Function positionKey

positionKey(state: ChessState): string
The position half of FEN: placement, side to move, castling rights and en passant square, without either move clock. Two positions share a key exactly when the same moves are legal from both, which is what makes it the lookup key for an opening book (board/openings.ts builds and probes keys with this function on both sides, so the en passant square is taken as stored rather than re-derived).
Parameters
- state: ChessState
Returns string
Example
```
import { positionKey, startingChess } from '@datamoc/mwg/board';

console.log(positionKey(startingChess()));
// 'rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq -'
```
- Defined in src/board/chess.ts:99