# Architecture

## The shape

```
				 ┌──────────────────────────────────────────┐
				 │  src/sim/  -- plain RefCounted objects    │
   authoritative │  SimWorld, SimPlayer, SimEnemy, SimBoss,  │ replica
   (server)      │  BulletPool, emitters                     │ (client)
				 │  no nodes · no physics · no rendering     │
				 └───────────────┬──────────────────────────┘
								 │
		┌────────────────────────┼────────────────────────┐
		│                        │                        │
┌───────▼────────┐      ┌────────▼────────┐      ┌────────▼────────┐
│ src/instances/ │      │    src/net/     │      │  src/view/ ui/  │
│ lobby, dungeon │      │ codec, server & │      │ read-only draw  │
│ progression    │      │ client runtimes │      │                 │
└────────────────┘      └────────┬────────┘      └─────────────────┘
								 │
						┌────────▼────────┐
						│ src/autoload/   │
						│ net.gd (RPC)    │
						└─────────────────┘
```

Dependencies point inwards only. `src/sim/` imports nothing from `net`, `view`,
`ui` or `instances`, and never touches the `Net` autoload. That is what makes
the simulation runnable in a unit test, in a headless server and inside a client
without a single conditional.

## Why the simulation is not made of nodes

Godot's instinct is `CharacterBody2D` + `Area2D` + `MultiplayerSynchronizer`.
This project deliberately does not, for four reasons:

1. **Bullet count.** Hundreds of live bullets. A node and an `Area2D` each would
   cost more than the entire simulation does. `BulletPool` is parallel packed
   arrays; a tick over ~350 bullets costs 0.24 ms including AI and hit checks.
2. **Headless cost.** The dedicated server allocates no nodes and touches no
   physics server.
3. **Testability.** A `SimWorld` is `SimWorld.new()`. The suite drives thousands
   of ticks in ~2 s with no SceneTree, no awaits and no frame timing.
4. **Replication control.** `MultiplayerSynchronizer` is excellent for
   prototyping and wrong for this shape: it would replicate per-bullet state and
   saturate the link. See [NETCODE.md](NETCODE.md).

The trade is that there is no physics engine, so collision is hand-written.
Since Stage 1 that means a tile grid ([MapGrid](../src/sim/map_grid.gd)):
circle-vs-tile for actors, a point test for bullets, and Bresenham line of sight
shared by fog and aggro. Actor-vs-actor hits stay circle-vs-circle, which is the
genre convention anyway.

That was the right call rather than a compromise: the same grid answers
collision, sight, and per-peer interest management as array lookups. On arbitrary
polygons all three become intersection tests, and the fog query in particular
stops being cheap enough to run every frame.

What it costs: geometry is axis-aligned and 32px-quantised, and bullets must
travel less than one tile per tick or the point test steps over walls (pinned by
`test_bullet_speeds_stay_below_the_tunnelling_threshold`).

## Where the layers sit now

Stage 1 and 2 added two things worth knowing before reading any file:

- **Geometry is per-world.** There is no global arena. `SimWorld.map` is a
  [MapGrid](../src/sim/map_grid.gd); the hub and every dungeon have their own,
  and the client holds a *partial* copy streamed to it in chunks.
- **`src/meta/` is server-only.** Accounts, characters, levels and experience
  live there. The simulation reads a player's level and maximum health; it never
  writes progression. One writer means a level cannot disagree with the
  experience that earned it.

## Tick

Everything runs on Godot's physics tick, pinned to 60 Hz in `src/main.gd`
(asserted in code, because `project.godot` drops settings equal to the engine
default). `SimConfig` holds every constant both sides must agree on; per-machine
options live in `GameOpts` and must never affect the simulation.

## Content pipeline

```
BulletEmitter (Resource, abstract)
├── RingEmitter          count, arc, spin per shot   -> rings and spirals
├── AimedSpreadEmitter   fan at nearest player       -> punishes standing still
├── WallGapEmitter       curtain with a sliding gap  -> forces a committed dodge
└── ArcSweepEmitter      rotating arms               -> forces tracking

EnemyDef  = stats + movement enum + emitters
BossPhase = hp threshold + telegraph + looping timeline of emitters
BossDef   = stats + ordered phases
```

`SimWorld._run_emitters()` is shared by enemies and bosses, so any pattern can
be dropped on either. A boss is four phases layering one idea at a time; a new
boss is a new function in `src/content/content.gd` and zero simulation changes.

## Entry point

One executable, one `src/main.tscn`. `GameOpts.parse()` reads the command line
after `--`:

- `--server` → `ServerRuntime`, no view, `max_fps` pinned to the tick rate.
- `--join` / `--bot` → connect immediately, skipping the menu.
- otherwise → the connect menu, which can also start a listen server.

## File map

| File | Role |
| --- | --- |
| `src/core/sim_config.gd` | Every constant server and client must agree on. |
| `src/core/movement.gd` | Pure movement + overlap helpers. Shared by prediction. |
| `src/core/game_log.gd`, `game_opts.gd` | Static; usable from tools and tests. |
| `src/sim/sim_world.gd` | The simulation. Authority flag decides what runs. |
| `src/sim/bullet_pool.gd` | Struct-of-arrays bullet storage and integration. |
| `src/sim/input_frame.gd` | The only thing a client may assert about itself. |
| `src/net/net_codec.gd` | Snapshot / event / input binary codecs. |
| `src/net/server_runtime.gd` | Instances, ticking, transfers, broadcast. |
| `src/net/client_runtime.gd` | Prediction, reconciliation, interpolation, bot input. |
| `src/instances/instance.gd` | Lobby hub and dungeon progression. |
| `src/autoload/net.gd` | ENet lifecycle, RPCs, local loopback for listen servers. |
| `src/sim/map_grid.gd` | Tile grid: collision, line of sight, chunked streaming. |
| `src/sim/map_gen.gd` | Dungeon generation. `build()` is the only entry point both sides use. |
| `src/content/rooms.gd` | Hand-authored stamps: the hub and each boss arena, as text. |
| `src/content/items.gd` | Every item. `Items.ORDER` doubles as the wire format. |
| `src/sim/sim_loot.gd` | An item on the ground. World-shared, or owned by one peer. |
| `src/meta/progression.gd` | XP curve and what a level is worth. Pure functions. |
| `src/meta/character.gd`, `character_store.gd` | Characters and their JSON persistence. Server-owned. |
| `src/meta/auth_provider.gd` | Identity, shaped like Steamworks so it swaps out. |
| `src/net/net_codec.gd` | Snapshot / event / input / roster / character / map-chunk codecs. |
| `src/net/server_runtime.gd` | Instances, ticking, transfers, interest, progression, map streaming. |
| `src/view/bullet_renderer.gd` | The bullet field: one MultiMesh per bullet kind. |
| `src/view/art.gd` | Every atlas rect and sound path, in one table. |
| `src/view/debug_draw.gd` | F1 overlay: what the simulation collides against. |
| `src/ui/character_select.gd` | Roster screen: pick or create. |