rutster/LEARNING.md

# LEARNING.md — to learn concept X, read file Y

This index maps a Rust concept you might be learning to the file where
slice 1 makes the concept concrete. Each entry is a worked example you
can read in `cargo doc --open` plus the source file itself.

## Concepts + pointers

- **Newtype pattern (type-safety via single-field wrappers)** →
  `crates/rutster-call-model/src/lib.rs` — `ChannelId(Uuid)`. The newtype
  stops us from mixing up a `ChannelId` with some future `SessionId` at
  the type-system level. Compile-enforced where a comment could only ask.

- **`enum` for closed state sets + exhaustive `match`** →
  `crates/rutster-call-model/src/lib.rs` — `ChannelState` (New →
  Connecting → Connected → Closing → Closed). Exhaustiveness checking
  forces every `match` to consider each state; adding a state later
  surfaces every site that needs handling.

- **Sans-IO pattern (no I/O inside the library; input via method calls,
  output via return values)** → `crates/rutster-media/src/loop_driver.rs`
  — the str0m poll loop. `Rtc::handle_input` takes a network packet as a
  struct argument, not from a socket the library owns; `poll_output`
  returns `Transmit` packets the caller sends. Fully testable without a
  network — str0m integration tests use this property to drive faster
  than realtime.

- **Trait design for extension points (a futures-compatible seam)** →
  `crates/rutster-media/src/pcm.rs` — the `AudioSource` / `AudioSink`
  traits. Slice 1 wires an `EchoAudioPipe` between them; step 2 swaps
  that for a real WSS tap client without touching `RtcSession`. The
  traits describe *what* the splice point does, not *how* it's filled.

- **Error enums with `thiserror` + hot-path match-and-continue** →
  `crates/rutster-media/src/lib.rs` (`MediaError`) and
  `crates/rutster-media/src/opus_codec.rs` (`OpusDecoder::decode` returns
  `Option<PcmFrame>`). Cold path: `thiserror`-derived enum + `?`. Hot
  path: match-and-continue, never `?`, never panic — "drop + observe,
  don't crash" (spec §3.8).

- **`Arc<Mutex<T>>` vs `Arc<RwLock<T>>` — when each is right** →
  `crates/rutster/src/session_map.rs`. The `RtcSession` lives behind
  `Arc<Mutex<...>>` because every access mutates it (str0m's `&mut self`
  contract) — `RwLock`'s read-mode would be useless. Comment on the
  struct explains the trade-off.

- **`DashMap` for sharded concurrent maps** →
  `crates/rutster/src/session_map.rs`. `DashMap` shards its inner map so
  two handlers operating on different `ChannelId`s don't contend;
  `HashMap` wrapped in a single `Mutex` would serialize every access.

- **str0m 0.21's single-mutation invariant** →
  `crates/rutster-media/src/loop_driver.rs`. Mutate (handle_input /
  Writer::write) → drain `poll_output` to `Output::Timeout` → next
  mutate. Violating this leaves str0m in an inconsistent state.

- **tokio graceful shutdown via signal handlers** →
  `crates/rutster/src/main.rs` (`shutdown_signal`). Ctrl-C / SIGTERM
  drops the AppState; the AppState drops the DashMap; the DashMap drops
  every RtcSession. No in-flight call preservation in slice 1.

- **`include_str!` for embedding static assets** →
  `crates/rutster/src/routes.rs` (`include_str!("../static/index.html")`).
  The HTML test client is compiled into the binary at build time — no
  separate file to ship, no disk IO to serve it.

- **`mpsc` + `oneshot` for cold-path task supervision** → `crates/rutster/src/tap_engine.rs`
  — how a spawned tokio task is supervised + cancelled via `oneshot::Receiver`.
- **`VecDeque` as a bounded playout ring with drop-oldest policy** → `crates/rutster-tap/src/tap_audio_pipe.rs`
  — why a manual ring (not `mpsc`) when the overflow policy is drop-oldest, not drop-newest.
- **Async WS connect + `Sink`/`Stream` traits** → `crates/rutster-tap/src/tap_client.rs`
  — `tokio_tungstenite::connect_async`, `WebSocketStream`, the `SinkExt`/`StreamExt`
  extension traits, `tokio::select!` over inbound + outbound + close.
- **`Box<dyn Trait + Send>` field widening (the seam test)** → `crates/rutster-media/src/rtc_session.rs`
  — why the `pipe` field type changed from `EchoAudioPipe` to
  `Box<dyn AudioSource + AudioSink + Send>` so `loop_driver`'s call sites
  are byte-identical (slice-2 §8.5 #6).
- **Zero-sized marker newtype for state flags** → `crates/rutster-call-model/src/lib.rs`
  — `TapHandle(())` compiles `Option<TapHandle>` to a single `bool`; no
  runtime cost for the type-system marker.

### Slice 3 — OpenAI Realtime brain

- **`async-trait` patterns / async fns in trait objects** →
  `crates/rutster/src/tool_registry.rs` — the `Tool` trait's `async fn call`
  and how `async-trait` lowers it to `Pin<Box<dyn Future + Send>>` (the same
 boxing the slice-1 `pipe` widening used, now applied to trait methods).
- **OpenAI Realtime adapter + event translation (pure-function layer)** →
  `crates/rutster-brain-realtime/src/translator.rs` — the §4.2 event mapping
  table as pure `fn`s: tap `audio_in` ⇄ OpenAI `input_audio_buffer.append`,
  `response.audio.delta` ⇄ `audio_out`, `function_call_arguments.done` ⇄
  `function_call`. Testable without a network (the pump is in a separate file).
- **Tap protocol additive extension + forward-compat via `#[serde(other)]`** →
  `crates/rutster-tap/src/protocol.rs` — how slice-3 adds `speech_started`,
  `speech_stopped`, `function_call`, `function_call_output`, `tools.update`
  without breaking slice-2 brains: unknown variants deserialize to a catch-all
  arm instead of erroring (forward-compat by construction).
- **Side-channel mpsc for FOB-boundary dispatch** →
  `crates/rutster/src/session_map.rs` — how `drive_all_sessions` drains a
  function-call mpsc side channel into the tap WS writer without blocking the
  20 ms media loop. The pattern: hot loop polls `try_recv`; cold path spawns.
- **HTTP request builder for WS subprotocol handshake** →
  `crates/rutster-brain-realtime/src/openai_client.rs` — why we hand-roll the
  WS upgrade `Request` instead of letting `connect_async` build it: to set
  `Authorization` + `OpenAI-Beta` headers on the WS handshake (tungstenite
  doesn't expose subprotocol/auth headers via the simple `connect_async(url)`
  entry point — see `openai_headers` / `openai_realtime_url`).

## How to read

1. `cargo doc --open` — every module has a `//!` doc comment; the doc
   tree is the high-level map.
2. Pick a concept above; open the named file. The first occurrence of
   each non-obvious pattern has a `//` comment explaining *why*.
3. Cross-ref back to the spec sections cited inline (`spec §3.8`,
   `ADR-0002`, etc.) for the architecture-level rationale.