# 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

Four things are 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, experience,
  inventories and upgrades live there. The simulation reads a player's level and
  maximum health and moves items between the ground and a bag; it never writes
  progression or touches the store. It announces what happened and
  `ServerRuntime` banks it. One writer means a level cannot disagree with the
  experience that earned it.
- **Derived, never stored.** Level comes from lifetime experience; combat
  numbers come from the upgrade list ([PlayerStats](../src/sim/player_stats.gd));
  maximum health comes from the level and the upgrades. Nothing that can be
  recomputed is persisted, so nothing saved can disagree with what produced it.
- **`src/core/settings.gd` is the exception to "shared".** Everything else in
  `src/core/` is agreed by both sides; settings are the player's own machine
  and never reach the server.

## 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.
`SimWorld._move_boss()` is the same idea for movement: the mode is a field on
the phase, and adding a boss that walks needs no code.

Emitters are **stateless**. They are shared resources — two bosses of the same
kind would otherwise stomp each other's timers — so anything an emitter needs to
remember between two ticks has to be derived instead. `TelegraphedStrikeEmitter`
computes its strike positions from the volley number for exactly this reason.

## 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

Grouped by layer. The rule the whole thing hangs on: **`src/sim/` imports
nothing from `net`, `view` or `ui`, and never touches `Net`.**

### Shared constants and helpers

| 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/core/settings.gd` | Client-local preferences. Never reaches the server. |
| `src/core/credits.gd` | Third-party assets and their licences. Two are CC BY, so this is a legal requirement. |

### The simulation

| File | Role |
| --- | --- |
| `src/sim/sim_world.gd` | The simulation. The 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/sim/player_stats.gd` | Combat numbers derived from a character's upgrades. |
| `src/sim/poison_track.gd` | Damage over time. O(1) per actor per tick however many doses. |
| `src/sim/sim_loot.gd` | An item on the ground: world-shared, or owned by one peer. |
| `src/sim/sim_portal.gd` | A dungeon entrance, and which dungeon it opens. |
| `src/sim/map_grid.gd` | Tile grid: collision, line of sight, chunked streaming. |
| `src/sim/map_gen.gd` | Dungeon generation. **Server-side only** — the client is never given the seed. |
| `src/sim/patterns/` | Bullet emitters. Stateless; the authoring surface for every fight. |

### Content (data, in code)

| File | Role |
| --- | --- |
| `src/content/content.gd` | Every enemy and boss. `ALL_ENEMIES` / `ALL_BOSSES` are what the tools and tests iterate. |
| `src/content/items.gd` | Every item. `Items.ORDER` doubles as the wire format. |
| `src/content/upgrades.gd` | The seven upgrades and their draw weights. |
| `src/content/dungeons.gd` | The kinds of run. `ORDER` is a wire format *and* the hub's portal order. |
| `src/content/rooms.gd` | Hand-authored stamps: the hub and each boss arena, as text. |
| `src/actors/` | The `Resource` definitions those tables build: enemy, boss, item, loot, upgrade, dungeon. |

### Server-owned state

| File | Role |
| --- | --- |
| `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. |
| `src/meta/auth_provider.gd` | Identity, shaped like Steamworks so it swaps out. |
| `src/instances/instance.gd` | The hub and one dungeon run: a world plus a peer list. |

### Network

| File | Role |
| --- | --- |
| `src/net/net_codec.gd` | Every binary codec: snapshot, events, input, roster, characters, map chunks, portals, upgrade state. |
| `src/net/server_runtime.gd` | Instances, ticking, transfers, interest, progression, persistence, map streaming. |
| `src/net/client_runtime.gd` | Prediction, reconciliation, interpolation, bot input. |
| `src/net/protocol.gd` | Wire version and constants. Bump `VERSION` whenever a layout changes. |
| `src/autoload/net.gd` | ENet lifecycle, RPCs, local loopback for listen servers. The only autoload. |

### View and UI (read-only; decides nothing)

| File | Role |
| --- | --- |
| `src/view/world_view.gd` | Everything that is not a bullet. Fog, actors, loot, telegraphs. |
| `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/ui_theme.gd` | The control theme, built in code from the UI pack. |
| `src/view/debug_draw.gd` | F1 overlay: what the simulation collides against. |
| `src/view/game_scene.gd`, `sfx.gd` | Wiring the view to whatever client `Net` currently has. |
| `src/ui/hud.gd` | Bars, inventory, prompts. Drawn, not built from controls. |
| `src/ui/main_menu.gd`, `game_menu.gd` | Connect screen and the in-game menu. |
| `src/ui/character_select.gd` | Roster screen: pick or create. |
| `src/ui/upgrade_screen.gd` | The quartermaster's three choices, and what you hold. |
| `src/ui/settings_screen.gd`, `credits_screen.gd` | Controls, volumes, attribution. |