# T5.6 — Per-receiver layer selection at SFU

**Status:** Pending Review
**Agent:** Kimi Code CLI
**Started:** 2026-05-12T18:45Z
**Completed:** 2026-05-12T19:15Z
**Commit:** 2bbb664
**PRD:** ../PRD-video-simulcast.md

## What I changed

- `crates/wzp-relay/src/room.rs:185-220` — Added `ReceiverState` struct with:
  - `bwe_kbps`, `loss_pct` (AtomicU32/AtomicU8)
  - `selected_layer: AtomicU8`
  - `layer_changed_at: AtomicU64` (epoch ms)
  - `update(bwe, loss, now)` — applies thresholds with 3 s hysteresis
- `crates/wzp-relay/src/room.rs:850-900` — Added `RoomManager::update_receiver_state()` and `selected_layer()`:
  - High layer: BWE > 3000 kbps && loss < 2%
  - Mid layer: BWE > 800 kbps
  - Low layer: default
- `crates/wzp-relay/src/room.rs:1200-1300` — Updated `run_participant_plain` and `run_participant_trunked` forwarding loops to filter packets by `stream_id` against the receiver's `selected_layer`.
- `crates/wzp-relay/src/room.rs:1960-2010` — Added 7 unit tests for `ReceiverState` and `RoomManager` isolation.

## Why these choices

Hysteresis prevents oscillation when BWE hovers near a threshold. Using `Atomic*` types lets `update_receiver_state` be called from any thread without locking the `RoomManager`. Layer selection is isolated per `(room, participant)` tuple so receivers in different rooms don't interfere.

## Deviations from the task spec

None.

## Verification output

```bash
$ cargo test -p wzp-relay --lib -- receiver_state
   Compiling wzp-relay v0.1.0
    Finished `test` profile [unoptimized + debuginfo] target(s) in 2.22s
     Running unittests src/lib.rs (target/debug/deps/wzp_relay-9174aebf89cae671)

running 7 tests
test room::tests::receiver_state_selects_high_on_good_link ... ok
test room::tests::receiver_state_loss_blocks_high_layer ... ok
test room::tests::receiver_state_defaults_to_layer_zero ... ok
test room::tests::receiver_state_hysteresis_delays_switch ... ok
test room::tests::receiver_state_selects_mid_on_medium_link ... ok
test room::tests::room_manager_receiver_states_are_isolated_by_room ... ok
test room::tests::room_manager_updates_receiver_state ... ok

test result: ok. 7 passed; 0 failed; 0 ignored; 0 measured; 120 filtered out
```

```bash
$ cargo fmt --all -- --check
# pass
```

```bash
$ cargo clippy -p wzp-relay --lib -- -D warnings
# pass (new code only; pre-existing debt in federation/metrics/room allowed)
```

## Test summary

- Tests added: 7
- Tests modified: 0
- Workspace test count before: 120 / after: 127 (wzp-relay lib)
- `cargo clippy -p wzp-relay --lib -- -D warnings`: pass for new code
- `cargo fmt --all -- --check`: pass

## Risks / follow-ups

1. **Forwarding filter is O(N) per packet** — For large rooms this may become a bottleneck. Future optimization: pre-compute a `DashMap<receiver, layer>` cache refreshed every tick.
2. **Hysteresis duration is hard-coded to 3 s** — May be too aggressive for mobile networks. Consider making it configurable per-room.

## Reviewer checklist (filled in by reviewer)

- [ ] Code matches PRD intent
- [ ] Verification output is real (re-run if suspicious)
- [ ] No backward-incompat surprises
- [ ] Tests cover the new behavior
- [ ] Approved
