# API Surface: OECS

This document describes the public API surface of the OECS library. It serves
as a contract for implementation and a reference for consumers.

All types reside in the `OECS` namespace unless otherwise noted.

---

## Entity

```csharp
namespace OECS;

public readonly struct Entity : IEquatable<Entity>
{
    public static Entity Null { get; }        // ID=0, Version=0
    public uint Id { get; }                    // lower 24-bit identifier
    public uint Version { get; }               // upper 8-bit generation
    public bool IsNull { get; }                // true for Entity.Null

    public bool Equals(Entity other);
    public override bool Equals(object? obj);
    public override int GetHashCode();
    public override string ToString();         // "Entity(42:v3)"
    public static bool operator ==(Entity left, Entity right);
    public static bool operator !=(Entity left, Entity right);
}
```

---

## World

```csharp
namespace OECS;

public class World : IDisposable
{
    // --- Lifecycle ---
    public World();

    // --- Entity Management ---
    public Entity CreateEntity();
    public void DestroyEntity(Entity entity);
    public bool IsAlive(Entity entity);

    // --- Component Management ---
    public void AddComponent<T>(Entity entity, T component) where T : struct;
    public void RemoveComponent<T>(Entity entity) where T : struct;
    public ref T GetComponent<T>(Entity entity) where T : struct;
    public T ReadComponent<T>(Entity entity) where T : struct;
    public bool TryGetComponent<T>(Entity entity, out T value) where T : struct;
    public bool HasComponent<T>(Entity entity) where T : struct;

    // --- Singleton ---
    public void SetSingleton<T>(T component) where T : struct;
    public ref T GetSingleton<T>() where T : struct;
    public T ReadSingleton<T>() where T : struct;
    public bool HasSingleton<T>() where T : struct;
    public void RemoveSingleton<T>() where T : struct;

    // --- Queries ---
    // See QueryExtensions for Select / FindEntity / FindEntities.

    // --- Commands ---
    public CommandQueue Commands { get; }
    public void ExecuteCommands();

    // --- Reactivity ---
    public void MarkModified<T>(Entity entity) where T : struct;
    public void PostChanges();

    public Observable<EntityChange> ObserveEntityChanges();
    public Observable<EntityChange> ObserveComponentChanges<T>()
        where T : struct;
    public Observable<EntityChange> ObserveQuery<T1>(Query<T1> query = default)
        where T1 : struct;
    public Observable<EntityChange> ObserveQuery<T1, T2>(Query<T1, T2> query = default)
        where T1 : struct where T2 : struct;

    // --- Relationships ---
    public IReadOnlyCollection<Entity> GetSources<T>(Entity target)
        where T : struct, IRelationship;

    // --- Cleanup ---
    public void Dispose();
}
```

---

## WorldQueryExtensions

Extension methods on `World` providing `foreach`-compatible iteration and
entity lookup. Select returns `ref struct` enumerators for zero-allocation
iteration with `ref` access to components.

```csharp
namespace OECS;

public static class WorldQueryExtensions
{
    // Select — returns a ref struct enumerator for foreach.
    public static Select1<T1> Select<T1>(
        this World world, Query<T1> query = default) where T1 : struct;
    // Overloads for 2–6 component types:
    public static Select2<T1, T2> Select<T1, T2>(...) where T1 : struct where T2 : struct;
    // ... up to Select6<T1..T6>

    // FindEntity — returns the first matching entity or Entity.Null.
    public static Entity FindEntity<T1>(
        this World world, Query<T1> query = default) where T1 : struct;
    public static Entity FindEntity<T1, T2>(...) where T1 : struct where T2 : struct;
    public static Entity FindEntity<T1, T2, T3>(...) where T1 : struct where T2 : struct where T3 : struct;

    // FindEntities — returns all matching entities as List<Entity>.
    public static List<Entity> FindEntities<T1>(...) where T1 : struct;
    public static List<Entity> FindEntities<T1, T2>(...) where T1 : struct where T2 : struct;
    public static List<Entity> FindEntities<T1, T2, T3>(...) where T1 : struct where T2 : struct where T3 : struct;
}
```

Each ref struct (Select1 through Select6) exposes:
- `Entity Entity` — the current entity handle.
- `ref T1 Item1`, `ref T2 Item2`, ... — read-write references to components.
- `bool MoveNext()` — advances and returns true while items remain.
- `GetEnumerator()` — returns `this` for `foreach` compatibility.
- `Dispose()` — ends the iteration scope (flushes deferred mutations).

Usage:
```csharp
// Simple scan: all entities with Position.
foreach (var it in world.Select<Position>())
{
    it.Item1.X += 1; // ref mutates component in-place
}

// With Without filter:
var query = new Query<Position>().Without<Frozen>();
foreach (var it in world.Select(query))
{
    // it.Entity, it.Item1
}

// Find first entity:
var hand = world.FindEntity<PlayerHand>();

// Multi-component:
foreach (var it in world.Select<Position, Velocity>())
{
    it.Item1.X += it.Item2.X * dt;
}
```

---

## Query

Describes a query over the ECS world. The `With` types are encoded as generic
parameters. Optional `Without` filters are added via the fluent API.

```csharp
namespace OECS;

public struct Query<T1> where T1 : struct
{
    public Query<T1> Without<W>() where W : struct;
}

// Overloads for 2–6 With types:
public struct Query<T1, T2> where T1 : struct where T2 : struct { ... }
// ... up to Query<T1, T2, T3, T4, T5, T6>
```

Usage:
```csharp
// Two required types, one excluded:
var query = new Query<Position, Velocity>().Without<Frozen>();
foreach (var it in world.Select(query)) { ... }

// Simple scan — no Without filter needed:
foreach (var it in world.Select<Card>()) { ... }
```

---

## ISystem

A system is a unit of logic that runs during a tick. It receives the `World`
and decides what to do — typically reading singletons, iterating queries,
or enqueuing commands.

```csharp
namespace OECS;

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

Systems do **not** declare a query on the interface. Instead, they either
read singletons directly (`world.ReadSingleton<T>()`) or use the
`WorldQueryExtensions` (`Select`, `FindEntity`, `FindEntities`) with a
`Query<T1..T6>` they build inline. This keeps the interface minimal and
gives systems full flexibility over what they inspect at runtime.

---

## ITickedSystem

An optional extension for systems that need tick metadata (delta time or
logical tick marker).

```csharp
namespace OECS;

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

`SystemGroup` checks each system at runtime: if it implements
`ITickedSystem`, the `Run(World, Tick)` overload is called; otherwise
`Run(World)` is called. Both overloads must be implemented.

---

## Tick

```csharp
namespace OECS;

public readonly struct Tick
{
    public TickType Type { get; }
    public float DeltaTime { get; }

    public static Tick Timed(float deltaTime);
    public static Tick Logical();
}

public enum TickType
{
    Timed,
    Logical
}
```

---

## SystemGroup

```csharp
namespace OECS;

public class SystemGroup
{
    public SystemGroup(World world);
    public void Add(ISystem system);
    public void Remove(ISystem system);
    public void RunTimed(float deltaTime);
    public void RunLogical();
    public int Count { get; }
}
```

Systems execute in registration order. `SystemGroup` automatically drains
commands and posts changes after each system and after the full tick.

---

## ICommand

```csharp
namespace OECS;

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

Implementations should be `[MessagePackObject]` structs so they can be
serialized and replayed.

---

## CommandQueue

```csharp
namespace OECS;

public class CommandQueue
{
    public void Enqueue<T>(T command) where T : struct, ICommand;
    public void ExecuteAll(World world);
    public void Clear();
    public void ClearErrors();
    public int Count { get; }
    public 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.

---

## IRelationship

```csharp
namespace OECS;

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

---

## Relationship\<TSelf, TTarget\>

A convenience base struct for relationship components. The type parameters are
phantom types that differentiate relationship kinds at the type level, enabling
type-safe queries and reverse lookups.

```csharp
namespace OECS;

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

---

## EntityChange

```csharp
namespace OECS;

public readonly struct EntityChange : IEquatable<EntityChange>
{
    public Entity Entity { get; }
    public ChangeKind Kind { get; }
    public Type? ComponentType { get; }  // null for entity-level changes
}

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

---

## WorldSerializer

Saves and loads a `World` to/from a MessagePack stream. Components are
serialized by their runtime type using the source-generated `ComponentRegistry`.

```csharp
namespace OECS;

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

---

## WorldSnapshot / EntitySnapshot / ComponentEntry

Serialization types used by `WorldSerializer`. These are public but intended
primarily for serialization infrastructure.

```csharp
namespace OECS;

[MessagePackObject]
public class WorldSnapshot
{
    [Key(0)] public EntitySnapshot[] Entities { get; set; }
}

[MessagePackObject]
public class EntitySnapshot
{
    [Key(0)] public uint Id { get; set; }
    [Key(1)] public uint Version { get; set; }
    [Key(2)] public ComponentEntry[] Components { get; set; }
    [IgnoreMember]
    public Entity Entity { get; }
}

[MessagePackObject]
public class ComponentEntry
{
    [Key(0)] public string TypeName { get; set; }
    [Key(1)] public byte[] Data { get; set; }
}
```

---

## Usage Example

Components and commands should be `public record struct` types with explicit
fields/properties (not positional syntax). See the `writing-games` skill for
the rationale.

```csharp
using OECS;

// Define components
[MessagePackObject]
public record struct Position
{
    [Key(0)] public float X;
    [Key(1)] public float Y;
}

[MessagePackObject]
public record struct Velocity
{
    [Key(0)] public float X;
    [Key(1)] public float Y;
}

// Define a system
public class MovementSystem : ITickedSystem
{
    public void Run(World world) => Run(world, Tick.Logical());

    public void Run(World world, Tick tick)
    {
        float dt = tick.DeltaTime;

        foreach (var it in world.Select<Position, Velocity>())
        {
            it.Item1.X += it.Item2.X * dt;
            it.Item1.Y += it.Item2.Y * dt;
        }
    }
}

// Wire it up
var world = new World();
var group = new SystemGroup(world);
group.Add(new MovementSystem());

// Create entities
var player = world.CreateEntity();
world.AddComponent(player, new Position { X = 0, Y = 0 });
world.AddComponent(player, new Velocity { X = 1, Y = 0 });

// Observe changes via R3
world.ObserveComponentChanges<Position>()
    .Subscribe(change => Console.WriteLine($"{change.Entity} moved"));

// Run a tick
group.RunTimed(0.016f); // ~60 FPS
```

---

## Internal Types (not part of public API)

These types are implementation details and may change without notice:

| Type | Purpose |
|---|---|
| `SparseSet<T>` | Dense/sparse array pair for component storage. |
| `ComponentStore` | Registry of `SparseSet<T>` instances by type. |
| `ChangeBuffer` | Accumulates `EntityChange` during system run, posts to R3 subjects. |
| `ChangeSet` | Deduplicated change accumulator within a single batch. |
| `RelationshipIndex` | Reverse lookup from target entity to source entities. |
| `EntityAllocator` | Free-list + bump allocator for entity IDs. |
| `WorldQueryExtensions` | Extension methods with ref struct iterators and entity lookup. |
| `ComponentRegistry` | Source-generated registry of all component types for serialization. |
| `ComponentDescriptor` | Source-generated per-type descriptor with serialize/deserialize callbacks. |