# Working agreements

Read this before changing anything. It is written for coding agents, and it is just as valid for
humans. [`docs/architecture.md`](docs/architecture.md) explains *why* the pieces are shaped this
way; this file is *how to work in them*.

## Orientation

| I want to change… | Go to |
| --- | --- |
| game rules, movement, entities | `src/HSchool.Simulation` |
| what the client receives | `src/HSchool.Protocol` **and** `src/HSchool.Client/src/net/protocol.ts` **and** `docs/protocol.md` |
| connection handling, endpoints | `src/HSchool.Server` |
| what runs locally | `src/HSchool.AppHost/AppHost.cs` |
| rendering, input, HUD | `src/HSchool.Client/src` |

## Commands

```bash
dotnet build HSchool.slnx
```

```bash
dotnet test
```

```bash
npm --prefix src/HSchool.Client test
```

```bash
npm --prefix src/HSchool.Client run build
```

```bash
dotnet run --project src/HSchool.AppHost
```

`run-aspire.cmd` is the same command for Windows users who want a double-clickable entry point —
keep the two in sync if the AppHost path ever moves.

`dotnet run --project src/HSchool.AppHost` starts the server *and* the Vite dev server and opens
the Aspire dashboard. The Vite port is assigned per run (`npm run dev -- --port <random>`), so read
the client URL off the dashboard instead of assuming 5173.

Do not start a dev server with a bare `npm run dev` when you meant to run the whole app — the
client only finds the backend through the Aspire-injected `SERVER_HTTP` environment variable, or
the `localhost:5180` fallback that matches the server's own launch profile.

## Invariants

These are the rules that keep the base coherent. Breaking one is a design decision, not a detail —
say so explicitly in the change description.

1. **The server is authoritative.** The client sends intents and draws what it is told. No game
   logic in `src/HSchool.Client`.
2. **The protocol lives in three places at once.** `ProtocolCodec.cs`, `protocol.ts` and
   `docs/protocol.md` change in the same commit. A layout change bumps
   `ProtocolConstants.Version` / `PROTOCOL_VERSION`. Tests on both sides assert byte offsets —
   if one of them has to change, so do the other two.
3. **Only the loop thread touches `GameWorld`.** Everything inbound goes through
   `GameCommandQueue`; everything outbound goes through the per-client outbox. No locks around the
   ECS world, no `Task.Run` into it.
4. **The simulation knows nothing about the network.** `HSchool.Simulation` must not reference
   ASP.NET Core, sockets or logging infrastructure. It stays testable without a host.
5. **Fixed timestep.** Systems get `SimulationContext.DeltaTime`, never wall-clock time and never
   `DateTime.Now`. Same inputs, same results — `Simulation_IsDeterministicForTheSameInputs` guards it.
6. **Everything from the wire is untrusted.** Validate lengths and ranges in the handler before
   anything reaches the simulation.

## Conventions

**C#**

- File-scoped namespaces, `var` where the type is obvious, primary constructors for services.
- Private fields are `_camelCase`; `.editorconfig` enforces it.
- Nullable is on everywhere. Don't add `!` to silence it; fix the flow.
- Internal by default in `HSchool.Server`; public only where another project consumes it.
- New tunables go on `SimulationOptions` with a default, not as a constant buried in a system.

**TypeScript**

- `strict` is on, no `any`, no non-null `!` assertions.
- Relative imports carry the `.ts` extension (bundler resolution is configured for it).
- Modules stay thin: `net/` speaks protocol, `game/` renders, `main.ts` wires them together.
- No framework. If a UI need appears, plain DOM first.

**Both**

- Comments explain *why*, not *what*. Assume the reader can read code.
- Match the surrounding style rather than introducing a new one.

## Testing policy

- Simulation changes need a `GameWorld` test. They are fast, hermetic and do not need a host.
- Protocol changes need a round-trip test **and** a byte-layout assertion on both sides.
- Server wiring, endpoints and the WebSocket handshake belong in `tests/HSchool.AppHost.Tests`.
  That suite shares one AppHost across all tests (`AppHostFixture`) — keep it that way, booting
  per test costs about ten seconds each.
- Never assert on the *first* snapshot after a join without checking the entity is in it; the
  frame in flight may predate the spawn. Use `ReceiveSnapshotWithAsync`.

## Dependencies

- NuGet versions are centrally managed in `Directory.Packages.props`. Add the version there and a
  bare `<PackageReference Include="..." />` in the project.
- Transitive pinning is on, so a downgrade warning means you bump the central version rather than
  adding a per-project override.
- Keep the dependency count low. Arch, PixiJS, Aspire and the test runners are the whole budget;
  anything new needs a reason in the change description.

## Things that will bite you

- `PeriodicTimer` does not catch up on its own. The accumulator in `GameLoopService` does, capped
  at 5 steps — do not "simplify" it away.
- Arch recycles entity ids. Replicate `NetworkId`, never `Entity.Id`.
- Snapshots are full-state: an entity missing from a frame is *despawned* by the client. Filtering
  entities out of a snapshot is how you accidentally delete them on screen.
- The client outbox drops the oldest frame under pressure. That is correct for snapshots and wrong
  for anything that must arrive exactly once — such a message would need its own path.
- `erasableSyntaxOnly` is off in `tsconfig.app.json` on purpose: constructor parameter properties
  are used throughout.