godot-genre-sandbox

What this skill does

# Genre: Sandbox

Physical simulation, emergent play, and player creativity define this genre.

## NEVER Do (Expert Anti-Patterns)

### Performance & Scalability
- NEVER use individual `RigidBody` nodes for every block; strictly use **Static Colliders** for the world and reserve physics for dynamic props.
- NEVER simulate the entire world every frame; strictly process **"Dirty" chunks** with active changes. Sleeping chunks must consume zero CPU.
- NEVER update `MultiMesh` buffers every frame; strictly **batch changes** and only rebuild the buffer when a modification completes (e.g., player stops painting).
- NEVER use standard Godot `Nodes` for every grid cell; strictly use **PackedInt32Arrays** or typed Dictionaries to keep RAM overhead minimal.
- NEVER raycast against every individual voxel for placement; strictly use **Grid Quantization** (`floor(pos/size)`) for direct O(1) cell calculation.
- NEVER render every block face in a chunk; strictly generate an `ArrayMesh` that only pushes **visible exterior faces** to the GPU (Culling/Greedy Meshing).

### Data & Persistence
- NEVER save raw arrays of every block transform; strictly use **Run-Length Encoding (RLE)** (e.g., "Air x 50,000") to compress uniform spaces.
- NEVER load massive terrain chunks synchronously; strictly use `ResourceLoader.load_threaded_request()` to prevent frame stutter.
- NEVER use standard text `.tscn` files for voxel datasets; strictly use **binary `.res` files** for 10x faster parsing.
- NEVER ignore **Floating-Point Precision limits** (32,768 units); strictly implement floating-origin shifting for massive worlds.

### Systems & Architecture
- NEVER hardcode element interactions (`if water and fire`); strictly use a **Property System** where interactions emerge from material attributes (flammability, density).
- NEVER trust client-side placement in multiplayer; strictly require the **Server to validate** bounds and resources.
- NEVER manipulate the SceneTree from background generation threads; strictly use `call_deferred()` or Mutex locks for safety.
- NEVER leave orphaned chunks in memory; strictly track loaded regions and call `queue_free()` on discarded branches.

---

## 🛠 Expert Components (scripts/)

### Original Expert Patterns
- [voxel_chunk_manager.gd](scripts/voxel_chunk_manager.gd) - Professional chunk management using `MultiMeshInstance3D` with batch update logic.
- [cellular_automata_liquid.gd](scripts/cellular_automata_liquid.gd) - Optimized simulation of liquids and powders using property-based density checks.
- [voxel_world.gd](scripts/voxel_world.gd) - Top-level world controller for grid state, tool-based editing, and chunk lifecycle.

### Modular Components
- [sandbox_patterns.gd](scripts/sandbox_patterns.gd) - Utility collection for async chunk loading, multithreading, and origin shifting.

## Architecture Patterns

### 1. Element System (Property-Based Emergence)
Model material properties, not behaviors. Interactions emerge from overlapping properties.

```gdscript
# element_data.gd
class_name ElementData extends Resource

enum Type { SOLID, LIQUID, GAS, POWDER }
@export var id: String = "air"
@export var type: Type = Type.GAS
@export var density: float = 0.0      # For liquid flow direction
@export var flammable: float = 0.0    # 0-1: Chance to ignite
@export var ignition_temp: float = 400.0
@export var conductivity: float = 0.0  # For electricity/heat
@export var hardness: float = 1.0     # Mining time multiplier

# EDGE CASE: What if two elements have same density but different types?
# SOLUTION: Use secondary sort (type enum priority: SOLID > LIQUID > POWDER > GAS)
func should_swap_with(other: ElementData) -> bool:
    if density == other.density:
        return type > other.type  # Enum comparison: SOLID(0) > GAS(3)
    return density > other.density
```

### 2. Cellular Automata Grid (Falling Sand Simulation)
Update order matters. Top-down prevents "teleporting" godot-particles.

```gdscript
# world_grid.gd
var grid: Dictionary = {}  # Vector2i -> ElementData
var dirty_cells: Array[Vector2i] = []

func _physics_process(_delta: float) -> void:
    # CRITICAL: Sort top-to-bottom to prevent double-moves
    dirty_cells.sort_custom(func(a, b): return a.y < b.y)
    
    for pos in dirty_cells:
        simulate_cell(pos)
    dirty_cells.clear()

func simulate_cell(pos: Vector2i) -> void:
    var cell = grid.get(pos)
    if not cell: return
    
    match cell.type:
        ElementData.Type.LIQUID, ElementData.Type.POWDER:
            # Try down, then down-left, then down-right
            var targets = [pos + Vector2i.DOWN, 
                           pos + Vector2i(- 1, 1), 
                           pos + Vector2i(1, 1)]
            for target in targets:
                var neighbor = grid.get(target)
                if neighbor and cell.should_swap_with(neighbor):
                    swap_cells(pos, target)
                    mark_dirty(target)
                    return
        
        ElementData.Type.GAS:
            # Gases rise (inverse of liquids)
            var targets = [pos + Vector2i.UP,
                           pos + Vector2i(-1, -1),
                           pos + Vector2i(1, -1)]
            # Same swap logic...

# EDGE CASE: What if multiple godot-particles want to move into same cell?
# SOLUTION: Only mark target dirty, don't double-swap. Next frame resolves conflicts.
```

### 3. Tool System (Strategy Pattern)
Decouple input from world modification.

```gdscript
# tool_base.gd
class_name Tool extends Resource
func use(world_pos: Vector2, world: WorldGrid) -> void: pass

# tool_brush.gd
extends Tool
@export var element: ElementData
@export var radius: int = 1

func use(world_pos: Vector2, world: WorldGrid) -> void:
    var grid_pos = Vector2i(floor(world_pos.x), floor(world_pos.y))
    
    # Circle brush pattern
    for x in range(-radius, radius + 1):
        for y in range(-radius, radius + 1):
            if x*x + y*y <= radius*radius:  # Circle boundary
                var target = grid_pos + Vector2i(x, y)
                world.set_cell(target, element)

# FALLBACK: If element placement fails (e.g., occupied by indestructible block)?
# Check world.can_place(target) before set_cell(), show visual feedback.
```

### 4. Chunk-Based Rendering (3D Voxels)
Only render visible faces. Use greedy meshing to merge adjacent blocks.

```gdscript
# See scripts/voxel_chunk_manager.gd for full implementation

# EXPERT DECISION TREE:
# - Small worlds (<100k blocks): Single MeshInstance with SurfaceTool
# - Medium worlds (100k-1M blocks): Chunked MultiMesh (see script)
# - Large worlds (>1M blocks): Chunked + greedy meshing + LOD
```

## Save System for Sandbox Worlds

```gdscript
# chunk_save_data.gd
class_name ChunkSaveData extends Resource

@export var chunk_coord: Vector2i
@export var rle_data: PackedInt32Array  # [type_id, count, type_id, count...]

# EXPERT TECHNIQUE: Run-Length Encoding
static func encode_chunk(grid: Dictionary, chunk_pos: Vector2i, chunk_size: int) -> ChunkSaveData:
    var data = ChunkSaveData.new()
    data.chunk_coord = chunk_pos
    
    var run_type: int = -1
    var run_count: int = 0
    
    for y in range(chunk_size):
        for x in range(chunk_size):
            var world_pos = chunk_pos * chunk_size + Vector2i(x, y)
            var cell = grid.get(world_pos)
            var type_id = cell.id if cell else 0  # 0 = air
            
            if type_id == run_type:
                run_count += 1
            else:
                if run_count > 0:
                    data.rle_data.append(run_type)
                    data.rle_data.append(run_count)
                run_type = type_id
                run_count = 1
    
    # Flush final run
    if run_count > 0:
        data.rle_data.append(run_type)
        data.rle_data.append(run_count)
    
    return data

# COMPRESSION RESULT: Empty chunk (16×16 = 256 blocks of air)
# Without RLE: 256 integers = 1024 bytes
# With RLE: [0, 256] = 8 bytes (128x c