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Entity-Management.md

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+# Entity Management
+
+Entities are game objects that implement behavior and live on Grids. While Grids handle rendering and mediate interactions, Entities encapsulate game logic like movement, combat, and AI.
+
+## Quick Reference
+
+**Parent System:** [[Grid-System]]
+
+**Key Types:**
+- `mcrfpy.Entity` - Game entities on grids
+- `mcrfpy.Grid` - Spatial container for entities
+- `mcrfpy.EntityCollection` - Collection of entities on a grid
+
+**Key Files:**
+- `src/UIEntity.h` / `src/UIEntity.cpp`
+- `src/UIEntityCollection.h` / `.cpp`
+
+**Related Issues:**
+- [#115](../../issues/115) - SpatialHash for fast queries (Open)
+- [#117](../../issues/117) - Memory Pool for entities (Open)
+
+---
+
+## What Are Entities?
+
+Entities are game objects that:
+- **Live on a Grid** (0 or 1 grid at a time)
+- **Have a sprite** for visual rendering
+- **Have grid position** (integer cell coordinates)
+- **Implement behavior** (movement, AI, combat, inventory)
+
+**Key distinction:** Entities implement behavior. Grids mediate interaction between entities and render them to screen.
+
+---
+
+## Entity-Grid Relationship
+
+The Entity-Grid relationship mirrors the UIDrawable parent-child pattern:
+
+| Relationship | Property | Automatic Behavior |
+|--------------|----------|-------------------|
+| Entity → Grid | `entity.grid` | Set when added to `grid.entities` |
+| Grid → Entities | `grid.entities` | Collection of all entities on grid |
+
+```python
+import mcrfpy
+
+# Create grid and entity
+grid = mcrfpy.Grid(grid_size=(50, 50), pos=(0, 0), size=(400, 400))
+player = mcrfpy.Entity(pos=(10, 10), sprite_index=0)
+
+# Before adding: entity has no grid
+print(player.grid)  # None
+
+# Add to grid
+grid.entities.append(player)
+
+# After adding: bidirectional link established
+print(player.grid == grid)  # True
+print(player in grid.entities)  # True
+
+# Removing breaks the link
+grid.entities.remove(player)
+print(player.grid)  # None
+```
+
+**Important:** An entity can only be on 0 or 1 grids at a time. Adding to a new grid automatically removes from the old one.
+
+---
+
+## Entity Properties
+
+### Position
+
+```python
+# Grid coordinates (integer cells)
+entity.x = 15
+entity.y = 20
+entity.pos = (15, 20)  # Tuple form
+
+# Draw position (float, for animation interpolation)
+print(entity.draw_pos)  # Actual render position
+```
+
+### Sprite
+
+```python
+entity.sprite_index = 5  # Index in texture sprite sheet
+```
+
+### Visibility
+
+```python
+entity.visible = True
+entity.opacity = 0.8  # 0.0 to 1.0
+```
+
+### Grid Reference
+
+```python
+current_grid = entity.grid  # Read-only, set by collection operations
+```
+
+---
+
+## EntityCollection
+
+`grid.entities` is an `EntityCollection` with list-like operations:
+
+```python
+# Add entities
+grid.entities.append(entity)
+grid.entities.extend([entity1, entity2, entity3])
+grid.entities.insert(0, entity)  # Insert at index
+
+# Remove entities
+grid.entities.remove(entity)
+entity = grid.entities.pop()  # Remove and return last
+entity = grid.entities.pop(0)  # Remove and return at index
+
+# Query
+count = len(grid.entities)
+idx = grid.entities.index(entity)
+n = grid.entities.count(entity)
+found = grid.entities.find("entity_name")  # Find by name
+
+# Iteration
+for entity in grid.entities:
+    print(entity.pos)
+```
+
+---
+
+## Entity Lifecycle
+
+### Creation
+
+```python
+# Basic creation
+entity = mcrfpy.Entity(pos=(10, 10), sprite_index=0)
+
+# With name for later lookup
+entity = mcrfpy.Entity(pos=(10, 10), sprite_index=0, name="player")
+```
+
+### Adding to Grid
+
+```python
+grid.entities.append(entity)
+# entity.grid is now set to grid
+# Entity will be rendered with the grid
+```
+
+### Movement
+
+```python
+# Direct position change
+entity.pos = (new_x, new_y)
+
+# Animated movement
+mcrfpy.Animation("x", target_x, 0.3, "easeOutQuad").start(entity)
+mcrfpy.Animation("y", target_y, 0.3, "easeOutQuad").start(entity)
+```
+
+### Removal
+
+```python
+# Method 1: Remove from collection
+grid.entities.remove(entity)
+
+# Method 2: Entity.die() - removes from parent grid
+entity.die()
+
+# After removal: entity.grid is None
+```
+
+### Transfer Between Grids
+
+```python
+def transfer_entity(entity, to_grid, new_pos):
+    """Move entity to a different grid."""
+    entity.die()  # Remove from current grid
+    entity.pos = new_pos
+    to_grid.entities.append(entity)
+```
+
+---
+
+## Common Patterns
+
+### Player Entity
+
+```python
+class Player:
+    def __init__(self, grid, start_pos):
+        self.entity = mcrfpy.Entity(pos=start_pos, sprite_index=0, name="player")
+        grid.entities.append(self.entity)
+
+    def move(self, dx, dy):
+        new_x = self.entity.x + dx
+        new_y = self.entity.y + dy
+
+        # Check walkability via grid
+        point = self.entity.grid.at(new_x, new_y)
+        if point and point.walkable:
+            self.entity.pos = (new_x, new_y)
+            return True
+        return False
+```
+
+### Enemy Entity
+
+```python
+class Enemy:
+    def __init__(self, grid, pos, aggro_range=10):
+        self.entity = mcrfpy.Entity(pos=pos, sprite_index=1)
+        self.aggro_range = aggro_range
+        self.health = 100
+        grid.entities.append(self.entity)
+
+    def update(self, player_pos):
+        dx = player_pos[0] - self.entity.x
+        dy = player_pos[1] - self.entity.y
+        dist = (dx*dx + dy*dy) ** 0.5
+
+        if dist < self.aggro_range:
+            self.chase(player_pos)
+        else:
+            self.wander()
+
+    def chase(self, target):
+        # Use pathfinding
+        path = self.entity.path_to(target)
+        if path and len(path) > 1:
+            next_cell = path[1]  # path[0] is current position
+            self.entity.pos = next_cell
+
+    def wander(self):
+        import random
+        dx = random.choice([-1, 0, 1])
+        dy = random.choice([-1, 0, 1])
+
+        new_pos = (self.entity.x + dx, self.entity.y + dy)
+        point = self.entity.grid.at(*new_pos)
+        if point and point.walkable:
+            self.entity.pos = new_pos
+```
+
+### Item Entity
+
+```python
+class Item:
+    def __init__(self, grid, pos, item_type):
+        self.entity = mcrfpy.Entity(pos=pos, sprite_index=10 + item_type)
+        self.item_type = item_type
+        grid.entities.append(self.entity)
+
+    def pickup(self, collector):
+        """Called when another entity picks up this item."""
+        collector.inventory.append(self.item_type)
+        self.entity.die()  # Remove from grid
+```
+
+For more interaction patterns (click handling, selection, context menus), see [[Grid-Interaction-Patterns]].
+
+---
+
+## Pathfinding
+
+Entities have built-in pathfinding via libtcod:
+
+```python
+# A* pathfinding to target
+path = entity.path_to((target_x, target_y))
+# Returns list of (x, y) tuples, or empty if no path
+
+if path:
+    next_step = path[1]  # path[0] is current position
+    entity.pos = next_step
+```
+
+Pathfinding respects `GridPoint.walkable` properties set on the grid.
+
+---
+
+## Performance Considerations
+
+**Current:** Entity queries are O(n):
+```python
+# Finding entities at position requires iteration
+def entities_at(grid, x, y):
+    return [e for e in grid.entities if e.x == x and e.y == y]
+```
+
+**Workarounds:**
+- Keep entity counts reasonable (< 200 for best performance)
+- Use timer callbacks for AI updates, not per-frame
+- Cache query results when possible
+
+**Future:** [#115](../../issues/115) SpatialHash will provide O(1) position queries.
+
+See [[Performance-and-Profiling]] for optimization guidance.
+
+---
+
+## Related Systems
+
+- [[Grid-System]] - Spatial container for entities
+- [[Grid-Interaction-Patterns]] - Click handling, selection, context menus
+- [[Animation-System]] - Smooth entity movement
+- [[Performance-and-Profiling]] - Entity performance metrics
+
+---
+
+*Last updated: 2025-11-29*