# Implementation Plan: OECS (Observable ECS)

## Overview

This document lays out the phased implementation of OECS as a .NET class library
(DLL). Each phase produces a shippable increment; later phases build on earlier
ones. Architecture decisions are captured in `docs/architecture.md`.

## Target

- .NET 8 (LTS), `net8.0` (C# 12)
- Dependencies: `MessagePack` (serialization), `R3` (reactivity)
- Output: `OECS.dll`
- Source generator: `OECS.SourceGen.dll` (component registry for serialization)

---

## Phase 1 — Core Foundation (Week 1)

**Goal:** Create/destroy entities, add/remove components, iterate sparse sets.

### 1.1 Project Scaffold

```
OECS.sln
├── OECS/OECS.csproj
├── OECS.SourceGen/OECS.SourceGen.csproj
├── OECS.Tests/OECS.Tests.csproj
├── Game.Blackjack/Blackjack.csproj
├── Game.Blackjack.Tests/Game.Blackjack.Tests.csproj
└── Game.TicTacToe/...
```

- `OECS.csproj` targets `net8.0`, references `MessagePack` and `R3`.
- `OECS.Tests.csproj` references `OECS` + `xunit` + `FluentAssertions`.

### 1.2 Entity

`readonly struct Entity : IEquatable<Entity>`

| Decision | Rationale |
|---|---|
| 32-bit `uint` backing | Keeps size small; 24-bit ID (16.7M) + 8-bit version (256 gens) is ample for observable-first use cases. |
| `Entity.Null` sentinel (value `0`) | Entity ID 0 is reserved; version 0 means "never alive." |
| `Id` (24-bit) and `Version` (8-bit) properties | Expose for debugging; opaque otherwise. |
| `ToString()` → `"Entity(42:v3)"` | Debuggability. |

Internal constructor; `World` is the only factory.

### 1.3 SparseSet\<T\>

```
class SparseSet<T> where T : struct
```

| Field | Purpose |
|---|---|
| `T[] dense` | Packed component values (no holes). |
| `Entity[] denseEntities` | Parallel array: which entity owns each dense slot. |
| `int[] sparse` | Maps entity ID → dense index; `-1` = absent. |

Operations: `Add(Entity, T)`, `Remove(Entity)`, `ref T Get(Entity)`,
`bool Contains(Entity)`, `int Count`, `void Clear()`.

`dense` and `sparse` arrays grow geometrically (×2) on overflow.

**Why sparse sets over archetypes?** The design prioritizes cheap add/remove
(for reactivity) over raw iteration throughput. Archetypes would require moving
entities between archetypes on component change, which complicates change
tracking.

### 1.4 ComponentStore

```
class ComponentStore
```

Holds a `Dictionary<Type, object>` mapping component types to their
`SparseSet<T>`. Provides typed generic methods:

- `void Add<T>(Entity, T)`
- `void Remove<T>(Entity)`
- `ref T Get<T>(Entity)`
- `T Read<T>(Entity)` — copy without auto-mark
- `bool TryGet<T>(Entity, out T)`
- `bool Has<T>(Entity)`
- `void RemoveAll(Entity)` — called on entity destruction

### 1.5 World

```
class World
```

| Responsibility | Detail |
|---|---|
| Entity allocation | Free-list of recycled IDs; bump allocator for new IDs. |
| Component access | Delegates to `ComponentStore`. |
| Entity destruction | Returns ID to free list, increments version, removes all components. |

API surface:

```csharp
Entity CreateEntity();
void DestroyEntity(Entity entity);
void AddComponent<T>(Entity entity, T component) where T : struct;
void RemoveComponent<T>(Entity entity) where T : struct;
ref T GetComponent<T>(Entity entity) where T : struct;
T ReadComponent<T>(Entity entity) where T : struct;
bool TryGetComponent<T>(Entity entity, out T value) where T : struct;
bool HasComponent<T>(Entity entity) where T : struct;
bool IsAlive(Entity entity);
```

### 1.6 Tests

- Entity creation returns unique IDs.
- Entity destruction recycles IDs with incremented version.
- `IsAlive` returns false for destroyed entities.
- Add/remove/has component round-trips correctly.
- Sparse set iteration visits all added components.
- Removing a component mid-iteration is safe (deferred or swap-remove).

---

## Phase 2 — Queries & Systems (Week 2)

**Goal:** Define queries, register systems, run ticks.

### 2.1 Query Description

A query is defined by:

- A set of "with" component types.
- A set of "without" component types.

```
class QueryDescriptor
{
    IReadOnlySet<Type> With { get; }
    IReadOnlySet<Type> Without { get; }
}
```

### 2.2 QueryBuilder

Fluent API returned by `World.Query()`:

```csharp
world.Query()
    .With<Position>()
    .With<Velocity>()
    .Without<Frozen>()
    .Build()  // → QueryDescriptor
```

### 2.3 Query Execution

`World` provides two iteration styles:

**ForEach callbacks** (1–6 component types):
```csharp
void ForEach<T1, T2>(QueryDescriptor query, ForEachAction<T1, T2> action);
```

**Ref struct iterators** via `EntityIterator.Select<T>()` extensions (1–3 component types):
```csharp
using var iter = world.Select<Position, Velocity>(query);
while (iter.MoveNext()) { ... }
```

Both accept an optional `QueryDescriptor`. When omitted, all entities with
the given component types are iterated (no `Without` filter).

The smallest "with" sparse set is used as the driver; other sets are probed
for membership. The singleton entity (ID 1) is always skipped.

### 2.4 ISystem

```
interface ISystem
{
    void Run(World world);
}
```

Systems do **not** declare a query on the interface. Instead, they either
read singletons directly or build queries internally and iterate with
`ForEach` / `Select`. This keeps the interface minimal and gives systems
full flexibility.

**Why no auto-injection?** The explicit API makes the iteration cost
visible and avoids the need for source generators or reflection for
system dispatch.

### 2.5 ITickedSystem

```
interface ITickedSystem : ISystem
{
    void Run(World world, Tick tick);
}
```

Systems that need tick metadata (delta time or logical tick marker)
implement this. Both `Run` overloads must be implemented; `SystemGroup`
dispatches to the appropriate one at runtime.

### 2.6 System Registration & Ordering

```
class SystemGroup
{
    void Add(ISystem system);          // registration order = execution order
    void Remove(ISystem system);
    void RunTimed(float deltaTime);
    void RunLogical();
    int Count { get; }
}
```

Systems run in registration order. `SystemGroup` automatically:
- Drains commands before the tick, after each system, and after the full tick.
- Posts changes after each system and after the full tick.

### 2.7 Tick

```
readonly struct Tick
{
    TickType Type { get; }   // Timed or Logical
    float DeltaTime { get; } // 0 for logical ticks
}
```

### 2.8 Tests

- Query with single component returns matching entities.
- Query with multiple components returns intersection.
- `Without<T>` excludes entities with T.
- Adding/removing components updates query results.
- Systems run in registration order.
- Destroyed entities don't appear in queries.
- `ITickedSystem` receives correct tick data.

---

## Phase 3 — Commands (Week 3)

**Goal:** Serializable command queue with deferred execution.

### 3.1 ICommand

```
interface ICommand
{
    void Execute(World world);
}
```

Commands are structs implementing `ICommand`. They are typically
`[MessagePackObject]` for serialization. Not stored in ECS sparse
sets — they live in a queue.

### 3.2 CommandQueue

```
class CommandQueue
{
    void Enqueue<T>(T command) where T : struct, ICommand;
    void ExecuteAll(World world);  // FIFO, fully drains queue
    void Clear();
    void ClearErrors();
    int Count { get; }
    IReadOnlyList<Exception> Errors { get; }
}
```

`Enqueue<T>` uses a constrained generic to avoid boxing at the call site.
Commands enqueued during `ExecuteAll` are processed in the same drain cycle.

### 3.3 Integration with World

`World` owns a `CommandQueue`. After each system runs (or after the full tick),
the queue is drained automatically by `SystemGroup`. Manual drain is also
available:

```csharp
world.ExecuteCommands(); // manual drain
```

Systems enqueue commands via `world.Commands.Enqueue(...)`.

### 3.4 Error Handling

If a command's `Execute` throws, the exception is caught and stored. The queue
continues processing remaining commands. After `ExecuteAll`, any errors are
available via `CommandQueue.Errors`.

### 3.5 Tests

- Commands execute in FIFO order.
- A command can read/write ECS state.
- A command enqueued during `ExecuteAll` runs in the same drain cycle.
- Exceptions are collected, not lost.
- Serialization round-trip preserves command data.

---

## Phase 4 — Relationships (Week 4)

**Goal:** Relationship components with auto-managed source/target and reverse
lookup.

### 4.1 IRelationship

```
interface IRelationship
{
    Entity Source { get; }
    Entity Target { get; }
}
```

### 4.2 Relationship\<TSelf, TTarget\>

```
[MessagePackObject]
struct Relationship<TSelf, TTarget> : IRelationship
    where TSelf : struct where TTarget : struct
{
    [Key(0)] Entity Source { get; set; }
    [Key(1)] Entity Target { get; set; }
}
```

The `IRelationship` marker interface lets `ComponentStore` detect relationships
and maintain the reverse index. The `TSelf`/`TTarget` phantom types
differentiate relationship kinds at the type level.

Alternatively, implement `IRelationship` directly on your own struct (see the
Blackjack `Holds` and `InDeck` types).

### 4.3 Reverse Index

`World` maintains a reverse index per relationship type, mapping target →
set of source entities. Updated automatically when relationship components
are added/removed.

### 4.4 Reverse Lookup API

```csharp
IReadOnlyCollection<Entity> GetSources<T>(Entity target) where T : struct, IRelationship;
```

### 4.5 Cascading Behavior

When an entity is destroyed:
- All relationships where it is the **source** are removed (components dropped).
- All relationships where it is the **target** are removed (components dropped
  from source entities).
- The reverse index is cleaned up.

### 4.6 Tests

- Adding a relationship updates the reverse index.
- Removing a relationship updates the reverse index.
- Destroying a source entity cleans up its relationships.
- Destroying a target entity cleans up incoming relationships.
- Reverse lookup returns correct sources.

---

## Phase 5 — Reactivity (Week 5–6)

**Goal:** Change tracking, marking, posting, and R3 observable queries.

### 5.1 Change Kinds

```
enum ChangeKind
{
    EntityAdded,
    EntityRemoved,
    ComponentAdded,
    ComponentRemoved,
    ComponentModified
}

struct EntityChange
{
    Entity Entity { get; }
    ChangeKind Kind { get; }
    Type? ComponentType { get; } // null for entity-level changes
}
```

### 5.2 ChangeBuffer

```
class ChangeBuffer
{
    void Mark<...>(...);
    void Post();  // pushes to R3 subjects, then clears
}
```

### 5.3 Automatic vs. Manual Marking

| Change Type | Marking |
|---|---|
| Entity created | Auto |
| Entity destroyed | Auto |
| Component added | Auto |
| Component removed | Auto |
| Component **modified** | **Manual** via `world.MarkModified<T>(entity)` |

Rationale: structural changes are always detectable. Value mutations inside a
`ref T` are not — the sparse set has no way to know the caller changed the
value. Requiring an explicit `MarkModified` call is the simplest correct
approach.

**`ReadComponent<T>`** (returns a copy) and **`ReadSingleton<T>`** never
auto-mark. Use these when you only need to inspect state without signaling
changes.

### 5.4 Posting Model

- During a system's `Run`, changes accumulate in a pending buffer.
- After each system's `Run`, `Post()` is called automatically by `SystemGroup`.
- After the full tick, `Post()` is called once more.
- When not in a system run, changes are **not** posted automatically — the
  caller must call `world.PostChanges()`.

### 5.5 R3 Integration

`World` exposes observables:

```csharp
Observable<EntityChange> ObserveEntityChanges();
Observable<EntityChange> ObserveComponentChanges<T>() where T : struct;
Observable<EntityChange> ObserveQuery(QueryDescriptor query);
```

These are backed by `Subject<EntityChange>` instances. Subscribers receive
batched changes after each `Post()`.

### 5.6 Subscription Lifecycle

Subscriptions return `IDisposable`. UI code ties this to component lifecycle:

```csharp
world.ObserveComponentChanges<Health>()
    .Subscribe(change => UpdateHealthBar(change))
    .AddTo(componentDisposables); // R3's AddTo
```

### 5.7 Tests

- Entity creation posts `EntityAdded`.
- Entity destruction posts `EntityRemoved`.
- Component add/remove posts corresponding changes.
- `MarkModified` posts `ComponentModified`.
- Changes are batched per `Post()` call.
- Subscribers receive changes in order.
- Disposing a subscription stops notifications.

---

## Phase 6 — Singletons (Week 6)

**Goal:** Singleton entity and ergonomic accessors.

### 6.1 Singleton Entity

`World` reserves entity ID `1` as the singleton entity. It is never destroyed
and is excluded from normal queries (all iterators skip ID 1).

### 6.2 Singleton Accessors

```csharp
void SetSingleton<T>(T component) where T : struct;
ref T GetSingleton<T>() where T : struct;
T ReadSingleton<T>() where T : struct;
bool HasSingleton<T>() where T : struct;
void RemoveSingleton<T>() where T : struct;
```

These are convenience wrappers around `AddComponent`/`GetComponent` on the
singleton entity. `GetSingleton` returns a `ref` (auto-marks during iteration);
`ReadSingleton` returns a copy (never auto-marks).

### 6.3 Query Exclusion

Queries automatically exclude the singleton entity. If a user genuinely wants
to include it, they can query it by its entity ID directly.

### 6.4 Tests

- `SetSingleton`/`GetSingleton` round-trips.
- Singleton entity does not appear in normal queries.
- Removing a singleton works.
- Singleton survives tick execution.

---

## Phase 7 — Source Generator & Serialization (Week 7)

**Goal:** Compile-time component registry for serialization without reflection.

### 7.1 OECS.SourceGen

A Roslyn incremental source generator that scans for all types used as
generic arguments to `World` methods (`AddComponent<T>`, `GetComponent<T>`,
`SetSingleton<T>`, etc.) and generates a `ComponentRegistry` class with
per-type serialize/deserialize callbacks via MessagePack.

### 7.2 WorldSerializer

```csharp
static class WorldSerializer
{
    static void Save(World world, Stream stream);
    static void Load(World world, Stream stream);
}
```

Uses `ComponentRegistry` to serialize/deserialize all entities and their
components to/from a MessagePack stream. `IRelationship.Source` is fixed
up to the owning entity on load.

### 7.3 Snapshot Types

`WorldSnapshot`, `EntitySnapshot`, and `ComponentEntry` are the public
serialization DTOs.

---

## Phase 8 — Polish & Documentation (Week 8)

### 8.1 XML Docs

All public API surface gets `<summary>` XML documentation comments.

### 8.2 README

Quick-start guide with a minimal example: create world, register system, run
tick, observe changes.

### 8.3 NuGet Packaging

`OECS.csproj` includes package metadata:
- `PackageId`: `OECS`
- `Description`: "Observable ECS for C# — an entity component system focused on
  a clean reactive API surface."
- `PackageTags`: `ecs;reactive;observable;gamedev`
- Bundles `OECS.SourceGen.dll` as an analyzer for consumers.

### 8.4 Testing Games

Test games (`Game.Blackjack`, `Game.TicTacToe`) serve as integration tests
and design validation. See `docs/testing-games.md` for the testing strategy.

---

## Dependency Graph

```
Phase 1 (Core)
  └─→ Phase 2 (Queries & Systems)
        └─→ Phase 3 (Commands)
        └─→ Phase 4 (Relationships)
              └─→ Phase 5 (Reactivity)
                    └─→ Phase 6 (Singletons)
                          └─→ Phase 7 (Source Gen & Serialization)
                                └─→ Phase 8 (Polish)
```

Phases 3 and 4 can be done in parallel; Phase 5 depends on both.

---

## Open Questions

1. **Parallel system execution?** Deferred. The design is single-threaded by
   default. If needed, systems could declare read/write component access for
   automatic parallel scheduling — but this adds significant complexity.

2. **Multiple worlds?** The design supports it naturally — `World` is a class,
   you can instantiate multiple. No cross-world references are supported.

3. **Component import from CSV/MasterMemory?** The `design.md` mentions this as
   a potential feature. Not yet implemented.