FEN

ChessCore parses and serializes standard FEN, and exposes three FEN-shaped accessors, each suited to a different purpose.

Parsing

let position = Position(fen:
    "rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq e3 0 1")

init(fen:) is failable. It requires at least the four core fields (placement, side, castling, en-passant) and eight ranks, and it recomputes the king squares. It returns nil on malformed input.

The three accessors

Accessor	What it contains	Use for
fen	full standard FEN, including the halfmove and fullmove counters	display, storage, round-tripping
positionKey	FEN without the move counters (board + side + castling + EP)	the legality / transposition key
consistentFEN	metadata fields made consistent with the placement	any FEN leaving for a parser you don't control

print(position.fen)          // "...b KQkq e3 0 1"
print(position.positionKey)  // "...b KQkq e3"     (no counters)

Crossing the engine boundary

Stockfish's FEN parser asserts on inconsistent metadata and aborts the process (assert(is_ok(s))). Always hand it consistentFEN, which zeros out castling rights that don't match the placement and drops phantom en-passant targets:

engine.send("position fen \(position.consistentFEN)")
engine.send("go depth 20")

Real en passant

enPassantTarget is the raw FEN field. capturableEnPassantTarget reports the EP square only when a capture is genuinely available — the X-FEN / Polyglot "real en passant" rule. Use it for transposition matching, since two positions that differ only in a phantom EP target should be treated as the same position.

if let ep = position.capturableEnPassantTarget {
    print("a real en-passant capture is possible on", ep.algebraic)
}

Insufficient material

if position.hasInsufficientMaterial {
    // K vs K, K + single minor vs K, or K+B vs K+B with same-colored bishops
}