# Gacrux: Pseudo-Legal to Legal Move Conversion Plan

## Current State
- Move generators produce **pseudo-legal** moves (follow piece movement rules but ignore checks/pins)
- `position_make_move()` has multiple bugs
- No attack detection or legal move filtering exists
- Perft test conditions are inverted

## Phase 1: Fix Existing Bugs

### `src/engine/moves.c` — `position_make_move()`
1. **Lines 94, 100**: Add missing semicolons after `assert(0)`
2. **Lines 54-55**: Black long castle modifies `WHITE_ROOK` → should be `BLACK_ROOK`
3. **Lines 71-73**: Black short castle modifies `WHITE_KING`/`WHITE_ROOK` → should be `BLACK_KING`/`BLACK_ROOK`
4. **Lines 96, 98**: `move.from` (u8) compared against `u64` bitboard constants → use square indices 63 and 56
5. **Missing turn toggle**: `position->turn` never flips after a move

### `test_perft_starting_position`
6. **Lines 169-176**: `==` → `!=` (test currently passes when counts are wrong)

## Phase 2: Attack Detection

Create `src/engine/moves/attack.c` with:
```c
bool square_attacked(position_t position, square_t square, u8 by_color);
```

Checks if any piece of `by_color` attacks `square`:
- **Pawn attacks**: Check diagonally forward for enemy pawns
- **Knight attacks**: Use existing `knight_moves[64]` lookup
- **King attacks**: Use existing `920078ULL` pattern
- **Sliding attacks**: Walk rays from target square (for now, ray-walking)

## Phase 3: Legal Move Filtering

Modify `get_moves()` (or add `get_legal_moves()`) to:
1. Generate all pseudo-legal moves
2. For each move, make it on a copy of the position
3. Find the friendly king in the resulting position
4. Check if the friendly king is attacked by the opponent
5. If attacked → remove the move (swap with last, decrement length)

**Castling pre-checks** (in `king.c`):
- King not currently in check
- Squares king passes through are not attacked
- No pieces between king and rook

## Phase 4: Magic Bitboards (Performance Optimization)

Replace ray-walking with O(1) magic lookups for sliding piece attacks.

### Data Structure
```c
typedef struct {
    bitboard_t mask;     // relevant occupancy bits (excludes edges)
    bitboard_t *attacks; // pointer into attack table
    bitboard_t magic;    // the magic multiplier
    int shift;           // 64 - popcount(mask)
} magic_t;
```

### Attack Tables
- `rook_attacks[0x19000]` (~100KB)
- `bishop_attacks[0x1480]` (~5KB)

### Index Computation
```c
unsigned index = ((occupied & magic.mask) * magic.magic) >> magic.shift;
return magic.attacks[index];
```

### Initialization
At startup, for each square:
1. Compute mask (pseudo-attacks minus edges)
2. Enumerate all subsets of the mask
3. Compute true attacks via ray-walking (reference)
4. Find magic number via PRNG search

### Usage
```c
// Move generation
bitboard_t attacks = get_rook_attacks(from_square, all_occupied);
attacks &= ~friendly_pieces;

// Attack detection
bool square_attacked(...) {
    if (get_rook_attacks(sq, occ) & (enemy_rooks | enemy_queens)) return true;
    if (get_bishop_attacks(sq, occ) & (enemy_bishops | enemy_queens)) return true;
    // ...
}
```

## Implementation Order
1. Fix bugs (Phase 1)
2. Add `square_attacked()` with ray-walking
3. Add legal move filtering
4. Fix castling legality
5. Verify perft d0-d7
6. Uncomment + verify Kiwipete perft
7. Add magic bitboards (Phase 4)