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CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Commands
+
+```bash
+# Run all tests
+dart test
+
+# Run a single test file
+dart test test/position_test.dart
+
+# Run a specific test by name
+dart test --name "some test name"
+
+# Analyze code (lint)
+dart analyze
+
+# Format code
+dart format .
+
+# Run benchmarks
+dart run benchmark/dartchess_benchmark.dart
+
+# Run perft (move generation correctness/performance test)
+dart run example/perft.dart
+```
+
+## Architecture
+
+This is a pure Dart chess rules library (`package:dartchess`) supporting standard chess and several variants. It targets native platforms only (not web).
+
+### Core data flow
+
+`Setup` (FEN parse) → `Position` (validated legal state) → `Move` (play/apply) → new `Position`
+
+### Key layers
+
+**`models.dart`** — fundamental value types: `Side`, `Role`, `Piece`, `File`, `Rank`, `Square`, `Move` (sealed: `NormalMove` / `DropMove`), `Rule` (the variant enum), `Outcome`, and exception types (`FenException`, `PositionSetupException`).
+
+**`square_set.dart`** — `SquareSet` is a 64-bit integer bitboard (little-endian rank-file mapping). All move generation and attack computation operates on `SquareSet` values. Prefer bitwise operations over iterating squares when working in this layer.
+
+**`attacks.dart`** — precomputed attack tables for kings and knights; bishop/rook/queen attacks via hyperbola quintessence (sliding piece attacks with `occupied` mask). Exposes `kingAttacks`, `knightAttacks`, `pawnAttacks`, `bishopAttacks`, `rookAttacks`, `queenAttacks`.
+
+**`board.dart`** — `Board` stores piece placement as a set of overlapping `SquareSet`s (one per side, one per role). Immutable; queries like `board.bySide(side)`, `board.byRole(role)`, `board.kingOf(side)`, `board.attacksTo(square, attacker)`.
+
+**`castles.dart`** — `Castles` tracks unmoved rooks and the path squares needed for castling legality. Supports Chess960 (king-to-rook encoding for castling moves).
+
+**`setup.dart`** — `Setup` is a non-validated position read from FEN. Parses/emits FEN, including pocket notation for Crazyhouse and remaining checks for ThreeCheck.
+
+**`position.dart`** — the heart of the library. `Position` is an immutable abstract base class; each variant subclasses it: `Chess`, `Antichess`, `Atomic`, `Crazyhouse`, `KingOfTheHill`, `ThreeCheck`, `RacingKings`, `Horde`. Concrete private implementations (`_Chess`, etc.) are returned by `fromSetup`. Key API:
+- `Position.setupPosition(rule, setup)` — variant-aware factory
+- `pos.legalMoves` — `IMap<Square, SquareSet>` (king-to-rook encoding for castling)
+- `pos.play(move)` / `pos.playUnchecked(move)` — returns new position
+- `pos.parseSan(san)` / `pos.makeSan(move)` — SAN I/O
+- `pos.isCheckmate`, `pos.isStalemate`, `pos.isGameOver`, `pos.outcome`
+- Castling moves are encoded as king-to-rook; use `makeLegalMoves()` (from `utils.dart`) to also include the traditional king-to-destination squares.
+
+**`pgn.dart`** — `PgnGame<T>` holds headers + a `PgnNode<T>` tree of moves (with variations, comments, NAGs, shapes, evaluations). `PgnGame.parsePgn` / `PgnGame.parseMultiGamePgn` for parsing; `game.makePgn()` for serialization. Use `PgnNode.transform` to walk the tree and attach computed data (e.g. FEN per node) without mutating nodes.
+
+**`utils.dart`** — `makeLegalMoves(pos)` returns `Map<Square, Set<Square>>` adding traditional castling destinations alongside king-to-rook destinations.
+
+**`debug.dart`** — `toSfen` helpers for printing boards in ASCII; primarily for testing and debugging.
+
+### Immutability
+
+All `Position`, `Board`, `Setup`, `Castles` instances are immutable (`@immutable`). The library uses `fast_immutable_collections` (`IMap`, `ISet`) for collections stored in positions.
+
+### Chess960
+
+Castling is internally encoded king-to-rook throughout. `makeLegalMoves` adds the standard king-to-g/c destination for display purposes. When playing a castling move from UI, both encodings are accepted by `isLegal`.