f201892c9c
Grounds the native-Rust direction in what now exists: port inside-out, lowest risk first, with tests/fixtures/track-format.json as the acceptance gate. Stage 1 (the track-format crate as a third conformance adapter) is the concrete next PR - host-testable in a container, no hardware. Toolchain goes in a container per the develop-in-container rule, not the host. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
70 lines
3.8 KiB
Markdown
70 lines
3.8 KiB
Markdown
# Rust port — staged plan
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This is the plan for the native-Rust direction first discussed alongside the A/B-bootloader
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idea. **What changed since then:** the track format is now formally specced
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(`docs/track-format.md`) with a golden-vector conformance suite (`tests/run.mjs`). That suite is
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the thing that makes a port *safe* — any Rust engine has a precise, executable definition of
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"correct" to validate against, the same one `engine.js` and `app.py` already pass.
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## The core idea
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Port from the **inside out**, lowest-risk first. The pure logic (track codec, then the
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scheduler) is host-testable with zero hardware and is gated by the existing golden vectors. Only
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once that's proven do we touch drivers, A/B, and the actual firmware. We do **not** rip out
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CircuitPython until the Rust engine passes the vectors *and* the drivers are proven on hardware.
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## Stages
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### Stage 0 — toolchain in a container
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Add a Rust toolchain image (mirroring `hardware/eda/`): a `Containerfile` with `rustup`, the
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`thumbv8m.main-none-eabihf` target (RP2350 is Cortex-M33), `flip-link`, `probe-rs`, `elf2uf2`.
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Driven by a `run.sh` like the EDA one. **Never on the host.**
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### Stage 1 — `track-format` crate ← the concrete first PR
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A pure, `no_std`-compatible crate: `parse(&str) -> Track` and `serialize(&Track) -> String`,
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plus a `normalize()` that emits the neutral structure from `docs/track-format.md` §5. Then a
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`cargo test` that reads `tests/fixtures/track-format.json` and asserts each case's `norm` and
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round-trip — i.e. a **third adapter alongside `js_adapter.mjs` / `py_adapter.py`**. When this is
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green, the Rust engine provably agrees with web + device on every groove, euclid, swing, ghost,
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polymeter, and the playback-flow tokens. No hardware, fully testable in the container.
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This is the highest-value slice: small, gated by work already done, and it proves the toolchain.
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### Stage 2 — scheduler/engine
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Port the look-ahead step scheduler. `engine.js` already marks these primitives
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`PORTS TO FIRMWARE`, and `app.py`'s `tick()` is the same model. Host-test it by asserting click
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*timings* for known patches (e.g. swing ratios, polymeter bar lengths) — still no hardware.
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### Stage 3 — drivers (hardware)
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On `embassy` / `rp-hal`:
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- ST7789 240×320 display → `mipidsi` + `embedded-graphics` (mature; the parts are well-supported).
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- I²S to the PCM5102A → RP2350 PIO.
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- WS2812 → `ws2812-pio`. USB-MIDI → `usbd-midi` / `embassy-usb`.
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- GT911 touch (Kit) over I²C.
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### Stage 4 — native A/B + secure boot
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Replace the `.mpy`-level A/B hack (`code.py` loads `app.mpy`, rolls back to `app.bak`) with the
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**RP2350 bootrom's native** partition-table A/B + signed boot, configured via `picotool` (the
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chip already provides this — see the earlier hardware discussion). The Rust app is the image in
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the slot; rollback and version selection move into silicon.
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## What you keep / lose
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- **Gain:** memory safety, native A/B + secure boot, performance headroom, one typed model instead
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of three hand-written parsers.
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- **Lose:** the live one-click `.mpy` push (Rust is compile→flash→reboot). The editor's *data*
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live-sync (tempo/pattern/setlist mirroring) still works — it's a data protocol. Only live
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*logic* edits go away, and an embedded `wasm3`/script module could buy those back if wanted.
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## Acceptance gate
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Every codec/engine change must pass `tests/fixtures/track-format.json`. The Rust crate joins
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`js`/`py` as a runner adapter, so "same groove on web, device, and the Rust build" is enforced,
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not hoped for.
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## Recommendation
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Do **Stage 1 in a container next** — it's small, testable today (given a toolchain), reuses the
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suite, and produces a real artifact to judge the Rust path on before committing to drivers or a
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firmware rewrite. Defer Stages 3–4 until Stage 1–2 are green and you've decided the live-push
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tradeoff is acceptable.
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