Merge 7b870e8f3c into b26fc39fce

Prevent invalidation during removal of pairs with deleted targe

.gitignore

@@ -65,3 +65,7 @@ drafts/
 
 # Luau tools
 profile.*
+
+# Patch files
+
+*.patch

.luaurc

@@ -1,8 +1,9 @@
 {
     "aliases": {
         "jecs": "jecs",
-        "testkit": "test/testkit",
-        "mirror": "mirror"
+        "testkit": "tools/testkit",
+        "mirror": "mirror",
+        "tools": "tools",
     },
     "languageMode": "strict"
 }

README.md

@@ -62,3 +62,32 @@ Can be found under /benches/visual/query.luau
 Inserting 8 components to an entity and updating them over 50 times.
 ![Insertions](assets/image-4.png)
 Can be found under /benches/visual/insertions.luau
+
+## Installation
+
+### Using Wally
+
+Add jecs to your `wally.toml` file:
+
+```toml
+[dependencies]
+jecs = "ukendio/jecs@0.1.0"
+```
+
+Then run `wally install` in your project directory.
+
+### Manual Installation
+
+Download the latest release from the [releases page](https://github.com/ukendio/jecs/releases).
+
+## Documentation
+
+For complete documentation, visit our [documentation site](https://ukendio.github.io/jecs/).
+
+## Contributing
+
+Contributions are welcome. Please feel free to submit a Pull Request.
+
+## License
+
+This project is licensed under the MIT License - see the [LICENSE](LICENSE) file for details.

benches/visual/despawn.bench.luau

@@ -5,41 +5,60 @@ local ReplicatedStorage = game:GetService("ReplicatedStorage")
 local Matter = require(ReplicatedStorage.DevPackages.Matter)
 local ecr = require(ReplicatedStorage.DevPackages.ecr)
 local jecs = require(ReplicatedStorage.Lib)
+local pair = jecs.pair
 local newWorld = Matter.World.new()
 local ecs = jecs.World.new()
+local mirror = require(ReplicatedStorage.mirror)
+local mcs = mirror.World.new()
 
-local A, B = Matter.component(), Matter.component()
-local C, D = ecs:component(), ecs:component()
+local C1 = ecs:component()
+local C2 = ecs:entity()
+ecs:add(C2, pair(jecs.OnDeleteTarget, jecs.Delete))
+local C3 = ecs:entity()
+ecs:add(C3, pair(jecs.OnDeleteTarget, jecs.Delete))
+local C4 = ecs:entity()
+ecs:add(C4, pair(jecs.OnDeleteTarget, jecs.Delete))
+local E1 = mcs:component()
+local E2 = mcs:entity()
+mcs:add(E2, pair(jecs.OnDeleteTarget, jecs.Delete))
+local E3 = mcs:entity()
+mcs:add(E3, pair(jecs.OnDeleteTarget, jecs.Delete))
+local E4 = mcs:entity()
+mcs:add(E4, pair(jecs.OnDeleteTarget, jecs.Delete))
+
+local registry2 = ecr.registry()
 
 return {
 	ParameterGenerator = function()
-		local matter_entities = {}
-		local jecs_entities = {}
-		local entities = {
-			matter = matter_entities,
-			jecs = jecs_entities,
-		}
+		local j = ecs:entity()
+		ecs:set(j, C1, true)
+		local m = mcs:entity()
+		mcs:set(m, E1, true)
 		for i = 1, 1000 do
-			table.insert(matter_entities, newWorld:spawn(A(), B()))
-			local e = ecs:entity()
-			ecs:set(e, C, {})
-			ecs:set(e, D, {})
-			table.insert(jecs_entities, e)
+			local friend1 = ecs:entity()
+			local friend2 = mcs:entity()
+
+			ecs:add(friend1, pair(C2, j))
+			ecs:add(friend1, pair(C3, j))
+			ecs:add(friend1, pair(C4, j))
+
+			mcs:add(friend2, pair(E2, m))
+			mcs:add(friend2, pair(E3, m))
+			mcs:add(friend2, pair(E4, m))
 		end
-		return entities
+		return {
+			m = m,
+			j = j,
+		}
 	end,
 
 	Functions = {
-		Matter = function(_, entities)
-			for _, entity in entities.matter do
-				newWorld:despawn(entity)
-			end
+		Mirror = function(_, a)
+			mcs:delete(a.m)
 		end,
 
-		Jecs = function(_, entities)
-			for _, entity in entities.jecs do
-				ecs:delete(entity)
-			end
+		Jecs = function(_, a)
+			ecs:delete(a.j)
 		end,
 	},
 }

demo/src/ReplicatedStorage/track.luau

@@ -0,0 +1,48 @@
+local events = {}
+
+local function trackers_invoke(event, component, entity, ...)
+	local trackers = events[event][component]
+	if not trackers then
+		return
+	end
+
+	for _, tracker in trackers do
+		tracker(entity, data)
+	end
+end
+
+local function trackers_init(event, component, fn)
+	local ob = events[event]
+
+	return {
+		connect = function(component, fn)
+			local trackers = ob[component]
+			if not trackers then
+				trackers = {}
+				ob[component] = trackers
+			end
+
+			table.insert(trackers, fn)
+		end,
+		invoke = function(component, ...)
+			trackers_invoke(event, component, ...)
+		end
+	}
+	return function(component, fn)
+		local trackers = ob[component]
+		if not trackers then
+			trackers = {}
+			ob[component] = trackers
+		end
+
+		table.insert(trackers, fn)
+	end
+end
+
+local trackers = {
+	emplace = trackers_init("emplace"),
+	add = trackers_init("added"),
+	remove = trackers_init("removed")
+}
+
+return trackers

docs/api/jecs.md

@@ -16,6 +16,12 @@ jecs.Wildcard: Entity
 ```
 Builtin component type. This ID is used for wildcard queries.
 
+## w
+```luau
+jecs.w: Entity
+```
+An alias for `jecs.Wildcard`. This ID is used for wildcard queries, providing a shorter syntax in query operations.
+
 ## Component
 ```luau
 jecs.Component: Entity
@@ -28,6 +34,54 @@ jecs.ChildOf: Entity
 ```
 Builtin component type. This ID is for creating parent-child hierarchies.
 
+## Name
+```luau
+jecs.Name: Entity
+```
+Builtin component type. This ID is used to associate a string name with an entity, typically for debugging or display purposes.
+
+## OnAdd
+```luau
+jecs.OnAdd: Entity<(entity: Entity) -> ()>
+```
+Builtin component hook. When set on a component, the provided function is called whenever that component is added to an entity.
+
+## OnRemove
+```luau
+jecs.OnRemove: Entity<(entity: Entity) -> ()>
+```
+Builtin component hook. When set on a component, the provided function is called whenever that component is removed from an entity.
+
+## OnSet
+```luau
+jecs.OnSet: Entity<(entity: Entity, data: any) -> ()>
+```
+Builtin component hook. When set on a component, the provided function is called whenever that component's value is set or changed on an entity.
+
+## OnDelete
+```luau
+jecs.OnDelete: Entity
+```
+Builtin component trait. Used with `pair()` to define behavior when a component or entity is deleted. Must be paired with an action component like `jecs.Delete` or `jecs.Remove`.
+
+## OnDeleteTarget
+```luau
+jecs.OnDeleteTarget: Entity
+```
+Builtin component trait. Used with `pair()` to define behavior when a relationship target is deleted. Must be paired with an action component like `jecs.Delete` or `jecs.Remove`.
+
+## Delete
+```luau
+jecs.Delete: Entity
+```
+Builtin action component used with `OnDelete` or `OnDeleteTarget` to specify that entities should be deleted in the cleanup process.
+
+## Remove
+```luau
+jecs.Remove: Entity
+```
+Builtin action component used with `OnDelete` or `OnDeleteTarget` to specify that components should be removed in the cleanup process.
+
 ## Rest
 ```luau
 jecs.Rest: Entity

docs/api/name.md

@@ -0,0 +1,130 @@
+# Name Component
+
+The `Name` component allows you to associate a string identifier with an entity.
+
+## Overview
+
+```luau
+jecs.Name: Entity<string>
+```
+
+The `Name` component is a built-in component in jecs that stores a string value. It has no special behavior beyond storing a name.
+
+## Usage
+
+### Setting a Name
+
+```luau
+local world = jecs.World.new()
+
+-- Create an entity
+local entity = world:entity()
+
+-- Assign a name to the entity
+world:set(entity, jecs.Name, "Player")
+```
+
+### Getting an Entity's Name
+
+```luau
+-- Retrieve an entity's name
+local name = world:get(entity, jecs.Name)
+print("Entity name:", name) -- Outputs: "Entity name: Player"
+```
+
+### Finding Entities by Name
+
+```luau
+-- Iterate over all entities with the Name component
+for entity, name in world:query(jecs.Name) do
+  if name == "Player" then
+    -- Found the entity named "Player"
+    -- Do something with entity...
+  end
+end
+```
+
+### Using Names with Hierarchies
+
+Names are particularly useful when working with entity hierarchies:
+
+```luau
+local world = jecs.World.new()
+local pair = jecs.pair
+
+-- Create a parent entity with a name
+local parent = world:entity()
+world:set(parent, jecs.Name, "Parent")
+
+-- Create child entities with names
+local child1 = world:entity()
+world:set(child1, jecs.Name, "Child1")
+world:add(child1, pair(jecs.ChildOf, parent))
+
+local child2 = world:entity()
+world:set(child2, jecs.Name, "Child2")
+world:add(child2, pair(jecs.ChildOf, parent))
+
+-- Print the hierarchy
+print("Parent:", world:get(parent, jecs.Name))
+for child in world:query(pair(jecs.ChildOf, parent)) do
+  print("  Child:", world:get(child, jecs.Name))
+end
+
+-- Output:
+-- Parent: Parent
+--   Child: Child1
+--   Child: Child2
+```
+
+## Helper Function
+
+For convenience, you can create a simple helper function to get an entity's name:
+
+```luau
+local function getName(world, entity)
+  return world:get(entity, jecs.Name) or tostring(entity)
+end
+
+-- Usage
+local name = getName(world, entity)
+```
+
+This function will return the entity's name if it has one, or fall back to the entity ID as a string.
+
+## Best Practices
+
+1. **Use Consistently**: Apply names to key entities systematically, especially for important game objects.
+
+2. **Be Descriptive**: Use meaningful names that describe the entity's role or purpose.
+
+3. **Handle Missing Names**: Always check if an entity has a name before trying to use it, or use a helper function like the one above.
+
+4. **Namespacing**: Consider using prefixes or namespaces for different systems (e.g., "UI:MainMenu", "Enemy:Boss").
+
+5. **Performance**: Remember that adding a Name component to every entity adds memory overhead. Use judiciously in performance-critical applications.
+
+## Example: Entity Debugging System
+
+Here's a simple debugging system that uses the Name component:
+
+```luau
+local function debugEntity(world, entity)
+  local name = world:get(entity, jecs.Name) or "Unnamed"
+  print("Entity", entity, "(" .. name .. ")")
+  
+  -- Print all components
+  for componentId in world:query(jecs.pair(jecs.Component, jecs.Wildcard)) do
+    if world:has(entity, componentId) then
+      local componentName = world:get(componentId, jecs.Name) or "Unknown"
+      local value = world:get(entity, componentId)
+      print("  Component:", componentName, "=", value)
+    end
+  end
+end
+
+-- Usage
+debugEntity(world, player)
+```
+
+This debugging function prints an entity's name along with all its components, making it easier to inspect entities during development. 

docs/api/world-entity-management.md

@@ -0,0 +1,215 @@
+# Entity Management
+
+This section covers methods for managing entities in the jecs World.
+
+## delete
+
+Deletes an entity (and its components/relationships) from the world entirely.
+
+```luau
+function World:delete(entity: Entity): void
+```
+
+### Parameters
+
+| Name | Type | Description |
+|------|------|-------------|
+| entity | Entity | The entity to delete from the world |
+
+### Behavior
+
+1. Invokes any `OnRemove` hooks for all components on the entity
+2. Triggers any cleanup actions based on `OnDelete` and `OnDeleteTarget` relationships
+3. Removes the entity from all archetypes
+4. Recycles the entity ID for future use
+
+### Example
+
+::: code-group
+
+```luau [luau]
+local world = jecs.World.new()
+
+-- Create an entity
+local entity = world:entity()
+world:set(entity, Position, {x = 0, y = 0})
+
+-- Delete the entity
+world:delete(entity)
+
+-- The entity no longer exists in the world
+assert(not world:contains(entity))
+```
+
+```typescript [typescript]
+import { World } from "@rbxts/jecs";
+
+const world = new World();
+
+// Create an entity
+const entity = world.entity();
+world.set(entity, Position, {x: 0, y: 0});
+
+// Delete the entity
+world.delete(entity);
+
+// The entity no longer exists in the world
+assert(!world.contains(entity));
+```
+
+:::
+
+### Child Entity Deletion
+
+When an entity has children (via the `ChildOf` relationship), deleting the parent can also delete the children if the appropriate cleanup policy is set:
+
+```luau
+-- Set up parent-child relationship
+local ChildOf = world:component()
+world:add(ChildOf, jecs.pair(jecs.OnDeleteTarget, jecs.Delete))
+
+local parent = world:entity()
+local child = world:entity()
+
+-- Make child a child of parent
+world:add(child, jecs.pair(ChildOf, parent))
+
+-- Deleting parent will also delete child
+world:delete(parent)
+assert(not world:contains(child))
+```
+
+### Component Deletion
+
+The `delete` method can also be used to delete a component definition:
+
+```luau
+local Temporary = world:component()
+world:add(Temporary, jecs.pair(jecs.OnDelete, jecs.Remove))
+
+-- Add Temporary to entities
+local e1 = world:entity()
+local e2 = world:entity()
+world:add(e1, Temporary)
+world:add(e2, Temporary)
+
+-- Delete the component definition
+world:delete(Temporary)
+
+-- Temporary is removed from all entities
+assert(not world:has(e1, Temporary))
+assert(not world:has(e2, Temporary))
+```
+
+## remove
+
+Removes a component from a given entity.
+
+```luau
+function World:remove(entity: Entity, component: Id): void
+```
+
+### Parameters
+
+| Name | Type | Description |
+|------|------|-------------|
+| entity | Entity | The entity to modify |
+| component | Id | The component or relationship to remove |
+
+### Behavior
+
+1. Invokes any `OnRemove` hook for the component being removed
+2. Removes the component from the entity
+3. May cause the entity to transition to another archetype
+
+### Example
+
+::: code-group
+
+```luau [luau]
+local world = jecs.World.new()
+
+-- Create components
+local Health = world:component()
+local Shield = world:component()
+
+-- Create an entity with both components
+local entity = world:entity()
+world:set(entity, Health, 100)
+world:set(entity, Shield, 50)
+
+-- Remove just the Shield component
+world:remove(entity, Shield)
+
+-- Entity still exists but no longer has Shield
+assert(world:contains(entity))
+assert(world:has(entity, Health))
+assert(not world:has(entity, Shield))
+```
+
+```typescript [typescript]
+import { World } from "@rbxts/jecs";
+
+const world = new World();
+
+// Create components
+const Health = world.component();
+const Shield = world.component();
+
+// Create an entity with both components
+const entity = world.entity();
+world.set(entity, Health, 100);
+world.set(entity, Shield, 50);
+
+// Remove just the Shield component
+world.remove(entity, Shield);
+
+// Entity still exists but no longer has Shield
+assert(world.contains(entity));
+assert(world.has(entity, Health));
+assert(!world.has(entity, Shield));
+```
+
+:::
+
+### Removing Relationships
+
+The `remove` method can also be used to remove relationship pairs:
+
+```luau
+local ChildOf = world:component()
+local parent = world:entity()
+local child = world:entity()
+
+-- Establish parent-child relationship
+world:add(child, jecs.pair(ChildOf, parent))
+
+-- Remove the relationship
+world:remove(child, jecs.pair(ChildOf, parent))
+
+-- Child is no longer related to parent
+assert(not world:has(child, jecs.pair(ChildOf, parent)))
+```
+
+## Differences Between delete and remove
+
+| Method | What It Affects | Entity Existence | Cleanup Policies |
+|--------|-----------------|------------------|------------------|
+| delete | Entity and all its components | Entity no longer exists | Triggers all cleanup policies |
+| remove | Only the specified component | Entity still exists | Only triggers component-specific OnRemove hook |
+
+### When to Use Each Method
+
+- Use `delete` when you want to completely remove an entity from the world
+- Use `remove` when you want to keep the entity but remove specific components
+- Use `delete` on a component definition to remove that component from all entities
+
+### Performance Considerations
+
+Both operations may cause archetype transitions, which have performance implications:
+
+- `delete` typically has more overhead because it has to remove all components
+- `remove` is generally faster for individual component removal
+- Both methods are optimized in jecs to be as efficient as possible
+
+For performance-critical code dealing with many entities, consider batching operations to minimize archetype transitions. 

docs/learn/faq/contributing.md

@@ -1,3 +1,84 @@
-## TODO
+# Contributing to jecs
 
-This is a TODO stub.
+This document provides guidelines for contributing to the project.
+
+## Getting Started
+
+1. Fork the repository
+2. Clone your fork
+3. Set up the development environment
+4. Create a branch for your changes
+
+## Development Setup
+
+```bash
+# Clone your fork
+git clone https://github.com/your-username/jecs.git
+cd jecs
+
+# Install dependencies
+wally install
+```
+
+## Making Changes
+
+1. Create a branch:
+   ```bash
+   git checkout -b feature/your-feature-name
+   ```
+
+2. Make your changes
+
+3. Add tests if applicable
+
+4. Run tests:
+   ```bash
+   lune run test
+   ```
+
+5. Commit your changes:
+   ```bash
+   git commit -m "Add feature: description"
+   ```
+
+## Submitting Changes
+
+1. Push your changes:
+   ```bash
+   git push origin feature/your-feature-name
+   ```
+
+2. Create a Pull Request
+
+3. Explain the changes and reference related issues
+
+## Code Style
+
+- Follow the existing code style
+- Use meaningful variable and function names
+- Write clear comments for complex logic
+- Keep functions focused on a single responsibility
+
+## Testing
+
+- Add tests for new features
+- Ensure all tests pass before submitting
+- Consider edge cases
+
+## Documentation
+
+- Update documentation for changed functionality
+- Document new features or APIs
+- Use clear and concise language
+
+## Issue Reporting
+
+1. Check if the issue already exists in the [Issues](https://github.com/ukendio/jecs/issues) section
+2. Create a new issue with a descriptive title and detailed information
+
+## Code of Conduct
+
+- Be respectful in communications
+- Provide constructive feedback
+- Accept constructive criticism
+- Focus on what is best for the community

docs/learn/faq/index.md

@@ -0,0 +1,94 @@
+# Frequently Asked Questions
+
+This section addresses common questions about jecs.
+
+## General Questions
+
+### What is jecs?
+
+jecs is a high-performance Entity Component System (ECS) library for Luau/Roblox.
+
+### How does jecs compare to other ECS libraries?
+
+jecs uses an archetype-based storage system (SoA) which is more cache-friendly than traditional approaches. For a detailed comparison with Matter, see the [Migration from Matter](../migration-from-matter.md) guide.
+
+### Can I use jecs outside of Roblox?
+
+Yes, jecs can be used in any Lua/Luau environment as it has zero dependencies.
+
+## Technical Questions
+
+### How many entities can jecs handle?
+
+jecs can handle up to 800,000 entities at 60 frames per second.
+
+### What are archetypes?
+
+Archetypes are groups of entities that have the exact same set of components. When you add or remove components from an entity, it moves to a different archetype.
+
+### How do I optimize performance with jecs?
+
+- Group entity operations to minimize archetype transitions
+- Use cached queries for frequently accessed data
+- Define components that are frequently queried together
+- Use tags instead of empty tables
+- Be mindful of archetype transitions
+
+### What are relationship pairs?
+
+Relationship pairs allow you to create connections between entities using the `pair()` function.
+
+## Common Issues
+
+### My entity disappeared after adding a component
+
+Check if you have cleanup policies set up with `OnDelete` or `OnDeleteTarget`.
+
+### Queries are returning unexpected results
+
+Make sure you're querying for the exact components you need. Use modifiers like `.with()` or `.without()` to refine queries.
+
+### Performance degrades over time
+
+This could be due to:
+- Memory leaks from not properly deleting entities
+- Excessive archetype transitions
+- Too many entities or components
+- Inefficient queries
+
+### How do I debug entity relationships?
+
+Use the `Name` component to give entities meaningful names:
+
+```lua
+-- Print all parent-child relationships
+for entity, target in world:query(jecs.pair(jecs.ChildOf, jecs.Wildcard)) do
+  print(world:get(entity, jecs.Name), "is a child of", world:get(target, jecs.Name))
+end
+```
+
+## Best Practices
+
+### When should I use component lifecycle hooks?
+
+Use lifecycle hooks (`OnAdd`, `OnRemove`, `OnSet`) for:
+- Initializing resources
+- Cleaning up resources
+- Reacting to data changes
+
+### How should I structure my ECS code?
+
+- Separate data (components) from behavior (systems)
+- Keep components small and focused
+- Design systems for specific component combinations
+- Use relationships to model connections
+- Document cleanup policies
+
+### Should I use multiple worlds?
+
+Multiple worlds are useful for:
+- Separating client and server logic
+- Creating isolated test environments
+- Managing different game states
+
+Component IDs may conflict between worlds if not registered in the same order. 

docs/learn/migration-from-matter.md

@@ -0,0 +1,218 @@
+# Migrating from Matter to jecs
+
+This guide is intended to help developers migrate from the Matter ECS library to jecs.
+
+## Key Differences
+
+### Architectural Differences
+
+- **Storage Implementation**: jecs uses an archetype-based storage system (SoA - Structure of Arrays). Matter uses a simpler component-based storage approach.
+- **Performance**: jecs is designed with a focus on performance, particularly for large-scale systems with hundreds of thousands of entities.
+- **Relationship Model**: jecs treats entity relationships as first-class citizens. Matter doesn't have built-in relationship features.
+- **Memory Usage**: jecs typically uses less memory for the same number of entities and components due to its optimized storage structure.
+
+### API Differences
+
+#### World Creation and Management
+
+**Matter:**
+```lua
+local Matter = require("Matter")
+local world = Matter.World.new()
+```
+
+**jecs:**
+```lua
+local jecs = require("jecs")
+local world = jecs.World.new()
+```
+
+#### Component Definition
+
+**Matter:**
+```lua
+local Position = Matter.component("Position")
+local Velocity = Matter.component("Velocity") 
+```
+
+**jecs:**
+```lua
+local Position = world:component()
+local Velocity = world:component()
+```
+
+#### Entity Creation
+
+**Matter:**
+```lua
+local entity = world:spawn(
+  Position({x = 0, y = 0}),
+  Velocity({x = 10, y = 5})
+)
+```
+
+**jecs:**
+```lua
+local entity = world:entity()
+world:set(entity, Position, {x = 0, y = 0})
+world:set(entity, Velocity, {x = 10, y = 5})
+```
+
+#### Queries
+
+**Matter:**
+```lua
+for id, position, velocity in world:query(Position, Velocity) do
+  -- Update position based on velocity
+  position.x = position.x + velocity.x * dt
+  position.y = position.y + velocity.y * dt
+end
+```
+
+**jecs:**
+```lua
+for entity, position, velocity in world:query(Position, Velocity) do
+  -- Update position based on velocity
+  position.x = position.x + velocity.x * dt
+  position.y = position.y + velocity.y * dt
+end
+```
+
+#### System Definition
+
+**Matter:**
+```lua
+local function movementSystem(world)
+  for id, position, velocity in world:query(Position, Velocity) do
+    position.x = position.x + velocity.x * dt
+    position.y = position.y + velocity.y * dt
+  end
+end
+```
+
+**jecs:**
+```lua
+local function movementSystem(world, dt)
+  for entity, position, velocity in world:query(Position, Velocity) do
+    position.x = position.x + velocity.x * dt
+    position.y = position.y + velocity.y * dt
+  end
+end
+```
+
+#### Entity Removal
+
+**Matter:**
+```lua
+world:despawn(entity)
+```
+
+**jecs:**
+```lua
+world:delete(entity)
+```
+
+#### Component Removal
+
+**Matter:**
+```lua
+world:remove(entity, Position)
+```
+
+**jecs:**
+```lua
+world:remove(entity, Position)
+```
+
+### jecs-Specific Features
+
+#### Relationships
+
+jecs has built-in support for entity relationships:
+
+```lua
+local ChildOf = world:component()
+local Name = world:component()
+
+local parent = world:entity()
+world:set(parent, Name, "Parent")
+
+local child = world:entity()
+world:add(child, jecs.pair(ChildOf, parent))
+world:set(child, Name, "Child")
+
+-- Query for all children of parent
+for e in world:query(jecs.pair(ChildOf, parent)) do
+  local name = world:get(e, Name)
+  print(name, "is a child of Parent")
+end
+```
+
+#### Wildcards
+
+jecs supports wildcard queries for relationships:
+
+```lua
+-- Query for all parent-child relationships
+for entity, target in world:query(jecs.pair(ChildOf, jecs.Wildcard)) do
+  print(world:get(entity, Name), "is a child of", world:get(target, Name))
+end
+
+-- Alternative using the shorter 'w' alias
+for entity, target in world:query(jecs.pair(ChildOf, jecs.w)) do
+  print(world:get(entity, Name), "is a child of", world:get(target, Name))
+end
+```
+
+#### Observer Hooks
+
+jecs provides component lifecycle hooks:
+
+```lua
+world:set(Position, jecs.OnAdd, function(entity)
+  print("Position added to entity", entity)
+end)
+
+world:set(Position, jecs.OnRemove, function(entity)
+  print("Position removed from entity", entity)
+end)
+
+world:set(Position, jecs.OnSet, function(entity, value)
+  print("Position set on entity", entity, "with value", value)
+end)
+```
+
+#### Automatic Cleanup with OnDelete and OnDeleteTarget
+
+jecs offers automated cleanup of related entities:
+
+```lua
+-- When a parent is deleted, all its children will be deleted too
+world:add(ChildOf, jecs.pair(jecs.OnDeleteTarget, jecs.Delete))
+
+-- When Health component is deleted, remove the entity's Shield component
+world:add(Health, jecs.pair(jecs.OnDelete, jecs.Remove))
+world:add(Shield, jecs.pair(jecs.OnDelete, jecs.Remove))
+```
+
+## Performance Considerations
+
+When migrating from Matter to jecs, consider these performance tips:
+
+1. **Batch Entity Operations**: Group entity operations when possible to minimize archetype transitions.
+2. **Use Cached Queries**: For frequently used queries, create cached versions.
+3. **Consider Component Layout**: Components frequently queried together should be defined together.
+4. **Use Tags When Appropriate**: For components with no data, use tags instead of empty tables.
+5. **Be Aware of Archetype Transitions**: Adding/removing components causes archetype transitions, which have performance implications for large-scale operations.
+
+## Migration Strategy
+
+1. **Start with World Creation**: Replace Matter's world creation with jecs.
+2. **Migrate Component Definitions**: Update component definitions to use jecs's approach.
+3. **Update Entity Creation**: Modify entity spawning code to use jecs's entity and set methods.
+4. **Adapt Queries**: Update queries, noting that iteration patterns are similar.
+5. **Implement Relationships**: Take advantage of jecs's relationship features where applicable.
+6. **Add Observer Hooks**: Implement lifecycle hooks to replace custom event handling in Matter.
+7. **Optimize**: Refine your implementation using jecs-specific features for better performance.
+
+By following this guide, you should be able to migrate your Matter ECS application to jecs while taking advantage of jecs's enhanced performance and features. 

docs/learn/observers.md

@@ -0,0 +1,246 @@
+# Observer APIs
+
+jecs provides observer hooks that allow you to respond to component lifecycle events and implement cleanup policies.
+
+## Component Lifecycle Hooks
+
+Component lifecycle hooks let you execute code when components are added, removed, or modified.
+
+### OnAdd
+
+The `OnAdd` hook is called when a component is added to an entity.
+
+```lua
+-- Define a component
+local Transform = world:component()
+
+-- Set an OnAdd hook for the Transform component
+world:set(Transform, jecs.OnAdd, function(entity)
+  print("Transform component added to entity", entity)
+end)
+
+-- The hook will be called when Transform is added to any entity
+local entity = world:entity()
+world:add(entity, Transform) -- OnAdd hook is triggered
+```
+
+TypeScript signature:
+```typescript
+type OnAddHook = (entity: Entity) => void;
+```
+
+### OnRemove
+
+The `OnRemove` hook is called when a component is removed from an entity.
+
+```lua
+-- Set an OnRemove hook for the Transform component
+world:set(Transform, jecs.OnRemove, function(entity)
+  print("Transform component removed from entity", entity)
+end)
+
+-- The hook will be called when Transform is removed from any entity
+world:remove(entity, Transform) -- OnRemove hook is triggered
+```
+
+TypeScript signature:
+```typescript
+type OnRemoveHook = (entity: Entity) => void;
+```
+
+### OnSet
+
+The `OnSet` hook is called when a component's value is set or changed on an entity.
+
+```lua
+-- Set an OnSet hook for the Transform component
+world:set(Transform, jecs.OnSet, function(entity, value)
+  print("Transform component set on entity", entity, "with value", value)
+end)
+
+-- The hook will be called when Transform's value is set on any entity
+world:set(entity, Transform, { position = {x = 10, y = 20} }) -- OnSet hook is triggered
+```
+
+TypeScript signature:
+```typescript
+type OnSetHook<T> = (entity: Entity, value: T) => void;
+```
+
+## Automatic Cleanup Policies
+
+jecs provides automatic cleanup policies through the `OnDelete` and `OnDeleteTarget` hooks paired with action components.
+
+### OnDelete
+
+The `OnDelete` trait specifies what happens when a component or entity is deleted. It must be paired with an action component like `Delete` or `Remove`.
+
+#### (OnDelete, Delete)
+
+When paired with `Delete`, this policy deletes entities that have the component when the component itself is deleted.
+
+```lua
+-- Define a component
+local Health = world:component()
+
+-- Add the (OnDelete, Delete) cleanup policy to Health
+world:add(Health, jecs.pair(jecs.OnDelete, jecs.Delete))
+
+-- Create entities
+local entity1 = world:entity()
+local entity2 = world:entity()
+
+-- Add Health component to entities
+world:add(entity1, Health)
+world:add(entity2, Health)
+
+-- When Health component is deleted, all entities with Health will be deleted
+world:delete(Health) -- Deletes entity1 and entity2
+```
+
+#### (OnDelete, Remove)
+
+When paired with `Remove`, this policy removes a component from entities when the component itself is deleted.
+
+```lua
+-- Define components
+local Poison = world:component()
+local PoisonEffect = world:component()
+
+-- Add the (OnDelete, Remove) cleanup policy to Poison
+world:add(Poison, jecs.pair(jecs.OnDelete, jecs.Remove))
+world:add(PoisonEffect, jecs.pair(jecs.OnDelete, jecs.Remove))
+
+-- Create entity
+local entity = world:entity()
+world:add(entity, Poison)
+world:add(entity, PoisonEffect)
+
+-- When Poison component is deleted, PoisonEffect will be removed from all entities
+world:delete(Poison) -- Removes PoisonEffect from entity
+```
+
+### OnDeleteTarget
+
+The `OnDeleteTarget` trait specifies what happens when a relationship target is deleted. It must be paired with an action component like `Delete` or `Remove`.
+
+#### (OnDeleteTarget, Delete)
+
+When paired with `Delete`, this policy deletes entities that have a relationship with the deleted target.
+
+```lua
+-- Define a ChildOf component for parent-child relationships
+local ChildOf = world:component()
+
+-- Add the (OnDeleteTarget, Delete) cleanup policy
+world:add(ChildOf, jecs.pair(jecs.OnDeleteTarget, jecs.Delete))
+
+-- Create parent and child entities
+local parent = world:entity()
+local child1 = world:entity()
+local child2 = world:entity()
+
+-- Establish parent-child relationships
+world:add(child1, jecs.pair(ChildOf, parent))
+world:add(child2, jecs.pair(ChildOf, parent))
+
+-- When the parent is deleted, all its children will be deleted too
+world:delete(parent) -- Deletes child1 and child2
+```
+
+#### (OnDeleteTarget, Remove)
+
+When paired with `Remove`, this policy removes a relationship component when its target is deleted.
+
+```lua
+-- Define a Likes component for relationships
+local Likes = world:component()
+
+-- Add the (OnDeleteTarget, Remove) cleanup policy
+world:add(Likes, jecs.pair(jecs.OnDeleteTarget, jecs.Remove))
+
+-- Create entities
+local bob = world:entity()
+local alice = world:entity()
+local charlie = world:entity()
+
+-- Establish relationships
+world:add(bob, jecs.pair(Likes, alice))
+world:add(charlie, jecs.pair(Likes, alice))
+
+-- When alice is deleted, all Likes relationships targeting her will be removed
+world:delete(alice) -- Removes Likes relationship from bob and charlie
+```
+
+## Best Practices
+
+1. **Use hooks for reactive logic**: Component hooks are perfect for synchronizing game state with visual representations or external systems.
+
+2. **Keep hook logic simple**: Hooks should be lightweight and focused on a single concern.
+
+3. **Consider cleanup policies carefully**: The right cleanup policies can prevent memory leaks and simplify your codebase by automating entity management.
+
+4. **Avoid infinite loops**: Be careful not to create circular dependencies with your hooks and cleanup policies.
+
+5. **Document your policies**: When using cleanup policies, document them clearly so other developers understand the entity lifecycle in your application.
+
+6. **Use OnAdd for initialization**: The OnAdd hook is ideal for initializing component-specific resources.
+
+7. **Use OnRemove for cleanup**: The OnRemove hook ensures resources are properly released when components are removed.
+
+8. **Use OnSet for synchronization**: The OnSet hook helps keep different aspects of your game in sync when component values change.
+
+## Example: Complete Entity Lifecycle
+
+This example shows how to use all observer hooks together to manage an entity's lifecycle:
+
+```lua
+local world = jecs.World.new()
+local pair = jecs.pair
+
+-- Define components
+local Model = world:component()
+local Transform = world:component()
+local ChildOf = world:component()
+
+-- Set up component hooks
+world:set(Model, jecs.OnAdd, function(entity)
+    print("Model added to entity", entity)
+    -- Create visual representation
+end)
+
+world:set(Model, jecs.OnRemove, function(entity)
+    print("Model removed from entity", entity)
+    -- Destroy visual representation
+end)
+
+world:set(Model, jecs.OnSet, function(entity, model)
+    print("Model set on entity", entity, "with value", model)
+    -- Update visual representation
+end)
+
+-- Set up cleanup policies
+world:add(ChildOf, pair(jecs.OnDeleteTarget, jecs.Delete))
+world:add(Transform, pair(jecs.OnDelete, jecs.Remove))
+
+-- Create entities
+local parent = world:entity()
+local child = world:entity()
+
+-- Set up relationships and components
+world:add(child, pair(ChildOf, parent))
+world:set(child, Model, "cube")
+world:set(child, Transform, {position = {x = 0, y = 0, z = 0}})
+
+-- Updating a component triggers the OnSet hook
+world:set(child, Model, "sphere")
+
+-- Removing a component triggers the OnRemove hook
+world:remove(child, Model)
+
+-- Deleting the parent triggers the OnDeleteTarget cleanup policy
+-- which automatically deletes the child
+world:delete(parent)
+```
+
+By effectively using observer hooks and cleanup policies, you can create more maintainable and robust ECS-based applications with less manual resource management. 

jecs.luau

@@ -46,7 +46,7 @@ export type Record = {
 }
 
 type IdRecord = {
-	columns: { number },
+	cache: { number },
 	counts: { number },
 	flags: number,
 	size: number,
@@ -480,7 +480,7 @@ local function world_target(world: World, entity: i53, relation: i24, index: num
 		nth = nth + count + 1
 	end
 
-	local tr = idr.columns[archetype_id]
+	local tr = idr.cache[archetype_id]
 
 	nth = archetype.types[nth + tr]
 
@@ -537,7 +537,7 @@ local function id_record_ensure(world: World, id: number): IdRecord
 
 		idr = {
 			size = 0,
-			columns = {},
+			cache = {},
 			counts = {},
 			flags = flags,
 			hooks = {
@@ -562,7 +562,7 @@ local function archetype_append_to_records(
 	local archetype_id = archetype.id
 	local archetype_records = archetype.records
 	local archetype_counts = archetype.counts
-	local idr_columns = idr.columns
+	local idr_columns = idr.cache
 	local idr_counts = idr.counts
 	local tr = idr_columns[archetype_id]
 	if not tr then
@@ -1063,7 +1063,7 @@ local function archetype_destroy(world: World, archetype: Archetype)
 
 	for id in records do
 		local idr = component_index[id]
-		idr.columns[archetype_id] = nil :: any
+		idr.cache[archetype_id] = nil :: any
 		idr.counts[archetype_id] = nil
 		idr.size -= 1
 		records[id] = nil :: any
@@ -1122,7 +1122,7 @@ do
 		if idr then
 			local flags = idr.flags
 			if bit32.band(flags, ECS_ID_DELETE) ~= 0 then
-				for archetype_id in idr.columns do
+				for archetype_id in idr.cache do
 					local idr_archetype = archetypes[archetype_id]
 
 					local entities = idr_archetype.entities
@@ -1134,7 +1134,7 @@ do
 					archetype_destroy(world, idr_archetype)
 				end
 			else
-				for archetype_id in idr.columns do
+				for archetype_id in idr.cache do
 					local idr_archetype = archetypes[archetype_id]
 					local entities = idr_archetype.entities
 					local n = #entities
@@ -1147,55 +1147,66 @@ do
 			end
 		end
 
-		local sparse_array = entity_index.sparse_array
 		local dense_array = entity_index.dense_array
 
 		if idr_t then
-			for archetype_id in idr_t.columns do
-				local children = {}
+			local children
+			local ids
+			local count = 0
+			local archetype_ids = idr_t.cache
+			for archetype_id in archetype_ids do
 				local idr_t_archetype = archetypes[archetype_id]
-
 				local idr_t_types = idr_t_archetype.types
-
-				for _, child in idr_t_archetype.entities do
-					table.insert(children, child)
-				end
-
-				local n = #children
+				local entities = idr_t_archetype.entities
+				local removal_queued = false
 
 				for _, id in idr_t_types do
 					if not ECS_IS_PAIR(id) then
 						continue
 					end
 					local object = ecs_pair_second(world, id)
-					if object == delete then
+					if object ~= delete then
+						continue
+					end
 					local id_record = component_index[id]
 					local flags = id_record.flags
 					local flags_delete_mask: number = bit32.band(flags, ECS_ID_DELETE)
 					if flags_delete_mask ~= 0 then
-							for i = n, 1, -1 do
-								world_delete(world, children[i])
+						for i = #entities, 1, -1 do
+							local child = entities[i]
+							world_delete(world, child)
 						end
 						break
 					else
-							local on_remove = id_record.hooks.on_remove
-							local to = archetype_traverse_remove(world, id, idr_t_archetype)
-							local empty = #to.types == 0
-							for i = n, 1, -1 do
-								local child = children[i]
-								if on_remove then
-									on_remove(child)
+					    if not ids then
+							ids = {}
 						end
-								local r = sparse_array[ECS_ENTITY_T_LO(child)]
-								if not empty then
-									entity_move(entity_index, child, r, to)
+						ids[id] = true
+						removal_queued = true
 					end
 				end
+
+				if not removal_queued then
+					continue
+				end
+				if not children then
+					children = {}
+				end
+				local n = #entities
+				table.move(entities, 1, n, count + 1, children)
+				count += n
+			end
+
+			if ids then
+				for id in ids do
+					for _, child in children do
+						world_remove(world, child, id)
 					end
 				end
 			end
 
-				archetype_destroy(world, idr_t_archetype)
+			for archetype_id in archetype_ids do
+				archetype_destroy(world, archetypes[archetype_id])
 			end
 		end
 
@@ -2096,7 +2107,7 @@ local function world_query(world: World, ...)
 		return q
 	end
 
-	for archetype_id in idr.columns do
+	for archetype_id in idr.cache do
 		local compatibleArchetype = archetypes[archetype_id]
 		if #compatibleArchetype.entities == 0 then
 			continue
@@ -2130,9 +2141,9 @@ local function world_each(world: World, id): () -> ()
 		return NOOP
 	end
 
-	local idr_columns = idr.columns
+	local idr_cache = idr.cache
 	local archetypes = world.archetypes
-	local archetype_id = next(idr_columns, nil) :: number
+	local archetype_id = next(idr_cache, nil) :: number
 	local archetype = archetypes[archetype_id]
 	if not archetype then
 		return NOOP
@@ -2144,7 +2155,7 @@ local function world_each(world: World, id): () -> ()
 	return function(): any
 		local entity = entities[row]
 		while not entity do
-			archetype_id = next(idr_columns, archetype_id) :: number
+			archetype_id = next(idr_cache, archetype_id) :: number
 			if not archetype_id then
 				return
 			end
@@ -2477,6 +2488,7 @@ export type World = {
 
 return {
 	World = World :: { new: () -> World },
+	world = World.new :: () -> World,
 
 	OnAdd = EcsOnAdd :: Entity<(entity: Entity) -> ()>,
 	OnRemove = EcsOnRemove :: Entity<(entity: Entity) -> ()>,
@@ -2500,6 +2512,8 @@ return {
 	ECS_GENERATION = ECS_GENERATION,
 	ECS_ID_IS_WILDCARD = ECS_ID_IS_WILDCARD,
 
+	ECS_ID_DELETE = ECS_ID_DELETE,
+
 	IS_PAIR = ECS_IS_PAIR,
 	pair_first = ecs_pair_first,
 	pair_second = ecs_pair_second,

mkdocs.yml

@@ -51,8 +51,64 @@ extra_css:
 extra_javascript:
   - assets/scripts/smooth-scroll.js
 
+plugins:
+  - search
+  - mike:
+      version_selector: true
+      canonical_version: latest
+  - mkdocstrings:
+      default_handler: python
+      handlers:
+        python:
+          import:
+            - https://installer.github.io/docs/python-objects.inv
+            - https://docs.python.org/3/objects.inv
+          selection:
+            members: true
+          rendering:
+            show_source: true
+            show_submodules: true
+          paths: [src]
+
 nav:
   - Home: index.md
+  - Learn:
+    - learn/index.md
+    - Getting Started:
+      - Installation: learn/getting-started/installation.md
+      - Quick start: learn/getting-started/quick-start.md
+    - Concepts:
+      - Entities & Components: learn/concepts/entities-components.md
+      - Queries: learn/concepts/queries.md
+      - Component Traits: learn/concepts/component-traits.md
+      - Command line: learn/concepts/command-line.md
+      - Addons: learn/concepts/addons.md
+    - Guides:
+      - learn/guides/index.md
+      - Hello, World: learn/guides/hello-world.md
+      - Reading Queries: learn/guides/reading-queries.md
+      - Pair Rules: learn/guides/pair-rules.md
+      - Tagging: learn/guides/tagging.md
+      - Removing Components: learn/guides/removing-components.md
+      - Bulk Operations: learn/guides/bulk.md
+      - Bulk Queries: learn/guides/bulk-queries.md
+      - Debugging: learn/guides/debugging.md
+      - Naming: learn/guides/naming.md
+      - Building Applications: learn/guides/building-applications.md
+      - Troubleshoot: learn/guides/troubleshoot.md
+    - Migration from Matter: learn/migration-from-matter.md
+    - Observer APIs: learn/observers.md
+    - FAQ: learn/faq/index.md
+  - API:
+    - api/jecs.md
+    - World: api/world.md
+    - Query: api/query.md
+    - Entity Management: api/world-entity-management.md 
+    - Name Component: api/name.md
+  - Contributing:
+    - contributing/index.md
+    - Roadmap: contributing/roadmap.md
+    - How to Contribute: learn/faq/contributing.md
   - Tutorials:
     - Get Started: tutorials/index.md
     - Installing Fusion: tutorials/get-started/installing-fusion.md

test/devtools_test.luau

@@ -0,0 +1,25 @@
+local jecs = require("@jecs")
+local pair = jecs.pair
+local ChildOf = jecs.ChildOf
+local lifetime_tracker_add = require("@tools/lifetime_tracker")
+local pe = require("@tools/entity_visualiser").prettify
+local world = lifetime_tracker_add(jecs.world(), {padding_enabled=false})
+local FriendsWith = world:component()
+local _1 = world:print_snapshot()
+local e1 = world:entity()
+local e2 = world:entity()
+world:delete(e2)
+
+local _2 = world:print_snapshot()
+local e3 = world:entity()
+world:add(e3, pair(ChildOf, e1))
+local e4 = world:entity()
+world:add(e4, pair(FriendsWith, e3))
+local _3 = world:print_snapshot()
+world:delete(e1)
+world:delete(e3)
+local _4 = world:print_snapshot()
+world:print_entity_index()
+world:entity()
+world:entity()
+local _5 = world:print_snapshot()

test/tests.luau

@@ -26,6 +26,50 @@ local N = 2 ^ 8
 type World = jecs.World
 type Entity<T=nil> = jecs.Entity<T>
 
+local c = {
+	white_underline = function(s: any)
+		return `\27[1;4m{s}\27[0m`
+	end,
+
+	white = function(s: any)
+		return `\27[37;1m{s}\27[0m`
+	end,
+
+	green = function(s: any)
+		return `\27[32;1m{s}\27[0m`
+	end,
+
+	red = function(s: any)
+		return `\27[31;1m{s}\27[0m`
+	end,
+
+	yellow = function(s: any)
+		return `\27[33;1m{s}\27[0m`
+	end,
+
+	red_highlight = function(s: any)
+		return `\27[41;1;30m{s}\27[0m`
+	end,
+
+	green_highlight = function(s: any)
+		return `\27[42;1;30m{s}\27[0m`
+	end,
+
+	gray = function(s: any)
+		return `\27[30;1m{s}\27[0m`
+	end,
+}
+
+local function pe(e)
+	local gen = ECS_GENERATION(e)
+	return c.green(`e{ECS_ID(e)}`)..c.yellow(`v{gen}`)
+end
+
+local function pp(e)
+	local gen = ECS_GENERATION(e)
+	return c.green(`e{ECS_ID(e)}`)..c.yellow(`v{jecs.ECS_ENTITY_T_HI(e)}`)
+end
+
 local function debug_world_inspect(world: World)
 	local function record(e): jecs.Record
 		return entity_index_try_get_any(world.entity_index, e) :: any
@@ -67,10 +111,61 @@ local function debug_world_inspect(world: World)
 	}
 end
 
+local dwi = debug_world_inspect
+
 local function name(world, e)
 	return world:get(e, jecs.Name)
 end
 
+TEST("#repro", function()
+	local world = world_new()
+
+	local function getTargets(relation)
+		local tgts = {}
+		local pairwildcard = pair(relation, jecs.Wildcard)
+		for _, archetype in world:query(pairwildcard):archetypes() do
+		local tr = archetype.records[pairwildcard]
+		local count = archetype.counts[pairwildcard]
+		local types = archetype.types
+		for _, entity in archetype.entities do
+		    for i = 0, count - 1 do
+		    local tgt = jecs.pair_second(world, types[i + tr])
+		    table.insert(tgts, tgt)
+		    end
+		end
+		end
+		return tgts
+	end
+
+	local Attacks = world:component()
+	local Eats = world:component()
+
+	local function setAttacksAndEats(entity1, entity2)
+		world:add(entity1, pair(Attacks, entity2))
+		world:add(entity1, pair(Eats, entity2))
+	end
+
+	local e1 = world:entity()
+	local e2 = world:entity()
+	local e3 = world:entity()
+	print(e1, e2, e3)
+	setAttacksAndEats(e3, e1)
+	setAttacksAndEats(e3, e2)
+	setAttacksAndEats(e1, e2)
+	print("---------------- delete e2 ---------------")
+	local d = dwi(world)
+	for archetype_id in world.component_index[pair(jecs.Wildcard, e2)].cache do
+		local archetype = world.archetypes[archetype_id].type
+		testkit.print(archetype)
+	end
+	world:delete(e2)
+	print("-----------------------------")
+	testkit.print(d.tbl(e1).types)
+	-- testkit.print(d.tbl(e3).types)
+	-- testkit.print(getTargets(Attacks))
+	-- testkit.print(getTargets(Eats))
+end)
+
 TEST("archetype", function()
 	local archetype_traverse_add = jecs.archetype_traverse_add
 	local archetype_traverse_remove = jecs.archetype_traverse_remove
@@ -1199,6 +1294,7 @@ TEST("world:delete", function()
 			world:delete(e)
 		end)
 
+		local d = debug_world_inspect(world)
 		for i, friend in friends do
 			CHECK(not world:has(friend, pair(FriendsWith, e)))
 			CHECK(world:has(friend, Health))

tools/ansi.luau

@@ -0,0 +1,33 @@
+return {
+	white_underline = function(s: any)
+		return `\27[1;4m{s}\27[0m`
+	end,
+
+	white = function(s: any)
+		return `\27[37;1m{s}\27[0m`
+	end,
+
+	green = function(s: any)
+		return `\27[32;1m{s}\27[0m`
+	end,
+
+	red = function(s: any)
+		return `\27[31;1m{s}\27[0m`
+	end,
+
+	yellow = function(s: any)
+		return `\27[33;1m{s}\27[0m`
+	end,
+
+	red_highlight = function(s: any)
+		return `\27[41;1;30m{s}\27[0m`
+	end,
+
+	green_highlight = function(s: any)
+		return `\27[42;1;30m{s}\27[0m`
+	end,
+
+	gray = function(s: any)
+		return `\27[30;1m{s}\27[0m`
+	end,
+}

tools/entity_visualiser.luau

@@ -0,0 +1,43 @@
+local jecs = require("@jecs")
+local ECS_GENERATION = jecs.ECS_GENERATION
+local ECS_ID = jecs.ECS_ID
+local ansi = require("@tools/ansi")
+
+local function pe(e: any)
+	local gen = ECS_GENERATION(e)
+	return ansi.green(`e{ECS_ID(e)}`)..ansi.yellow(`v{gen}`)
+end
+
+local function name(world: jecs.World, id: any)
+	return world:get(id, jecs.Name) or `${id}`
+end
+
+local function components(world: jecs.World, entity: any)
+	local r = jecs.entity_index_try_get(world.entity_index, entity)
+	if not r then
+		return false
+	end
+
+	local archetype = r.archetype
+	local row = r.row
+	print(`Entity {pe(entity)}`)
+	print("-----------------------------------------------------")
+	for i, column in archetype.columns do
+		local component = archetype.types[i]
+		local n
+		if jecs.IS_PAIR(component) then
+			n = `({name(world, jecs.pair_first(world, component))}, {name(world, jecs.pair_second(world, component))})`
+		else
+			n = name(world, component)
+		end
+		local data = column[row] or "TAG"
+		print(`| {n} | {data} |`)
+	end
+	print("-----------------------------------------------------")
+	return true
+end
+
+return {
+	components = components,
+	prettify = pe,
+}

tools/lifetime_tracker.luau

@@ -0,0 +1,212 @@
+local jecs = require("@jecs")
+local ECS_GENERATION = jecs.ECS_GENERATION
+local ECS_ID = jecs.ECS_ID
+local __ = jecs.Wildcard
+local pair = jecs.pair
+
+local prettify = require("@tools/entity_visualiser").prettify
+
+local pe = prettify
+local ansi = require("@tools/ansi")
+
+function print_centered_entity(entity, width: number)
+    local entity_str = tostring(entity)
+    local entity_length = #entity_str
+
+    local padding_total = width - 2 - entity_length
+
+    local padding_left = math.floor(padding_total / 2)
+    local padding_right = padding_total - padding_left
+
+    local centered_str = string.rep(" ", padding_left) .. entity_str .. string.rep(" ", padding_right)
+
+    print("|" .. centered_str .. "|")
+end
+
+local function name(world, e)
+	return world:get(world, e, jecs.Name) or pe(e)
+end
+local padding_enabled = false
+local function pad()
+	if padding_enabled then
+		print("")
+	end
+end
+
+local function lifetime_tracker_add(world: jecs.World, opt)
+	local entity_index = world.entity_index
+	local dense_array = entity_index.dense_array
+	local component_index = world.component_index
+
+	local ENTITY_RANGE = (jecs.Rest :: any) + 1
+
+	local w = setmetatable({}, { __index = world })
+
+	padding_enabled = opt.padding_enabled
+
+	local world_entity = world.entity
+	w.entity = function(self)
+		local will_recycle = entity_index.max_id ~= entity_index.alive_count
+		local e = world_entity(world)
+		if will_recycle then
+			print(`*recycled {pe(e)}`)
+		else
+			print(`*created {pe(e)}`)
+		end
+		pad()
+		return e
+	end
+	w.print_entity_index = function(self)
+		local max_id = entity_index.max_id
+		local alive_count = entity_index.alive_count
+		local alive = table.move(dense_array, 1+jecs.Rest::any, alive_count, 1, {})
+		local dead = table.move(dense_array, alive_count + 1, max_id, 1, {})
+
+		local sep = "|--------|"
+		if #alive > 0 then
+			print("|-alive--|")
+			for i = 1, #alive do
+				local e = pe(alive[i])
+				print_centered_entity(e, 32)
+				print(sep)
+			end
+			print("\n")
+		end
+
+		if #dead > 0 then
+			print("|--dead--|")
+			for i = 1, #dead do
+				print_centered_entity(pe(dead[i]), 32)
+				print(sep)
+			end
+		end
+		pad()
+	end
+	local timelines = {}
+	w.print_snapshot = function(self)
+		local timeline = #timelines + 1
+		local entity_column_width = 10
+		local status_column_width = 8
+
+		local header = string.format("| %-" .. entity_column_width .. "s |", "Entity")
+		for i = 1, timeline do
+			header = header .. string.format(" %-" .. status_column_width .. "s |", string.format("T%d", i))
+		end
+
+		local max_id = entity_index.max_id
+		local alive_count = entity_index.alive_count
+		local alive = table.move(dense_array, 1+jecs.Rest::any, alive_count, 1, {})
+		local dead = table.move(dense_array, alive_count + 1, max_id, 1, {})
+
+		local data = {}
+		print("-------------------------------------------------------------------")
+		print(header)
+
+		-- Store the snapshot data for this timeline
+		for i = ENTITY_RANGE, max_id do
+			if dense_array[i] then
+				local entity = dense_array[i]
+				local id = ECS_ID(entity)
+				local status = "alive"
+				if id > alive_count then
+					status = "dead"
+				end
+				data[id] = status
+			end
+		end
+
+		table.insert(timelines, data)
+
+		-- Create a table to hold entity data for sorting
+		local entities = {}
+		for i = ENTITY_RANGE, max_id do
+			if dense_array[i] then
+				local entity = dense_array[i]
+				local id = ECS_ID(entity)
+				-- Push entity and id into the new `entities` table
+				table.insert(entities, {entity = entity, id = id})
+			end
+		end
+
+		-- Sort the entities by ECS_ID
+		table.sort(entities, function(a, b)
+			return a.id < b.id
+		end)
+
+		-- Print the sorted rows
+		for _, entity_data in ipairs(entities) do
+			local entity = entity_data.entity
+			local id = entity_data.id
+			local status = "alive"
+			if id > alive_count then
+				status = "dead"
+			end
+			local row = string.format("| %-" .. entity_column_width .. "s     |", pe(entity))
+			for j = 1, timeline do
+				local timeline_data = timelines[j]
+				local entity_data = timeline_data[id]
+				if entity_data then
+					row = row .. string.format(" %-" .. status_column_width .. "s |", entity_data)
+				else
+					row = row .. string.format(" %-" .. status_column_width .. "s |", "-")
+				end
+			end
+			print(row)
+		end
+		print("-------------------------------------------------------------------")
+		pad()
+	end
+	local world_add = world.add
+	local relations = {}
+	w.add = function(self, entity: any, component: any)
+		world_add(world, entity, component)
+		if jecs.IS_PAIR(component) then
+			local relation = jecs.pair_first(world, component)
+			local target = jecs.pair_second(world, component)
+			print(`*added ({pe(relation)}, {pe(target)}) to {pe(entity)}`)
+			pad()
+		end
+
+	end
+
+	local world_delete = world.delete
+	w.delete = function(self, e)
+		world_delete(world, e)
+
+		local idr_t = component_index[pair(__, e)]
+		if idr_t then
+			for archetype_id in idr_t.cache do
+				local archetype = world.archetypes[archetype_id]
+				for _, id in archetype.types do
+					if not jecs.IS_PAIR(id) then
+						continue
+					end
+					local object = jecs.pair_second(world, id)
+					if object ~= e then
+						continue
+					end
+					local id_record = component_index[id]
+					local flags = id_record.flags
+					local flags_delete_mask: number = bit32.band(flags, jecs.ECS_ID_DELETE)
+					if flags_delete_mask ~= 0 then
+						for _, entity in archetype.entities do
+							print(`*deleted dependant {pe(entity)} of {pe(e)}`)
+							pad()
+						end
+						break
+					else
+						for _, entity in archetype.entities do
+							print(`*removed dependency ({pe(jecs.pair_first(world, id))}, {pe(object)}) from {pe(entity)}`)
+						end
+					end
+				end
+			end
+		end
+
+		print(`*deleted {pe(e)}`)
+		pad()
+	end
+	return w
+end
+
+return lifetime_tracker_add