight can move to either f3 or h3. The rank changes by 2 and the file by 1; or the file changes by 2 and the rank changes by 1. There are 8 places a knight could start from relative to its final location.

A bishop moves diagonally. The amount of change in the rank must be the same as the change in the file. There are 16 places a bishop can start from on the two diagonals that intersect the final location.

The queen's move combines bishop and rook: any number of spaces diagonally, horizontally or vertically. There are 16 places on the diagonals, plus 16 more places on the horizontals and verticals where the queen could originate. Pawns that reach the opposite side of the board are often promoted to queens, meaning there can be multiple queens late in the game.

The king is unique, there is only one. The king can only move one space hoizontally, vertically or diagonally.

The king and a rook can also engage in a move called castling: both pieces move. When the king and the closest rook (the one in file h) castle, this is king side and annoted O-O. The king moves from file e to file g and the rook from fiel h to file f. When the king and the queen's side rook (the one in file a) castle, this is annotated O-O-O. The king moves from file e to file c and the rook move from file a to file d. Castling can only be accomplished if (a) neither piece has moved and (b) space between them is unoccupied by other pieces. Part a of this rule requires that the game remember when a king or rook moves, and eliminate that side from available castling moves. Moving the rook in file a eliminates queen-side castling; moving the rook in file h eliminates king-side castling. Moving the king eliminates all castling.

Summary and Examples

Here's a suymmary of the algebraic notation symbols used for annotating chess games. This is followed by some examples.

Symbol Meaning
a-h file from white's left to right
1-8 rank from white to black
R, N, B, Q, K rook, knight, bishop, king, queen
- move (non-SAN)
x capture; the piece that was at this location is removed
+ check, a note that the king is threatened
# checkmate, a note that this is the reason for the end of the game
++ checkmate (non-SAN), a note that this is the reason for the end of the game
= promoted to; a pawn arriving at the opposite side of the board is promoted to another piece, often a queen.
0-0 castle on the king's side; swap the king and the rook in positions e1 and h1 (if neither has moved before this point in the game)
0-0-0 castle on the queen's side; swap the king and the rook in positions e1 and a1 (if neither has moved before this point in the game)
e.p. en passant capture (non-SAN), a note that a pawn was taken by another pawn passing it. When a pawn's first move is a two space move (from 7 to 5 for black or 2 to 4 for white) it can be captured my moving behind it to the 6th rank (white taking black) or 3rd rank (black taking white).
ep en passant capture (non-SAN), see e.p.
?, ??, !, !! editorial comments (non-SAN), weak, very weak, strong, very strong

Here's parts of an example game in abbreviated notation:

  1. e4 e5. White pawn moves to e4 (search e3 and e2 for the pawn that could do this); black pawn moves to e5 (search e6 and e7 for a pawn that could do this)

  2. Nf3 d6. White knight moves to f3 (search 8 positions: g1, h2, h4, g5, e5, d4, d2, e1 and g1 for the knight that could do this); black pawn moves to d6 (search d7 and d8 for the pawn).

  3. d4 Bg4. White pawn moves from d4 (search d3 and d2 for the pawn); black bishop moves to g4 (search the four diagonals: f5, e6, d7, c8; h5; h3; f3, e3, and d3 for the bishop that could do this).

  4. dxe5 Bxf3. A white pawn in d takes a piece at e5, the pawn must have been at d4, the black pawn at e5 is removed; a black bishop takes a piece at f3 (search the four radiating diagonals from f3: e4, d5, c6, b7, a8; g4, h5; g2, h1; e2, d1).

  5. Qxf3 dxe5. The white queen takes the piece at f3; the black pawn in d4 takes the piece in e5.

Here's a typical game in abbreviated notation:

1.c4 e6 2.Nf3 d5 3.d4 Nf6 4.Nc3 Be7 5.Bg5 0-0 6.e3 h6 7.Bh4 b6 
8.cxd5 Nxd5 9.Bxe7 Qxe7 10.Nxd5 exd5 11.Rc1 Be6 12.Qa4 c5 
13.Qa3 Rc8 14.Bb5 a6 15.dxc5 bxc5 16.0-0 Ra7 17.Be2 Nd7 
18.Nd4 Qf8 19.Nxe6 fxe6 20.e4 d4 21.f4 Qe7 22.e5 Rb8 
23.Bc4 Kh8 24.Qh3 Nf8 25.b3 a5 26.f5 exf5 27.Rxf5 Nh7 
28.Rcf1 Qd8 29.Qg3 Re7 30.h4 Rbb7 31.e6 Rbc7 32.Qe5 Qe8 
33.a4 Qd8 34.R1f2 Qe8 35.R2f3 Qd8 36.Bd3 Qe8 37.Qe4 Nf6 
38.Rxf6 gxf6 39.Rxf6 Kg8 40.Bc4 Kh8 41.Qf4 1-0

Here's a small game in full notation:

1.f2-f4 e7-e5 2.f4xe5 d7-d6 3.e5xd6 Bf8xd6 4.g2-g3 Qd8-g5 
5.Ng1-f3 Qg5xg3+ 6.h2xg3 Bd6xg3#

Algorithms for Resolving Moves

Algebraic notation is terse because it is focused on a human student of chess. It contains just enough information for a person to follow the game. Each individual move cannot be interpreted as a stand-alone (or “context-free” statement). Each move's description only makes sense in the context of the game state established by all the moves that came before it. Therefore, in order to interpret a log of chess moves, we also need to maintain the state of the chess board.

Given that we have a model of the chess board, Algorithm G can locate the pieces and execute the moves an entire game.

Procedure 42.1. Algoritm G

(Resolve chess moves in SAN notation, playing out the entire game.) We are given a block of text with a sequence of chess turns. Assume that line breaks have been removed and the game ending marker has been removed from the block of text.

  1. Parse turn into moves. Locate move number, white move and black move. Lines that don't have this form are some kind of commentary and can be ignored.

  2. Parse each move. For each move, parse the move locating the piece (R, B, N, Q, K, pawn if none of these), optional file (a-h) or rank (1-8) for the source, the optional x for a capture, the destination file (a-h) and rank (1-8), and other material like + or # for checks, = x for promotions, or !, !!, ?, ?? for editorial comments.

  3. Castling? If the move is simply 0-0 or O-O-O, move both the king (in file e) and the appropriate rook. For 0-0 it is the rook in file h moves to f, the king moves from e to g. For O-O-O it is the rook in file a moves to d, the king moves from e to c.

  4. Fully specified from location? If a two-character from-position is given, this is the starting location. Execute the move with step 7.

  5. Partially specified from location? If a one-character from-position is given (a-h or 1-8), restrict the search for the source to this rank for file. Use the piece-specific version of Algorithm S with rank or file restrictions for the search. After the starting location is found, execute the move with step 7.

  6. Omitted from location? Search all possible origins for the from-position for this piece. Each piece has a unique search pattern based on the piece's movement rules. Use the piece-specific version of Algorithm S with no restrictions for the search. After the starting location is found, execute the move with step 7.

  7. Execute move. Move the piece, updating the state of the board, removing captured pieces. Periodically during game processing, print the board position. The board, by the way, is always oriented so that a1 is a dark square in the lower-left.

  8. Next move. Loop to step 2, processing the black move after the white move in this turn. If the black move is omitted or is one of the ending strings, skip the black move.

  9. Next turn. Loop to step 1, processing the next turn. If the turn number is omitted or is one of the ending strings, this is the end of the game.

We have to design six different kinds of searches for possible starting pieces. These searches include pawns, rooks, knights, bishops queens and the king. We'll provide formal algorithms for pawns and rooks, and informal specifications for the other pieces.

Procedure 42.2. Algorithm P

(Search for possible pawn starting locations.) Given a destination location, piece color, and optional restrictions on starting rank or starting file.

  1. First move. If the destination is rank 4 (white) or rank 5 (black), search two spaces back for the first move of a pawn (from rank 7 or rank 2). If moving this piece will not put the king into check, this is the stating location.

  2. Previous space. Search the previous space (rank -1 for white, rank +1 for black) for a move. If moving this piece will not put the same color king into check, this is the stating location.

  3. Capture. Search the adjacent files one space back for a pawn which performed a capture. If moving this piece will not put the same color king into check, this is the stating location.

Procedure 42.3. Algorithm R

(Search for possible rook starting locations.) Given a destination location, piece color, and optional restrictions on starting rank or starting file.

  1. To Right. Set r ← +1.

  2. Target. Target position has file offset by r.

  3. Check. If this is off the board, or a non-rook was found, skip to 4. If moving this piece will not put the king into check, this is the stating location.

  4. Loop. Increment r. Repeat from step 2.

  5. To Left. Set r ← -1.

  6. Target. Target position has file offset by r.

  7. Check. If this is off the board, or a non-rook was found, skip to 8. If moving this piece will not put the king into check, this is the stating location.

  8. Loop. Decrement r. Repeat from step 5.

  9. To Black. Set r ← +1.

  10. Target. Target position has rank offset by r.

  11. Check. If this is off the board, or a non-rook was found, skip to 12. If moving this piece will not put the king into check, this is the stating location.

  12. Loop. Decrement r. Repeat from step 9.

  13. To White. Set r ← -1.

  14. Target. Target position has rank offset by r.

  15. Check. If this is off the board, or a non-rook was found, we have a problem. If moving this piece will not put the king into check, this is the stating location.

  16. Loop. Decrement r. Repeat from step 13.

Procedure 42.4. Algorithm N

(Search for possible knight starting locations.) Given a destination location, piece color, and optional restrictions on starting rank or starting file.

  1. Search the eight possible starting positions for a knight's move. Four of the positions have a file offset of +1 or -1 and rank offsets of +2 or -2. The other four positions have file offsets of +2 or -2 and rank offsets of +1 or -1.

  2. When evaluating a piece, moving this piece cannot put the king into check.

Procedure 42.5. Algorithm B

(Search for possible bishop starting locations.) Given a destination location, piece color, and optional restrictions on starting rank or starting file.

  1. Search radially out the diagonals until edge of board or an intervening piece or the correct bishop was found. This algorithm is similar to the rook algorithm, except the offsets apply to both rank and file. Applying +1 to both rank and file moves north-east; applying -1 to both rank and file moves south-west. Applying +1 to rank and -1 to file moves south east; applying -1 to rank and +1 to file moves north west.

  2. When evaluating a piece, moving this piece cannot put the king into check.

Procedure 42.6. Algorithm Q

Search for possible queen starting locations.) Given a destination location, piece color, and optional restrictions on starting rank or starting file.

  1. Search radially out the ranks, files and diagonals until edge of board or an intervening piece or the correct queen was found. This combines the bishop and rook algorithms.

  2. When evaluating a piece, moving this piece cannot put the king into check.

Procedure 42.7. Algorithm K

Search for possible king starting locations.) Given a destination location, piece color, and optional restrictions on starting rank or starting file.

  • Search the 8 adjacent locations. These are all combinations of -1, 0, +1 for rank offset and -1, 0, +1 for file offset; skipping the one combination with a 0 offset to rank and a 0 offset to file.


 
 
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