Chess

The chess package sets chess diagrams and game scores. Load it with:

\use{chess}

What makes it more than a board-drawing macro is that it plays the moves. A document writes a line of algebraic notation the way a book prints it, and asks for a diagram; the diagram is whatever position those moves reached, so it cannot disagree with them.

\newgame
\mainline{1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6 5.O-O Be7}
\centerline{\showboard}

Setting a position

\fenboard reads the placement field and, if it is there, the side to move:

\fenboard{r1bqkb1r/pppp1Qpp/2n2n2/4p3/2B1P3/8/PPPP1PPP/RNB1K1NR b}

The fields FEN carries after that — castling rights, the en-passant square, the move clocks — are read and dropped. Nothing needs them: castling is named explicitly in the notation, and an en-passant capture identifies itself.

\chessfen is the inverse. It prints where it stands, which is a quick way to check what a line of moves reached.

Playing moves

All three take the moves as they are written — 1.e4 e5 2.Nf3 or 1. e4 e5 2. Nf3, with the numbers or without them. The numbers in the input are not trusted for the numbers that get printed: \mainline counts those from the position, so a line beginning at Black’s twelfth move prints 12... whatever the source says.

\hidemoves is how a document reaches a position it wants to talk about without printing the moves that got there. \showmoves is the opposite — a sideline set in the text beside a main position it must not disturb:

\newgame
\hidemoves{1.d4 d5 2.c4 c6 3.Nf3 Nf6 4.Nc3 dxc4 5.a4 Bf5 6.e3 e6 7.Bxc4 Bb4 8.O-O Nbd7}
\showmoves{9.Qe2 Bg6 10.e4 O-O}
\centerline{\showboard}

The diagram there is the position after Black’s eighth move. The two moves \showmoves printed were set and then forgotten.

A word that names a result — 1-0, 0-1, 1/2-1/2, * — is set as written and ends the line.

What the notation leaves out

Algebraic notation names the square a piece arrives at and leaves the square it came from to be worked out from the position. Nf3 is whichever knight can reach f3. The package works it out the way a reader does — by looking outward from the destination along the moving piece’s own geometry — and then narrows what it finds:

1. The disambiguator, when the notation gives one. Rad1 is the rook on the a-file; R1d2 is the rook on the first rank.
2. Legality, when it does not. Notation omits the disambiguator when only one of the two candidates may legally move — which means the other is pinned. The package makes each candidate’s move on a copy of the position and discards the one that would leave its own king attacked.

Castling, promotion and en passant all work. En passant needs no remembered state: a pawn arriving on an empty square from another file can only be capturing in passing, so the pawn it takes is the one beside it.

Check and mate marks (+, #) and annotation marks (!, ?) are read and ignored — they say nothing about the move that the position does not already say.

When a move cannot be read

A move the position cannot account for is reported on the diagnostic channel and skipped:

chess: cannot read the move 'Ne4' in the position rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w

The rest of the line still plays, and the side to move still passes over — so a single mistyped move does not put every move after it on the wrong side. The diagram that follows will be visibly wrong rather than quietly so, which is the point.

Figurines

A printed move sets its piece letter as the piece, in the convention that gives White the outline figurines and Black the solid ones — so a move quoted on its own says whose it is. Set chessfigurines to 0 for plain letters.

The glyphs come from the Miscellaneous Symbols block (U+2654–265F) of the bundled chess face, which is Noto Sans Symbols 2. No text face covers those codepoints, and none is likely to, so the face is part of the font tree rather than the compiled-in core: a document using this package needs an installed texish, not a bare binary.

The diagram

\showboard draws one \picture box, which flows in the text like any other box — in a paragraph, in a \centerline, or in a cell of a larger layout. Everything about it is a variable, set with \set after \use{chess}:

A board small enough to sit in a line of text is \set chesssquare {9} with \set chesscoords {0}; a green board is \set chesslight {#eeeed2} and \set chessdark {#769656}.

How it works

There is no chess-specific engine primitive. The position is a \map from a square’s name to a piece letter in FEN’s convention — uppercase for White, lowercase for Black, . for empty — which makes the side owning a piece a matter of case. Reading a move is a walk over the characters; finding the piece that moved is \for over the knight’s eight leaps or a ray cast along each of a rook’s four directions; making a move is \mapset. Saving a position in order to test a move’s legality is an assignment, because a map is a value.

The whole package is packages/chess.texish, and scripts/chess-demo.script exercises all of it.