# ecs-observable

Entity-Component-System with an observable-style API for TypeScript. Built for games and simulations.

## Table of Contents

- [Installation](#installation)
- [Concepts](#concepts)
- [Quick Start](#quick-start)
- [API](#api)
  - [World](#world)
  - [Components](#components)
  - [Singleton Components](#singleton-components)
  - [Queries](#queries)
  - [Observable Queries](#observable-queries)
  - [Change Tracking](#change-tracking)
  - [Relationships](#relationships)
  - [Events](#events)
  - [Serialization](#serialization)
  - [Commands](#commands)
  - [Behaviour Trees](#behaviour-trees)
- [TypeScript Inference](#typescript-inference)

## Installation

```bash
npm install ecs-observable
```

## Concepts

| Concept      | Purpose |
|-------------|---------|
| **Entity**   | Opaque `number` handle. Has no data of its own. |
| **Component** | Plain object attached to an entity. Defined via `defineComponent()`. |
| **Query**     | Filter that finds entities matching a component signature. |
| **Relationship** | Directed edge between two entities (e.g. `ChildOf`, `Targeting`). |
| **World**     | Central container that stores entities, components, and relationships. |

## Quick Start

```ts
import { World, defineComponent, query } from "ecs-observable";

// 1. Define components
const Position = defineComponent("position", { x: 0, y: 0 });
const Velocity = defineComponent("velocity", { vx: 0, vy: 0 });

// 2. Create world
const world = new World();

// 3. Spawn entities and add components
const player = world.spawn();
world.add(player, Position, { x: 100, y: 200 });
world.add(player, Velocity, { vx: 2, vy: 0 });

const npc = world.spawn();
world.add(npc, Position, { x: 300, y: 150 });

// 4. Iterate with queries
for (const e of world.query(query(Position, Velocity))) {
  const pos = world.get(e, Position);
  const vel = world.get(e, Velocity);
  pos.x += vel.vx;
  pos.y += vel.vy;
}
```

---

## API

### World

```ts
const world = new World();
```

#### Entity Lifecycle

```ts
// Create
const e = world.spawn();

// Check
world.isAlive(e); // true

// Destroy (removes all components and relationships)
world.destroy(e);
world.isAlive(e); // false

// Count
world.entityCount; // number of live entities
```

Destroyed slots are recycled (with a generation bump) so stale entity handles do not match new ones.

---

### Components

Define a component with a name and defaults. The defaults provide the TypeScript shape and initial values.

```ts
const Health = defineComponent("health", { current: 100, max: 100 });
type Health = typeof Health.type; // → { current: number; max: number }
```

#### CRUD

```ts
const e = world.spawn();

// Add — returns a mutable reference initialized from defaults
const h = world.add(e, Health, { current: 50 }); // override defaults
h.current; // 50

// Get — returns the same mutable reference (throws if missing)
world.get(e, Health).current = 75;

// Try-get — safe access
const val = world.tryGet(e, Health); // Health | undefined

// Has — check presence
world.has(e, Health); // true

// Set — bulk replace (marks dirty automatically)
world.set(e, Health, { current: 90, max: 90 });

// Remove
world.remove(e, Health);
```

Get returns a **live mutable reference** — no defensive copies. Mutations are visible immediately.

---

### Singleton Components

For global state (score, board, config) that doesn't need per-entity tracking. A single internal entity is created lazily and reused for all singleton components.

```ts
const Score = defineComponent("score", { points: 0, level: 1 });

// Add (auto-creates the backing entity on first call)
world.addSingleton(Score);

// Get / set / check — no entity argument needed
world.getSingleton(Score).points += 100;
world.hasSingleton(Score);           // true
world.setSingleton(Score, { points: 0, level: 2 });

// Try-get
const s = world.tryGetSingleton(Score); // Score | undefined

// Mark dirty for change tracking
world.markDirtySingleton(Score);

// Remove (destroys the backing entity if it becomes bare)
world.removeSingleton(Score);
```

---

### Queries

Create filters with `query()`. Chain `.without()` to exclude components.

```ts
const Position = defineComponent("position", { x: 0, y: 0 });
const Velocity = defineComponent("velocity", { vx: 0, vy: 0 });
const Dead    = defineComponent("dead",    { timestamp: 0 });

// Entities with Position AND Velocity
query(Position, Velocity)

// Entities with Position but NOT Dead
query(Position).without(Dead)
```

#### Synchronous iteration

```ts
for (const e of world.query(query(Position, Velocity))) {
  const pos = world.get(e, Position);
  const vel = world.get(e, Velocity);
  // update pos from vel...
}
```

Efficient: iterates the smallest component store and cross-checks the others.

---

### Observable Queries

Subscribe to get live diffs when the result set changes.

```ts
world.observe(query(Position, Velocity)).subscribe(update => {
  // update.added    — Entity[]
  // update.removed  — Entity[]
  // update.changed  — Entity[]  (only after flush, see below)

  for (const e of update.added)   { /* e now matches */ }
  for (const e of update.removed) { /* e no longer matches */ }
});
```

Common pattern — maintain a rendering list:

```ts
const movers = new Set<number>();

world.observe(query(Position, Velocity)).subscribe(update => {
  for (const e of update.added)   movers.add(e);
  for (const e of update.removed) movers.delete(e);
});
```

Subscriptions are **seeded** on creation: existing matches are tracked without emitting spurious added events.

---

### Change Tracking

For observable queries to emit `changed`, you must explicitly mark dirty and flush.

```ts
const log: QueryUpdate[] = [];
world.observe(query(Position)).subscribe(u => log.push(u));

const e = world.spawn();
world.add(e, Position);

// Mutate and flush
world.get(e, Position).x = 99;
world.markDirty(e, Position);
world.flush();
// → log contains { added: [], removed: [], changed: [e] }
```

**`world.set()` marks dirty for you**, so `markDirty` is only needed after direct mutation via `world.get()`.

Flush is batched — call it once per frame after all systems have run.

---

### Relationships

Directed edges between entities. A source can target at most one entity per relationship type.

```ts
const ChildOf = defineRelationship("childOf");
const Parent  = defineRelationship("parent");
```

```ts
const parent = world.spawn();
const child  = world.spawn();

// Create
world.relate(child, ChildOf, parent);

// Read
world.getRelated(child, ChildOf);      // → parent
world.getRelatedTo(parent, ChildOf);   // → [child]

// Replace
world.relate(child, ChildOf, otherParent); // old edge removed automatically

// Remove
world.unrelate(child, ChildOf);
```

Destroying an entity cleans up all its edges bidirectionally.

#### Observable relationships

```ts
world.observeRelated(ChildOf).subscribe(update => {
  // update.added   — { source: Entity; target: Entity }[]
  // update.removed — { source: Entity; target: Entity }[]
  for (const { source, target } of update.added) {
    console.log("new child relationship");
  }
});
```

Like query observers, these are seeded with current edges on subscription.

---

### Events

The global event stream gives full visibility into everything happening in the world.

```ts
world.events$.subscribe(ev => {
  switch (ev.type) {
    case "spawned":
    case "destroyed":
      // ev.entity
      break;
    case "componentAdded":
    case "componentRemoved":
    case "componentChanged":
      // ev.entity, ev.component
      break;
    case "relationshipAdded":
    case "relationshipRemoved":
      // ev.source, ev.target, ev.relationship
      break;
  }
});
```

Events fire **immediately** on mutation (synchronous), before `flush()`.

---

### Serialization

```ts
// Export
const snapshot = world.toJSON();
const json     = JSON.stringify(snapshot);

// Import — must supply the known component/relationship definitions
const components: ComponentDef<any>[] = [Position, Velocity, Health];
const relationships: RelationshipDef[] = [ChildOf];

const loaded = World.fromJSON(JSON.parse(json), components, relationships);
```

Snapshots use stable string IDs (`"e0"`, `"e1"`, …). Bare entities (no components) are preserved. Holes from destroyed entities are collapsed.

**Round-trip example:**

```ts
const world = new World();
const e = world.spawn();
world.add(e, Position, { x: 42, y: 99 });
world.add(e, Health,    { current: 75, max: 100 });

const snap  = world.toJSON();
const world2 = World.fromJSON(snap, [Position, Health]);

world2.entityCount;                          // 1
const e2 = [...world2.query(query(Position))][0];
world2.get(e2, Position);                    // { x: 42, y: 99 }
world2.get(e2, Health);                      // { current: 75, max: 100 }
```

---

### Commands

Decouple input from game logic. Define command components, register handlers, spawn command entities from input — the `CommandQueue` drains and dispatches them each frame.

```ts
import { CommandQueue } from "ecs-observable/commands";

const MoveLeft  = defineComponent("moveLeft", {});
const MoveRight = defineComponent("moveRight", {});

const queue = new CommandQueue(world);

// Register handlers
queue.handle(MoveLeft, () => {
  player.x -= 1;
});

queue.handle(MoveRight, () => {
  player.x += 1;
});

// Input → spawn command entities
onKey("ArrowLeft", () => {
  const cmd = world.spawn();
  world.add(cmd, MoveLeft);
});

// Each frame — drain and dispatch
queue.execute();
```

Command entities are automatically destroyed after processing if they become bare.

---

### Behaviour Trees

Behaviour trees control game flow by composing tasks into a tree. Each node in the tree is an ECS entity with a `Task` component. Parent-child relationships are `ChildOf` edges. This means you can query, observe, and serialize the tree just like any other ECS data.

`buildTree()` takes a declarative definition and materializes it into entities, returning a fully-wired `TaskRunner`.

```ts
import { buildTree, Cancel } from "ecs-observable/bt";
```

#### Leaf patterns

**One-shot** — just return. Implicit success.
```ts
{ kind: "leaf", run: () => { doWork(); } }
```

**Fail** — throw any error.
```ts
{ kind: "leaf", run: () => { throw new Error("bad"); } }
```

**Cancel** — throw the `Cancel` symbol.
```ts
{ kind: "leaf", run: () => { throw Cancel; } }
```

**Ongoing** — generator function. Each `yield` suspends until next tick. The yielded value is the delay in ms (or nothing for next frame). Completion = success.
```ts
{ kind: "leaf", *run() {
    while (true) {
      const dt: number = yield;  // delta time from runner.tick(dt)
      timer.accumulator += dt;
      if (timer.accumulator >= timer.interval) {
        // ... act ...
      }
    }
} }
```

#### Composite nodes

```ts
{ kind: "sequential", children: [a, b, c] }  // left-to-right, all must succeed
{ kind: "selector",   children: [a, b, c] }  // left-to-right, first success wins
{ kind: "parallel",   children: [a, b, c] }  // all at once, all must succeed
{ kind: "random",     children: [a, b, c] }  // pick one child each activation
{ kind: "repeat",     child: a }             // decorator — re-run child forever
```

#### Full example

```ts
const runner = buildTree(world, {
  kind: "parallel",
  children: [
    {
      kind: "leaf",
      *run() {
        while (true) {
          const dt: number = yield;
          updatePhysics(dt);
        }
      },
    },
    {
      kind: "repeat",
      child: {
        kind: "sequential",
        children: [
          { kind: "leaf", run: () => { handleInput(); } },
          { kind: "leaf", run: () => { render(); } },
        ],
      },
    },
  ],
});

// Kick off
runner.schedule((runner as any).root);

// Game loop
setInterval(() => runner.tick(16), 16);
```

---

## TypeScript Inference

Components infer their type from the defaults object — no separate type declaration needed.

```ts
const Inventory = defineComponent("inventory", {
  items: [] as string[],
  gold: 0,
});

// world.add returns the typed component
const inv = world.add(e, Inventory, { gold: 42 });
inv.items.push("sword");        // ✅ string[]
inv.gold = 100;                 // ✅ number

// world.get also returns typed
world.get(e, Inventory).items;  // string[]
```