BikeControl/REQUIREMENTS.md

# BikeControl — Requirements Specification

**Status:** Draft v0.4
**Date:** 2026-08-05
**Target hardware:** Van Rysel D100 trainer + Zwift Cog + **Zwift Click v2**

> **v0.4 changes — the input decision is reversed.** Further research found that the Click
> v2 "unlock" is **performed by the rider in the free Zwift app**, not by the consuming
> application, and that it then works with *any* third-party app for ~24 h (§2.3). Combined
> with three open-source implementations of the Zwift protocol — one **GPL-3.0**, one
> **MIT** (§3.4–3.6) — **the app talks to the Click v2 directly over BLE.** v0.3's
> OpenBikeControl bridge is dropped: **there is no fallback input path.** New risk: the
> **left pod**, where third-party support is weakest, now carries all gradient and route
> control with nothing behind it (RISK-9).

---

## 1. Overview

BikeControl is a desktop-first (Android-later) application that connects to a Van Rysel
D100 smart trainer and a handlebar controller over Bluetooth Low Energy, and provides:

- **Virtual shifting** on the single-cog drivetrain, via the Click's shift paddles.
- **Handlebar gradient control**, via the Click's D-pad.
- **Gradient-adaptive routes** driven from GPX or hand-authored profiles.
- **Synthetic waveform profiles** — sine, square, ramp — applied to resistance, gradient
  or target power.
- **Ride recording** exported as a FIT file.
- **A GUI** for connection management and live telemetry visualisation.

### 1.1 Goals

| ID | Goal |
|----|------|
| G-1 | Ride a gradient profile end-to-end without touching the computer once started |
| G-2 | Control gearing and gradient entirely from the bars |
| G-3 | Produce a FIT file that imports cleanly into Strava / Garmin Connect |
| G-4 | Run on Linux desktop today; port to Android without rewriting the core |
| G-5 | Fail safe — never leave the trainer at high resistance after a fault |
| G-6 | Prefer documented, openly-licensed protocols; keep any proprietary protocol handling isolated and replaceable |

### 1.2 Non-goals

- Multiplayer, social features, or any network service.
- 3D world rendering or avatars.
- ANT+ support (§10.1).
- **Implementing the Click v2 unlock ourselves.** The rider performs it in the free Zwift
  app (§2.3); we only need to speak the protocol afterwards.
- Acting as a bridge for *other* training apps — that is what the existing BikeControl
  app does, and this project does not duplicate it.
- Any secondary input protocol. The Click v2 client is the only input path.

---

## 2. Hardware & Operating Assumptions

### 2.1 Van Rysel D100 ✅

**FTMS support confirmed** by a working MIT-licensed implementation built specifically for
this trainer (§3.2).

| Property | Value | Confidence |
|----------|-------|------------|
| Bluetooth profile | FTMS (`0x1826`) | **Confirmed** |
| Target power (`0x05`) | Supported; reference clamps 100–600 W | **Confirmed** |
| Target resistance (`0x04`) | Supported, sint16; reference caps at 100 | **Confirmed** |
| Target inclination (`0x03`) | Implemented in the reference as sint16 | **Confirmed** |
| Sim mode (`0x11`) | Not used by the reference | **Unconfirmed — TASK-1** |
| Concurrent BLE hosts | Assume **one** | Assumed |

> **A-1:** The reference drives grade via `SetTargetInclination` (`0x03`), not
> `SetIndoorBikeSimulationParameters` (`0x11`). TASK-1 resolves whether `0x11` works. Low
> impact either way, because the app owns the physics (FR-7.1).

### 2.1.1 Cadence is on the Zwift channel, not FTMS ✅ *(measured 2026-08-05)*

The D100's Fitness Machine Feature bits (`014000000ca00000`) advertise average
speed and power only — **no cadence** — and that is accurate for FTMS. The ten
undeclared trailing bytes in its Indoor Bike Data were investigated and carry no
cadence either: the only varying field is a fixed **73.8x the declared speed**
(wheel RPM restated), constant to 0.5% from 1.7 to 29.7 km/h.

Cadence is nevertheless available, on the **trainer's Zwift service**. Message
type `0x03` at ~1 Hz, five protobuf varints, of which two are independent:

| field | meaning | note |
|-------|---------|------|
| 1 | instantaneous power, W | verified: ~120 W then ~350 W across two effort phases |
| 2 | heart rate | always 0 with no strap paired |
| 3 | **cadence, 0.1 rpm** | verified: 620 -> 62.0 rpm, 1030 -> 103.0 rpm against held cadences |
| 4 | duplicate of field 1 | ratio exactly 1.00 on every frame |
| 5 | speed | a fixed **28.6x field 3** |

Field 5 being a constant multiple of field 3 is the *signature of the Cog*: one
sprocket means cadence and wheel speed are mechanically locked, so the ratio is
the gearing. Observing that constant is what identifies the pair.

Decoded by `bikecontrol_ble::zwift::decode_riding_data`, tested against captured
frames. **This unlocks torque-based virtual gearing** (tau = P/omega), replacing the
provisional gradient-offset table in `crates/core/src/gearing.rs`.

### 2.2 Zwift Cog

A single 14T cog — **no mechanical gears**. This is why virtual shifting (§5.4) is a core
requirement, not a nicety: without it the rider has exactly one gear.

### 2.3 Zwift Click v2 ⚠

The unit is a **v2** (two pods: navigation D-pad on the left, lettered face buttons on the
right, a shift paddle under each). This is the harder variant, and the difference is not
merely one of degree.

**The unlock is performed by the rider, not by the app.** This is the key finding of v0.4
and it changes everything. The Click v2's encryption context times out roughly a minute
after it leaves a Zwift session; refreshing it requires the official Zwift app. But once
refreshed, **the device works with *any* third-party app for ~24 hours**.

The rider's procedure — **free, no paid subscription required**:

1. Open the Zwift app (desktop or mobile) and log in.
2. Go to the device pairing screen; pair the Click.
3. Keep it connected 10–30 seconds while pressing a button.
4. Close Zwift completely.

> **A-2:** BikeControl 6.0.0 (June 2026) added a keep-alive that removes the daily unlock
> entirely — but only for the **right** controller, only for paid Pro users, and the
> implementation lives in a **private submodule**. We therefore assume the manual daily
> unlock. Replicating the keep-alive is explicitly out of scope; see §1.2.

### 2.3.1 Zwift BLE protocol ✅ **TASK-0 answered**

Originally established from three open-source implementations (§3.4–3.6), then **confirmed
against our own pods on 2026-08-05** with `probe zwift`. Where the two disagree, the
hardware wins and the difference is called out below.

| Item | Value | Confidence |
|------|-------|------------|
| Service **on the Click v2** | `0000FC82-0000-1000-8000-00805F9B34FB` | **Confirmed** |
| Service **on the D100 trainer** | `00000001-19CA-4651-86E5-FA29DCDD09D1` | **Confirmed** |
| Async (notify) | `00000002-19CA-…` — device events | **Confirmed** |
| Sync RX (write-without-response) | `00000003-19CA-…` — commands to device | **Confirmed** |
| Sync TX (read/indicate) | `00000004-19CA-…` — responses | **Confirmed** |
| `00000100-…`, `00000101-…` (write/notify), `00000102-…` (write-without-response/notify) | **TI OAD firmware update — do not write.** Their `0x2901` descriptors name them: `Img Identify`, `Img Block`, `OAD Extended Control` | **Confirmed, read 2026-08-20** |
| `00000006-…` | **absent** on our v2 | — |
| Manufacturer ID | 2378 (`0x094A`) | **Confirmed** |
| Device type byte | `0x0A` and `0x0B` — **one per pod, not a version marker** | **Confirmed** |
| Handshake | Write `52 69 64 65 4F 6E 00 09` to Sync RX | **Confirmed** |
| Handshake reply | `52 69 64 65 4F 6E 02 03` on Sync TX — `02 03`, **not** the documented `01 03` | **Confirmed** |
| Button state | `0x23` + one protobuf varint: a **32-bit active-low bitmask** | **Confirmed** |
| Battery | `0x19`; `19 10 64` = 100% | **Confirmed** |
| `0x37` two-varint button message | **never seen** on our v2 — presumably v1 | Superseded |

> **The two services are not interchangeable.** The Click v2 wraps the *same* `…-19CA-…`
> characteristics in the SIG-assigned service `0xFC82`; the trainer uses the original custom
> service UUID. Discovery must accept either.

**Button bitmask.** Idle is `0xFFFFFFFF`; a **clear** bit means pressed, and the pod streams
at roughly 10 Hz while anything is held. Two bits can be clear at once, so it is live state,
not an event code.

**Bit map — confirmed 2026-08-05.** Established by pressing each button in a known order,
reproduced across two full runs plus targeted spot checks:

| Bit | Button | Mask when held | | Bit | Button | Mask when held |
|-----|--------|----------------|---|-----|--------|----------------|
| 0 | `left` | `0xFFFFFFFE` | | 5 | `B` | `0xFFFFFFDF` |
| 1 | `up` | `0xFFFFFFFD` | | 6 | `Y` | `0xFFFFFFBF` |
| 2 | `right` | `0xFFFFFFFB` | | 7 | `Z` | `0xFFFFFF7F` |
| 3 | `down` | `0xFFFFFFF7` | | 8 | `−` paddle | `0xFFFFFEFF` |
| 4 | `A` | `0xFFFFFFEF` | | 12 | `+` paddle | `0xFFFFEFFF` |

D-pad on 0–3, face buttons on 4–7, paddles at 8 and 12. **Bits 9–11 are unclaimed** — no
button we have found drives them.

> `A` and `Z` disagreed between the first two captures and were settled by an alternating
> two-button test returning 4, 7, 4, 7, 4, 7. The first capture had been pressed out of
> order.

> **The pods are not disjoint — answered 2026-08-05.** All ten buttons arrived over the
> **single** pod at `f4:c4:59:03:a1:8e` (type byte `0x0B`), and with both pods connected the
> `+` paddle arrives **twice**: once from the pod it is printed on and once relayed by its
> twin. `−` arrives once. Measured from a ride log rather than a capture — one press of `+`
> moved the gear by two (7 → 9 → 11), one press of `−` by one.
>
> Two consequences, both handled in `controller::Buttons`. The app must **merge the pair into
> one controller**: a button is held when either pod says so, and only that aggregate's edges
> may act, or every `+` press shifts twice. And **a pod sending its twin's paddle is normal**,
> so it is evidence of a wrong-way-round pair only while that pod has never sent its own —
> otherwise the connection screen asks the rider to swap a pair that is filed correctly.

> **One link is the whole controller — confirmed 2026-08-21.** Pairing the `−` pod *alone*
> delivers all ten buttons on this hardware: its own paddle and D-pad, plus the `+` paddle and
> face buttons relayed from its twin. There is nothing a second link adds, and there is one
> thing it takes away — connected as a pair, the `−` pod stops reporting its own paddle.
>
> So the app pairs the `−` pod and stops there (`controller::take_plus_pod`). The `+` pod is a
> **substitute, not a second half**: it is connected on sight only when no `−` pod is known, or
> when a known one has been unreachable for `PLUS_GRACE`, so a flat `−` pod costs the rider a
> D-pad rather than a controller. `Buttons` stays exactly as it is — it is what makes the
> handover between the two configurations invisible.

### 2.3.2 The D100's own Zwift service — telemetry, not shifting

The trainer answers the same handshake (`RideOn 00 09` → `RideOn 02 00`) on the original
`00000001-19CA-…` service, and then streams a `0x03` message at ~1 Hz:

| Field | Pedalling | Coasting | Reading |
|-------|-----------|----------|---------|
| 1 | 177 → 180 → 120 | 6 | **Instantaneous power, watts** |
| 2 | 0 | 0 | always zero |
| 3 | 558 → 601 → 621 | 174 | monotonic counter |
| 4 | 177 → 180 → 120 | 6 | mirrors field 1 exactly |
| 5 | 15968 → 17184 → 17748 | 4976 | monotonic counter, constant 28.6× ratio to field 3 |

Fields 3 and 5 rise monotonically at a fixed ratio, so they are cumulative counters
(distance and revolutions, in unidentified units), not gears.

**Conclusion: this is a read channel.** Nothing here is a virtual-shifting *command*
surface, and no gear field appears. Establishing whether the trainer would *accept* a gear
command means fuzzing unknown writes to a device that controls resistance under a rider —
open-ended and not obviously safe. **Virtual shifting is therefore emulated app-side**
(§5.4), which the physics model was always designed for.

> Corroboration: `qdomyos-zwift` [#3678](https://github.com/cagnulein/qdomyos-zwift/issues/3678)
> reports Click shifting on a D100 dying after 30–60 s with the pod's LED dropping out —
> the §2.3 unlock-timeout signature. Labelled `wontfix`. Our unencrypted sessions ran past
> **three minutes** with button edges still arriving, so this path does not appear to hit
> that timeout — but a long ride is the real test.

**Encryption — not required.** ✅ **A-3 holds for the v2.** The pod completed the handshake
and streamed button and battery events in cleartext, and the app has never sent a key. The
ECDH P-256 → HKDF → AES-256-CCM path documented below is therefore *not* needed, and no
crypto crate has been added.

> **A pod offering a public key does not mean it wants encryption.** Captured on
> 2026-08-20: the `−` pod sent `ff 03 00 0a 21 02 …` — protobuf field 1, 33 bytes, a
> compressed P-256 point — **in the same session as 304 cleartext `0x23` button frames**.
> The key offer and the cleartext stream coexist. This cost an evening: a pod that had gone
> quiet for an unrelated reason was read as "it has switched to the encrypted path", twice,
> and neither reading survived contact with the capture. If a pod goes silent, the key
> frames are not the evidence — see the teardown leak in §7.

| Stage | Mechanism |
|-------|-----------|
| Key agreement | **ECDH on NIST P-256** (`prime256v1`); each side sends a public key |
| Key derivation | **HKDF** → 36 bytes: bytes 0–31 = AES key, bytes 32–35 = IV |
| Cipher | **AES-256-CCM**, 4-byte MAC |
| Framing | 4-byte big-endian counter prepended per message; nonce = IV ‖ counter |

Retained only in case a firmware update closes the unencrypted path. It maps onto pure-Rust
RustCrypto crates (`p256`, `hkdf`, `sha2`, `aes`, `ccm`), so reinstating it needs no C
dependency.

> **Open:** whether the daily Zwift-app unlock (§2.3) is needed at all on this path is
> **untested** — the successful session followed recent Zwift use, so it cannot yet be
> distinguished from an unlock that was simply still valid. Re-test after 24 hours away
> from the Zwift app.

### 2.4 Environmental assumptions

- **A-3:** BLE peripherals accept a single central connection. Only one host may hold the
  trainer, and only one may hold the Click.
- **A-4:** The trainer wakes on pedalling; the Click wakes on button press. Either may be
  absent from a scan until woken — the UI must say so rather than report "not found".

---

## 3. Prior Art, Reuse and the Input Decision

### 3.1 `OpenBikeControl/bikecontrol` — Zwift device reference ⚠

Flutter/Dart, 589★, actively maintained. A *bridge* app that translates controller input
into actions for other training apps. Not used as a component here, but its public source
documents Zwift device discrimination (manufacturer-data type bytes, v1 vs v2 response
codes) and the shape of the v2 unlock.

> **⚠ Licensing.** Current versions are a custom **Non-Commercial** licence (©
> OpenBikeControl UG): personal/educational use only, source-only redistribution under the
> same terms, no commercial use, no marketplace distribution, and the grant is
> **revocable**. Versions before the `gpl3` tag were **GPL-3.0** (copyleft). **No code is
> copied from it** — treat it as a map, and confirm bytes against the device.

### 3.2 `obostjancic/smart-trainer-control` — D100 FTMS reference ✅

React/TypeScript, Web Bluetooth, **MIT**. Literally *"Control a Van Rysel D100 smart
trainer from your browser"*. Working FTMS client, control-point encoders, and FIT and TCX
writers with tests.

**MIT means this logic can be ported freely, with attribution.** It pre-solves most of
TASK-1 and de-risks the FIT writer.

### 3.3 `OpenBikeControl/openbikecontrol-protocol` — considered, not used

An MIT open standard (service `d273f680-…`) for controllers to drive trainer apps, with
Python references for both sides. v0.3 proposed consuming it via a bridge process. **That
approach is dropped** — it required a second app running during every ride. Recorded here
only so the decision is not silently revisited.

### 3.4 `cagnulein/qdomyos-zwift` (QZ) — GPL-3.0 Zwift protocol implementation ⭐

C++/Qt, 826★, **GPL-3.0**. `src/zwift_play/` contains a complete working implementation:
`zapCrypto.h` (ECDH/HKDF/AES-CCM), `localKeyProvider.h` (P-256 keygen), `zapBleUuids.h`,
`zapConstants.h`, `abstractZapDevice.h` (handshake), and `zwiftclickremote.cpp`.

**This is the reference that makes direct Click v2 support tractable.** It is the source of
the crypto details in §2.3.1.

> **Licensing:** GPL-3.0 is copyleft. **Porting this code makes BikeControl GPL-3.0.** The
> underlying algorithm — ECDH P-256 + HKDF + AES-256-CCM — is standard cryptography and not
> itself copyrightable, so a clean implementation from the *documented* protocol (§2.3.1) is
> unencumbered. See OQ-9.

### 3.5 `ajchellew/zwiftplay` — the original reverse-engineering ⚠

Kotlin/Android + a C# Windows console app. The origin of most public knowledge here,
including the packet captures, the `RideOn` handshake, and the fourth (`…0006-…`)
characteristic.

> **⚠ No LICENSE file — all rights reserved.** Excellent *documentation*, but **no code may
> be copied from it.** Its README is the citable protocol description.

### 3.6 `jat255/zwift_click_handling` — MIT, Python, Linux/BlueZ ✅

A small **MIT** script using `bleak` on Linux that connects to a Click and logs button
press/release and battery level. Notably it connects **without encryption** and still
receives events (A-3), and it carries the protocol constants in reusable form.

**MIT and already proven on Linux/BlueZ** — the natural basis for the `probe` tool.

### 3.7 Decision: direct Click v2, no fallback

v0.3 routed all input through a bridge process because the unlock looked insurmountable.
The research above shows it is not: **the rider performs the unlock in the free Zwift app,
and the protocol afterwards is fully documented across three open implementations.**

```
Zwift Click v2 ──BLE, ECDH P-256 + HKDF + AES-256-CCM──► This app
      (rider unlocks daily in the free Zwift app)
```

| | |
|---|---|
| **No second app during rides** | The bridge's real cost was a process running every session. A once-daily 30-second unlock is a far smaller tax. |
| **The protocol is documented** | §2.3.1 is a complete spec, corroborated by three independent implementations. |
| **Standard crypto, pure Rust** | `p256`, `hkdf`, `sha2`, `aes`, `ccm` — no C dependency, no bespoke cryptography. |
| **Lower latency** | No intermediate hop in the NFR-1 budget. |
| **One code path** | No input abstraction to maintain, no divergence between paths. |

**Accepted consequence.** There is no second way in. If a Zwift firmware update breaks the
client, or the left pod proves unreliable (RISK-9), controller input is lost until the
client is fixed — the keyboard and on-screen controls (FR-3.19) are the only stopgap. This
is a deliberate trade of resilience for simplicity.

> **OQ-9 — licensing.** Porting QZ's crypto directly makes this project **GPL-3.0**. Writing
> it from the documented algorithm keeps the licence open. Which do you want? For a personal
> project GPL-3.0 costs nothing.

---

## 4. Technology Stack

**Tauri v2**, Rust core + web frontend. Confirmed; we port rather than import.

| Layer | Choice | Notes |
|-------|--------|-------|
| Shell | Tauri v2 | Desktop + Android from one codebase |
| Core | Rust | BLE, physics, profiles, gearing, FIT |
| BLE | `btleplug` directly | One `platform::Manager` on every target. On Android its "droidplug" backend is half Java: the classes are taken from the crate source at the `Cargo.lock` version (NFR-14), `platform::init` is called from `MainActivity.onCreate`, and the runtime permissions are requested there. `tauri-plugin-blec` was the v0.4 plan and would have supplied that plumbing, but it wraps btleplug in its own device model, which `crates/ble` would then have to be written against twice |
| Crypto | `p256`, `hkdf`, `sha2`, `aes`, `ccm` | Click v2 session (§2.3.1); pure Rust, no OpenSSL |
| Frontend | Web — Svelte recommended | Connection UI, telemetry |
| Charts | `uPlot` | Canvas, built for streaming series |
| FIT | Ported from §3.2 (MIT) | See RISK-3 |

**Architectural constraint.** The control loop lives in **Rust, not JavaScript**.
`tauri-plugin-blec` is designed to expose BLE to the frontend; we use its Rust-side API and
keep all device I/O, physics and state in the core. The frontend renders telemetry pushed
over Tauri events and issues intents as commands. This keeps the safety-critical path (§7)
independent of the webview and the core testable without a UI.

```
bikecontrol/
├── crates/
│   ├── core/        # physics, profiles, gearing, state machine — no BLE, no UI
│   ├── ble/         # FTMS client, Zwift Click v2 client (handshake + crypto)
│   ├── fit/         # FIT encoder (ported from §3.2)
│   └── probe/       # CLI for protocol discovery (Phase 0)
├── src-tauri/       # Tauri shell, commands, event bridge
└── ui/              # web frontend
```

---

## 5. Functional Requirements

### 5.1 Device discovery and connection (FR-1)

| ID | Requirement | Priority |
|----|-------------|----------|
| FR-1.1 | Scan for BLE peripherals; list name, address, RSSI, advertised services | Must |
| FR-1.2 | Identify trainers by FTMS service UUID and Click pods by the Zwift custom service UUID plus manufacturer-data type byte | Must |
| FR-1.3 | Connect to trainer and controller independently; either may connect first | Must |
| FR-1.4 | Track the left and right pods as separate connections, since each is an independent peripheral | Must |
| FR-1.5 | Remember paired devices and auto-connect on launch | Should |
| FR-1.5a | Remembering survives the process: trainer, `−` pod and heart rate monitor are written to `devices.json` in the app data directory when a link actually comes up, and a device the rider forgets is recorded as refused rather than merely dropped | Should |
| FR-1.5b | Auto-connect is driven by the scan, not by startup — the hardware is asleep at launch (A-4), so a remembered device is reconnected the moment it advertises. Bounded by `AUTO_ATTEMPTS`, so "gave up" (FR-1.11) is not contradicted two ticks later, and suspended for any device the rider disconnected by hand this session | Should |
| FR-1.6 | Auto-reconnect on unexpected disconnect, with backoff, without ending the ride | Must |
| FR-1.7 | Surface per-device connection state (scanning / connecting / connected / lost) | Must |
| FR-1.8 | When nothing is found, prompt to wake the device (per A-4) — pedal the trainer, press a Click button | Must |
| FR-1.9 | Allow riding with the trainer alone; on-screen and keyboard controls substitute | Must |
| FR-1.10 | A connect or reconnect attempt in flight is cancellable: disconnect and app exit abandon it rather than queue behind it, and any half-open GATT link is closed on the way out | Must |
| FR-1.11 | Auto-reconnect is bounded, for the controller as well as the trainer. Giving up is announced as a terminal state carrying the reason — never a silent return to `Idle`, and never an actor that stops without saying so | Must |
| FR-1.12 | Scanning resumes on its own whenever no trainer link is held or being attempted, so a disconnect or a failed connect does not leave a frozen device list | Should |

**Cancellation note (FR-1.10):** a connect is a long operation — up to `scan_timeout` before the
peripheral is even found, then connect, service discovery and the control handshake. Waiting for
it to finish before honouring a disconnect or a quit is what makes SAF-2 miss its budget, so the
attempt must be abandoned instead. Abandoning is not free: the D100 accepts one BLE host (A-3),
so an attempt that had already opened a link must close it, or the trainer stays unreachable
until it times out on its own.

### 5.2 Trainer control (FR-2)

| ID | Requirement | Priority |
|----|-------------|----------|
| FR-2.1 | Acquire FTMS control (`0x00`) before issuing commands | Must |
| FR-2.2 | Subscribe to Indoor Bike Data (`0x2AD2`); decode all present fields per the flags bitfield | Must |
| FR-2.3 | Set gradient via `SetTargetInclination` (`0x03`), or `0x11` if TASK-1 confirms support | Must |
| FR-2.4 | Set resistance via `SetTargetResistanceLevel` (`0x04`) | Must |
| FR-2.5 | Set target power via `SetTargetPower` (`0x05`) — needed for ERG waveforms | Must |
| FR-2.6 | Read and respect `Fitness Machine Feature` (`0x2ACC`) and `Supported Resistance Level Range` (`0x2AD6`); never send out-of-range values | Must |
| FR-2.7 | Handle control-point indications including error responses, not fire-and-forget | Must |
| FR-2.8 | Rate-limit control writes (≤4 Hz target) | Must |

**Decoding note (FR-2.2):** Indoor Bike Data is variable-length, determined by a leading
16-bit flags field. **Bit 0 is inverted** — instantaneous speed is present when the bit is
*clear*. Fields must be consumed strictly in specification order.

### 5.3 Controller input (FR-3)

Input comes from the **Zwift Click v2 over BLE, directly** (§2.3.1). There is no bridge and
no secondary protocol path.

**Direct Click v2 client**

| ID | Requirement | Priority |
|----|-------------|----------|
| FR-3.1 | Discover Click pods by Zwift custom service UUID plus manufacturer data; identify left vs right pod | Must |
| FR-3.2 | Perform the `RideOn` handshake and establish the encrypted session — ECDH P-256 → HKDF → AES-256-CCM (§2.3.1) | Must |
| FR-3.3 | Attempt the **unencrypted** path first (A-3) and fall back to encrypted — it is far simpler if the v2 permits it | Should |
| FR-3.4 | Connect **both pods concurrently**; each is an independent BLE peripheral | Must |
| FR-3.5 | Decode controller notifications into press/release events for every button on both pods | Must |
| FR-3.6 | Handle keepalive/empty messages (`0x15`) and maintain the session | Must |
| FR-3.7 | Decode battery level (`0x19`) per pod and warn when low | Should |

**Unlock handling — the app guides, the rider performs**

The unlock happens in the Zwift app (§2.3). This application never performs it, never asks
for Zwift credentials, and never talks to Zwift's servers. It detects the state and walks
the rider through the steps.

| ID | Requirement | Priority |
|----|-------------|----------|
| FR-3.8 | Detect the locked failure mode — notifications stop roughly a minute after connecting — and distinguish it from an ordinary dropout | Must |
| FR-3.9 | Track the last known-good unlock per pod and show remaining validity against the ~24 h window | Must |
| FR-3.10 | **Guide** the rider through the unlock with explicit ordered steps (open Zwift → log in → pairing screen → pair the Click → hold a button 10–30 s → close Zwift → return here) | Must |
| FR-3.11 | State plainly that no paid Zwift subscription is required | Should |
| FR-3.12 | Warn before a ride starts when the unlock is stale, expiring soon, or unknown | Must |
| FR-3.13 | Detect and confirm success automatically once events resume, closing the guidance without the rider having to declare it worked | Should |
| FR-3.14 | Offer "mark as unlocked" for riders who unlocked outside the app | Could |
| FR-3.15 | Never store Zwift credentials or contact Zwift services | Must |

**Input handling**

| ID | Requirement | Priority |
|----|-------------|----------|
| FR-3.16 | Debounce input; support press, release and hold-to-repeat | Must |
| FR-3.17 | All bindings user-configurable; the mapping below is the default | Should |
| FR-3.18 | Hold on D-pad up/down repeats the gradient step at a fixed rate | Should |
| FR-3.19 | Keyboard and on-screen controls mirror every action — needed to develop and test the app before the Click client works, and to keep a ride going if a pod dies mid-session | Must |

**Button mapping.** Per OQ-1: shift paddles shift, D-pad handles gradient.

| Click v2 control | Pod | Action |
|------------------|-----|--------|
| Shift paddle `+` | Right | Virtual gear **up** |
| Shift paddle `−` | Left | Virtual gear **down** |
| D-pad up | Left | Gradient **+0.5%** |
| D-pad down | Left | Gradient **−0.5%** |
| D-pad left | Left | **Previous** route / profile |
| D-pad right | Left | **Next** route / profile |
| Face button A | Right | Cycle control mode (§5.4) |
| Face button Z | Right | Reset gradient offset to zero |
| Face button B | Right | Pause / resume ride |
| Face button Y | Right | Insert lap marker |

> **RISK-9 applies here.** Every gradient and route control sits on the **left** pod, which
> is where third-party support is weakest (§10). With no bridge fallback, the left pod is a
> single point of failure for gradient control — TASK-0 must prove it works before this
> mapping is committed to.

### 5.4 Control modes (FR-4)

| ID | Mode | Behaviour | Priority |
|----|------|-----------|----------|
| FR-4.1 | **Virtual gearing** | Paddles shift a configurable virtual cassette; resistance follows gear × terrain | **Must** |
| FR-4.2 | **Manual grade** | D-pad adjusts simulated gradient in ±0.5% steps | Must |
| FR-4.3 | **Resistance** | Buttons step trainer resistance directly, ignoring physics | Must |
| FR-4.4 | **Route** | Gradient driven by route profile at current distance; paddles shift | Must |
| FR-4.5 | **Waveform** | Gradient/resistance/power driven by a synthetic profile (§5.6) | Must |
| FR-4.6 | **ERG** | Trainer holds a fixed target power | Should |

Gearing and gradient are **simultaneously active**, not alternatives — the paddles always
shift while the D-pad always trims gradient. "Mode" selects where the *base* gradient comes
from (manual, route, or waveform).

**Virtual shifting design (FR-4.1).** No mechanical shifting exists, and FTMS has no
virtual-shifting opcode. Approach:

1. Define a virtual cassette: N gears (default ~24), ratios configurable.
2. The app owns the physics (FR-7.1), so it knows the wheel force required for the current
   virtual speed and gradient.
3. The selected gear sets the cadence needed to hold that speed:
   `cadence = v / (gear_ratio × wheel_circumference)`.
4. The app sets trainer resistance so effort at that cadence matches the physics —
   effectively `resistance = f(gradient, speed, gear_ratio)`.
5. Mapping that target onto a D100 resistance level needs **empirical calibration** of the
   trainer's resistance curve (TASK-3).

| ID | Requirement | Priority |
|----|-------------|----------|
| FR-4.1.1 | Configurable gear count and ratios, with sensible defaults | Must |
| FR-4.1.2 | Display current gear prominently, with clear feedback on each shift | Must |
| FR-4.1.3 | Clamp at top and bottom gear; never wrap around | Must |
| FR-4.1.4 | Shift response ≤250 ms end-to-end (NFR-1) | Must |
| FR-4.1.5 | Persist gear selection across a reconnect | Should |

### 5.5 Routes (FR-5)

| ID | Requirement | Priority |
|----|-------------|----------|
| FR-5.1 | Import GPX; derive a distance/elevation profile | Must |
| FR-5.2 | Smooth GPX elevation before differentiating into gradients — raw GPS elevation is far too noisy to send to a trainer | Must |
| FR-5.3 | Clamp derived gradients to a configurable range (default −10%…+15%) | Must |
| FR-5.4 | Hand-authored routes as an ordered segment list in a human-editable file | Must |
| FR-5.5 | Interpolate between profile points so grade changes are continuous, not stepped | Must |
| FR-5.6 | Support looping, and support finishing at the end | Should |
| FR-5.7 | Show distance covered, remaining, and elevation profile with current position | Must |
| FR-5.8 | Cycle routes from the D-pad mid-ride (`0x12`/`0x13`) | Must |

### 5.6 Synthetic waveform profiles (FR-6)

| ID | Requirement | Priority |
|----|-------------|----------|
| FR-6.1 | Waveform types: **sine, square, triangle, sawtooth, ramp, constant** | Must |
| FR-6.2 | Apply to any of three channels: **gradient (%)**, **resistance level**, **target power (W)** | Must |
| FR-6.3 | Parameterise by amplitude, midpoint, period, phase, and duration or repeat count | Must |
| FR-6.4 | Support **time-based** and **distance-based** periods | Should |
| FR-6.5 | Compose into a sequence of blocks — warm-up ramp, sine intervals, cool-down | Must |
| FR-6.6 | Share a file format with hand-authored routes, so a profile may mix terrain and waveform blocks | Should |
| FR-6.7 | Render a preview chart before the ride; show position within it during | Must |
| FR-6.8 | Clamp generated values to the safe range at transmission (SAF-3) | Must |
| FR-6.9 | Smooth transitions between blocks so the trainer does not step discontinuously | Should |

**Illustrative format** (TBD):

```yaml
name: "Over-unders + hill repeats"
blocks:
  - { type: ramp,     channel: power,    from_w: 100, to_w: 200, duration_s: 600 }
  - { type: sine,     channel: power,    midpoint_w: 240, amplitude_w: 40,
      period_s: 120, repeats: 8 }
  - { type: segments, channel: gradient, loop: true, segments: [
        { distance_m: 800, gradient_pct: 6.5 },
        { distance_m: 400, gradient_pct: -3.0 } ] }
  - { type: constant, channel: power,    watts: 120, duration_s: 300 }
```

### 5.7 Ride engine (FR-7)

| ID | Requirement | Priority |
|----|-------------|----------|
| FR-7.1 | Compute virtual speed from measured power, gradient and mass — the app owns the physics rather than trusting the trainer's reported speed | Must |
| FR-7.2 | Integrate speed into distance, which drives route position | Must |
| FR-7.3 | Model inertia so speed changes feel natural rather than snapping to steady state | Must |
| FR-7.4 | Expose rider mass, bike mass, Crr, CdA and wheel circumference as configurable | Must |
| FR-7.5 | Use the trainer's reported speed as diagnostic/fallback only | Should |
| FR-7.6 | Track elapsed time, moving time, elevation gained, average and normalised power | Should |

```
F_propulsive = (P_measured × drivetrain_efficiency) / max(v, v_min)
F_gravity    = m × g × sin(atan(gradient))
F_rolling    = m × g × Crr × cos(atan(gradient))
F_aero       = ½ × ρ × CdA × v²
a            = (F_propulsive − F_gravity − F_rolling − F_aero) / m
v           += a × Δt      (clamped at ≥ 0)
```

Owning the physics is a prerequisite for virtual shifting, makes behaviour reproducible in
tests, and removes dependence on the trainer's internal mass assumptions.

### 5.8 Recording and export (FR-8)

| ID | Requirement | Priority |
|----|-------------|----------|
| FR-8.1 | Record a 1 Hz series: timestamp, power, cadence, speed, distance, gradient, gear, mode | Must |
| FR-8.2 | Export a valid FIT activity (file_id, session, lap, record, activity; correct CRC) | Must |
| FR-8.3 | Verify exported FIT files import into Strava and Garmin Connect | Must |
| FR-8.4 | Persist the raw series so a FIT can be regenerated after a crash | Must |
| FR-8.5 | Continue recording across a BLE dropout, marking the gap rather than aborting | Must |
| FR-8.6 | Write incrementally — a crash must not lose the session | Must |
| FR-8.7 | Record lap markers triggered from the controller (`0x35`) | Should |

> Only FIT export was requested. In-app ride *history* is not in scope for v1 — the FIT
> file is the ride artifact. FR-8.4's raw log is crash safety, not a history feature.

### 5.9 GUI (FR-9)

**Connection screen**

| ID | Requirement | Priority |
|----|-------------|----------|
| FR-9.1 | Live device list with signal strength and identified type | Must |
| FR-9.2 | Per-device connect / disconnect / forget, with clear state and error text | Must |
| FR-9.3 | Show FTMS control acquisition separately from BLE connection — connected ≠ controllable | Must |
| FR-9.4 | Show per-pod connection and unlock state, with a **guided walkthrough** of the Zwift unlock (FR-3.10) launched from here and from any locked-state warning | Must |

**Ride screen**

| ID | Requirement | Priority |
|----|-------------|----------|
| FR-9.5 | Large, legible readouts readable at ~1 m. **The screen is route-led, not power-led** — ETA, distance remaining, speed, gradient and elevation take visual priority; power, cadence and heart rate are present but subordinate | Must |
| FR-9.6 | Live streaming charts of power and gradient/target | Must |
| FR-9.7 | **The route elevation profile with current position marked is the primary visual element** of the ride screen, not a supporting chart. For waveform profiles, the profile preview serves the same role | Must |
| FR-9.15 | **Estimated time to finish (ETA)**, alongside distance covered and remaining | Must |
| FR-9.8 | Prominent display of active mode, current gear, and current target | Must |
| FR-9.9 | Visible feedback on every button press, so the rider knows input registered | Must |
| FR-9.10 | On-screen and keyboard equivalents for all controller actions | Must |
| FR-9.11 | Instantaneous power is noisy — show a rolling average alongside or instead | Should |
| FR-9.12 | Dark theme suitable for indoor training | Should |
| FR-9.16 | Estimated energy expenditure in kcal, from measured work and rider mass. One model shared by the live readout and the exported FIT, so the two agree; the trainer's own energy field is not used, because FTMS does not define what it means | Should |
| FR-9.17 | **Screen size is a measured input to the layout, not an assumption.** Viewport width, height, device pixel ratio and pointer coarseness are measured at startup and on every resize, rotation and fold, and a single pure function maps them to type sizes in pixels, column counts and which sections are shown. There is exactly one definition of "narrow" in the codebase; stylesheets consume the result and never restate a breakpoint of their own | Must |
| FR-9.18 | **Legibility floors are absolute.** No readout scales below the size at which it stops being readable from the riding position — the primary number never below 40 px, secondary never below 28 px. A viewport too small for the content sheds content instead: sparklines first, then the summary-style effort numbers, then the route detail row. The route profile (FR-9.7) and the live numbers are the last to go | Must |
| FR-9.19 | **Touch is a first-class input on Android.** Every control is at least 48 px on a coarse pointer, hover styling is suppressed, and keyboard hints are hidden — with a word on every control that carried only a key cap | Must |

**Post-ride**

| ID | Requirement | Priority |
|----|-------------|----------|
| FR-9.13 | Summary: duration, distance, elevation, average/max power, average cadence | Must |
| FR-9.14 | Save FIT to a chosen location, with the path confirmed | Must |

---

## 6. Non-Functional Requirements

| ID | Requirement |
|----|-------------|
| NFR-1 | **Input latency** — button press to trainer resistance change ≤ 250 ms |
| NFR-2 | **Telemetry** — process trainer notifications at native rate (1–4 Hz) without backlog |
| NFR-3 | **UI** — charts stay smooth for a 2-hour ride without unbounded memory growth |
| NFR-4 | **Resilience** — no BLE dropout, malformed packet or missing characteristic may crash the app |
| NFR-5 | **Portability** — `core` and `fit` compile for Android with no platform-specific code |
| NFR-6 | **Offline** — full functionality with no internet connection. The app never contacts Zwift or any other service; only the rider's separate daily unlock needs the internet |
| NFR-7 | **Startup** — launch to scanning in under 3 seconds |
| NFR-8 | **Observability** — all BLE traffic loggable at debug level, including decrypted Click frames, for protocol diagnosis |
| NFR-9 | **Shutdown** — every exit path completes the SAF-2 sequence and closes within 8 seconds, whatever the radio was doing when the rider quit |
| NFR-10 | **No busy-waiting** — a supervisor whose event source has closed drops it. No loop may spin on a permanently-ready future, and no arm of a `biased` select may starve the one that carries the shutdown command |
| NFR-11 | **Screen stays lit** — a live ride (running or paused) holds a system idle inhibitor, so the display never blanks or locks under a rider whose hands are on the bars. Held for the ride, not for the app: it is taken when the ride starts and released when it ends, however it ends. Failing to take it costs a blanked screen, never a ride. On Android this is `FLAG_KEEP_SCREEN_ON`, set for as long as the app is in the foreground: it needs no permission and lapses on backgrounding, so there is nothing to leak — but it is coarser than the desktop inhibitor, which is scoped to the ride |
| NFR-12 | **Reproducible builds** — every artifact (Linux `.deb`/AppImage, Arch `.pkg.tar.zst`, Android APK) is built in CI from a pinned container image, not from a developer's machine. The images are built from `Dockerfile.builder` and `Dockerfile.arch` in this repo |
| NFR-13 | **No untracked source** — `src-tauri/gen/` is generated and untracked, so every hand-written Android source lives in `src-tauri/android/` and is copied in after each `tauri android init`. CI fails if a source exists only under `gen/`, or differs from its tracked copy. The failure this prevents is silent: an APK that builds and installs and then behaves as if the file was never written |
| NFR-14 | **Version-locked BLE Java** — btleplug's Android backend is half Java, and a Java/Rust mismatch surfaces as a `NoSuchMethodError` at the first scan rather than a build error. Those classes are therefore taken from the crate sources at the versions in `Cargo.lock`, never from a separately published artifact |

---

## 7. Safety Requirements

| ID | Requirement |
|----|-------------|
| SAF-1 | On controller disconnect, hold the last target — never continue applying pending increments |
| SAF-2 | On app exit, crash, or ride end, reset the trainer to 0% grade / minimum resistance |
| SAF-3 | Clamp gradient, resistance and target power to configurable safe ranges **at the point of transmission**, regardless of source |
| SAF-4 | If the trainer stops acknowledging control-point writes, stop sending and alert the rider |
| SAF-5 | Never issue a step larger than one configured increment per input event |
| SAF-6 | Waveform parameter errors must not command an unsafe target (enforced by SAF-3) |
| SAF-7 | A stale or replayed Click frame must not re-trigger an action — enforce the session counter and reject out-of-order frames |
| SAF-8 | The shutdown sequence preempts in-flight BLE work. A connect or reconnect is abandoned, and every step of the reset is individually bounded, so SAF-2 finishes inside the NFR-9 budget rather than timing out |
| SAF-9 | Every BLE link the app owns — trainer *and* controller — is explicitly closed on exit. A link the process merely abandons can leave the peripheral held and unreachable on the next launch (A-3) |

### 7.1 How SAF-9 was violated, and what it looked like *(fixed 2026-08-20)*

Worth recording, because the symptom pointed everywhere except the cause.

`Actor::teardown` unsubscribed every notifying characteristic before disconnecting, each
under its own `DISCONNECT_TIMEOUT` of 2 s. A Click carries **five** of them, so the exit
path could spend 10 s on optional work against the controller's 3 s shutdown budget. The
supervisor gave up first, the process exited, and `disconnect` was never reached — leaving
BlueZ holding the pod with **no application running**. Confirmed directly: both apps
stopped, `bluetoothctl devices Connected` still listing it.

The next connect then inherited a half-dead session, and that is the part worth
remembering: **it streamed battery every ~5 s and no button frames at all.** A pod that
answers the handshake, reports battery, and ignores every press is indistinguishable from
broken hardware, and it was read that way — as a dead left pod (RISK-9), as a wrong bit
map, as pods filed into the wrong slot, and twice as a lapsed Zwift unlock. None of those
survived a capture. It also produced the connect failures that came with it,
`le-connection-abort-by-local` and `service discovery timed out`, and it came and went
depending on whether the previous exit had happened to time out.

Three rules fall out of it:

- **Nothing optional runs before the disconnect on an exit path.** Dropping the link clears
  the peripheral's CCCDs anyway, so the unsubscribes were politeness toward a connection
  about to be destroyed. The trainer's SAF-2 writes stay, because those are not optional.
- **A timed-out disconnect is logged with its consequence, not just its name.** The cost
  lands on the *next* run, which is exactly why this hid for so long.
- **Android is the worse case.** A leaked link there survives the app being swiped away.

---

## 8. Architecture

```
┌──────────────────────────────────────────────────────┐
│  Frontend (webview)                                  │
│  Connection manager · Gauges · uPlot · Profile editor│
└──────────────▲────────────────────────┬──────────────┘
        events │                        │ commands
┌──────────────┴────────────────────────▼──────────────┐
│  Rust core                                           │
│  ┌──────────┐ ┌──────────┐ ┌─────────┐ ┌──────────┐  │
│  │ Ride     │ │ Profile  │ │ Virtual │ │ Recorder │  │
│  │ engine   │◄┤ engine   │ │ gearing │ │ → FIT    │  │
│  │(physics) │ │(GPX/wave)│ └────┬────┘ └──────────┘  │
│  └────▲─────┘ └──────────┘      │ target             │
│  ┌────┴───────────┐  ┌──────────▼──────┐             │
│  │ FTMS client    │  │ Click v2 client │             │
│  │ (trainer, BLE) │  │ (BLE + crypto)  │             │
│  └────────▲───────┘  └────────▲────────┘             │
│           │  btleplug / blec  │                      │
└───────────┼───────────────────┼──────────────────────┘
            │                   │  ECDH P-256 → HKDF
            │                   │  → AES-256-CCM
     ┌──────┴──────┐     ┌──────┴──────────────┐
     │  D100       │     │ Click v2 L + R pods │
     └─────────────┘     └─────────────────────┘
                                 ▲
                         daily unlock performed
                         by the rider in Zwift
```

---

## 9. Delivery Plan

### Phase 0 — D100 protocol discovery *(blocking for Phase 1)*

| Task | Description |
|------|-------------|
| **TASK-1** | `probe` against the D100: enumerate services, dump `Fitness Machine Feature` and `Supported Resistance Level Range`, log decoded Indoor Bike Data, resolve whether `0x11` works (A-1) |
| **TASK-2** | Write control commands to the D100; confirm physical resistance change |

**Exit criteria:** telemetry decodes correctly and a written command produces a felt
resistance change.

### Phase 0b — Deferred discovery *(blocking for Phase 3 only)*

| Task | Description |
|------|-------------|
| **TASK-0** | **Prove the Click v2 — the riskiest thing in the project.** Unlock both pods in the Zwift app, then with `probe`: (a) try the **unencrypted** path (A-3); (b) if that fails, do the full `RideOn` + ECDH/HKDF/AES-CCM handshake; (c) log raw and decrypted frames against a known button sequence; (d) **confirm every button on *both* pods registers** (RISK-9); (e) measure how long events survive without a fresh unlock. Prototyping in Python against §3.6's MIT code is legitimate — the goal is knowledge, not shipped code |
| **TASK-3** | **Characterise the D100 resistance curve** — required to map virtual gears onto it (FR-4.1) |
| **TASK-4** | Android BLE spike (RISK-1). **Built, not yet proven on hardware:** the APK builds via btleplug's droidplug backend, and the manifest carries the Android 12+ runtime pair `BLUETOOTH_SCAN` + `BLUETOOTH_CONNECT` with `android:usesPermissionFlags="neverForLocation"` on the scan permission — which we can claim honestly only because every scan is filtered by service UUID — plus `ACCESS_FINE_LOCATION` capped at API 30 for older releases. `MainActivity` requests them at runtime before the first scan, not merely declares them. What remains is the actual spike: install on a phone and confirm a scan finds the D100 and both Click pods, and that a connect and a control-point write survive a screen-off |

> **TASK-0(d) satisfied — RISK-9 retired (2026-08-20).** Four connect/disconnect cycles
> against the left (`−`) pod with `probe zwift --buttons`: every button registers, on the
> bit §2.3.1 documents — D-pad `left`/`up`/`right`/`down` on bits 0–3, the `−` paddle on
> bit 8 (6, 30 and 14 presses across three rounds). The right pod was mapped the same way:
> `A`/`B`/`Y`/`Z` on bits 4–7 and the `+` paddle on bit 12, over 83 presses. Both halves of
> the bit table are now hardware-verified, and QS's `wontfix` for the left `−` does not
> reproduce here. **OQ-10 does not need answering** and the gradient mapping can stay on
> the left pod.
>
> The left pod did appear dead for most of an evening first. It was not: see the teardown
> leak in §7.

### Phase 1 — D100 + GUI + profiles ⭐ *(current focus)*

**The Click is deferred to Phase 3.** All control in this phase is on-screen; the app is
fully rideable from the keyboard and mouse before any Zwift protocol work begins. This
removes the only component with unknown protocol risk from the critical path.

FR-1 (trainer only), FR-2, FR-4.2/4.3/4.4/4.5, FR-5 (GPX), FR-6 (waveforms), FR-7
(physics), FR-3.19 (keyboard/on-screen), FR-9.1–9.3, 9.5–9.8, 9.10–9.12, §7 safety.

*Milestone: load a GPX or a sine-wave profile, ride it, and control gradient from the GUI.*

### Phase 2 — Recording
FR-8 (recording, FIT, crash safety), FR-9.13/9.14.
*Milestone: a ride lands in Strava.*

### Phase 3 — Zwift Click v2
TASK-0 (protocol discovery), then FR-3.1–3.18, FR-9.4, FR-4.1 (virtual gearing, which needs
TASK-3's resistance-curve calibration), RISK-9 resolution.
*Milestone: shift gears and trim gradient from the bars.*

### Phase 4 — Polish and stretch
FR-1.5 (auto-connect), FR-3.7 (battery), FR-3.14 (mark as unlocked), FR-3.17 (rebinding),
FR-4.6 (ERG), FR-9.11/9.12, Android.

---

## 10. Risks

| ID | Risk | Impact | Mitigation |
|----|------|--------|------------|
| **RISK-1** | `btleplug`'s Android backend is the least mature part of the stack | Android target lost | Partly retired: the target builds, the Java backend is version-locked to the crate (NFR-14) and the JNI init is wired and symbol-checked in CI. What is *not* yet proven is a scan and a connect on real hardware — TASK-4 |
| **RISK-2** | Virtual shifting feel may be poor if the D100's resistance curve is coarse or laggy | Core feature degraded | TASK-3 characterises it early; fall back to fewer, wider-spaced gears |
| **RISK-3** | Rust FIT *encoders* are thin — most crates read rather than write | FR-8.2 slips | Port the MIT writer from §3.2; TCX fallback |
| **RISK-4** | **No fallback input path (§3.7).** A firmware change or a protocol error leaves no second way in | Controller input lost entirely until the client is fixed | Accepted deliberately. Keep protocol handling isolated in `ble`; pin known-good behaviour in tests; keyboard/on-screen controls (FR-3.19) are the only stopgap |
| **RISK-5** | The Click v2 crypto is more involved than the D100 work — ECDH, HKDF, AES-CCM, session counters — and a subtle error yields silence rather than a clear failure | Phase 1 slips | Three reference implementations to check against (§3.4–3.6); `probe` logs raw and decrypted frames side by side (NFR-8) |
| **RISK-6** | The unencrypted path (A-3) may not work on a v2, forcing full crypto immediately | Less schedule slack | TASK-0 tests it first; the crypto path is specified either way |
| **RISK-7** | The unlock expires ~24 h, so a forgotten re-unlock blocks a ride | Frustration at session start | Detect and warn *before* the ride (FR-3.12), with a guided walkthrough (FR-3.10) |
| **RISK-8** | D100 lacks `0x11` sim mode | Reduced fidelity | Low impact — the app owns the physics |
| ~~**RISK-9**~~ **RETIRED 2026-08-20** | The left pod was where third-party support was weakest — QZ has an open `wontfix` where the left Click's `−` never registers | — | **Did not reproduce.** Every button on both pods verified on its documented bit (§5.3). The one evening the left pod *did* look dead was the teardown leak in §7, not the pod |

### 10.1 On ANT+

The D100 likely supports ANT+ FE-C, which permits multiple simultaneous connections and
would sidestep BLE contention. Excluded because it needs a USB stick on desktop and is
effectively dead on Android — the opposite of G-4. Revisit only if BLE contention proves
intolerable.

---

## 11. Open Questions

| ID | Question |
|----|----------|
| **OQ-2** | Gradient step size — is ±0.5% per D-pad press right? And what repeat rate on hold? |
| **OQ-3** | How many virtual gears, and what ratio spread? (Default: 24, roughly a 2×12 road setup.) |
| **OQ-4** | Should route mode auto-advance to the next route on completion, or stop and wait? |
| **OQ-5** | Is heart rate wanted? A BLE HRM strap is a small increment now and awkward to retrofit into the FIT writer later. |
| **OQ-6** | Which frontend framework? Svelte recommended. |
| **OQ-9** | *(§3.7)* Port QZ's GPL-3.0 crypto and licence this project GPL-3.0, or reimplement from the documented algorithm and stay unencumbered? |
| **OQ-10** | If TASK-0 shows the left pod is unreliable (RISK-9), do you want gradient remapped onto the right pod, or would you rather chase the left-pod bug? |

**Resolved:** OQ-1 (paddles shift, D-pad adjusts gradient) · OQ-7 (stay on Tauri, port the
MIT logic) · OQ-8 (no bridge, no fallback input path) · Click version (v2).

---

## 12. Glossary

| Term | Meaning |
|------|---------|
| **FTMS** | Fitness Machine Service — standard BLE profile for trainer telemetry and control |
| **Pod** | One half of a Click v2 — left (D-pad) or right (face buttons); each is an independent BLE peripheral |
| **Unlock** | Refreshing the Click v2's encryption context via the free Zwift app; lasts ~24 h |
| **ECDH / HKDF / AES-CCM** | The key agreement, key derivation and cipher used by the Zwift session (§2.3.1) |
| **ERG mode** | Trainer holds fixed target power regardless of cadence |
| **Sim mode** | Trainer applies resistance simulating a gradient; power varies with rider effort |
| **Virtual shifting** | Synthesising gear changes by varying trainer resistance, on a single-cog drivetrain |
| **Crr** | Coefficient of rolling resistance |
| **CdA** | Drag coefficient × frontal area |
| **Normalised power** | Weighted average power reflecting physiological cost of variable efforts |