Chessistics/PLAN.md

# Chessistics — Prototype Roadmap

## Phase 1: Core solvability (DONE)

- Black Box Sim pattern: commands self-apply via `DoApply()`/`AssertApplicationConditions()`
- 3 piece types: Rook (orthogonal range 2), Bishop (diagonal range 2), Knight (L-jump)
- Relay chain mechanic with shared relay points (collision-free alternating)
- Transfer system: production → pieces → demands, 4-adjacency, participation tracking
- Victory/defeat: all demands met vs deadline expired
- 57 tests passing: unit, solvability, full-level (Levels 1-3)

### Key findings from Phase 1

- **Transfer direction bug fixed**: receiver priority in piece-to-piece transfers now
  prefers pieces farther from production (pushes cargo forward, not backward).
- **GDD stock corrections**: Level 2 needs 6R+1B (GDD had 4R+1B), Level 3 simplified
  to single cargo type with 6R+1K (GDD had 4R+1B+2K for dual-cargo, which is infeasible).
- **Cross-route contamination**: TransferResolver has no cargo-type filtering — adjacent
  pieces from different routes transfer cargo regardless of type. On a 6x6 board, two
  diagonal routes cannot avoid cross-adjacency. Dual-cargo levels require engine support.

## Phase 2: Cargo-type aware transfers

**Goal**: Enable multi-route, multi-cargo-type levels on small boards.

- Add optional cargo-type filter to pieces (or routes), so a piece configured for Wood
  ignores adjacent Stone and vice-versa.
- Alternative: transfer resolver checks if cargo type matches the demand reachable from
  the receiver's relay chain (more complex, automatic).
- Simplest approach: production only gives to pieces whose relay chain leads toward a
  compatible demand. Requires route/chain tracking.
- Test: reintroduce Level 3 dual-cargo variant (Wood + Stone crossing 6x6 board with
  L-shaped wall).

## Phase 3: Surplus stock and puzzle difficulty tuning

**Goal**: Levels give more pieces than the minimum, creating genuine puzzle space.

- With forward-preferring transfers working, longer chains are viable.
- Design levels where the player has choice: multiple valid solutions with different
  efficiency scores (PiecesUsed, TurnsTaken, CellsOccupied).
- Add scoring/star system based on Metrics.
- Levels 4-6: increasing board size (8x8, 10x10), more complex wall layouts, multiple
  productions and demands.

## Phase 4: New piece — Pion (Pawn)

**Goal**: Add a one-directional piece for asymmetric relay constraints.

- Pion moves forward only (one direction, range 1).
- Cheap to place (low piece cost if scoring is added).
- Creates interesting constraints: must plan direction of cargo flow.
- Test levels specifically designed around Pion usage.

## Phase 5: Network levels and Dame (Queen)

**Goal**: Open-ended logistics puzzles with interconnected supply networks.

- Multiple productions feeding multiple demands through shared infrastructure.
- Dame piece: combines Rook + Bishop movement (range 2, all 8 directions).
- Powerful but expensive — forces cost/benefit tradeoffs.
- Larger boards (12x12+) with complex wall configurations.
- Potential for player-designed levels (level editor data format).

## Phase 6: Godot integration

**Goal**: Playable visual prototype.

- Board renderer: grid, walls, buildings, pieces.
- Drag-and-drop piece placement during Edit phase.
- Step/play/pause simulation controls.
- Event visualization: cargo movement, transfers, delivery animations.
- Victory/defeat screens with Metrics display.