udpscope/docs/claude/architecture.md

# Architecture: threads, the data path and the layers

The two threads and what they share, the four display modes, the display clock, and the layers to keep straight.

Part of [CLAUDE.md](../../CLAUDE.md).

`SPECS.md` is the requirement document. `README.md` documents what was actually built,
including deliberate deviations from the spec and known limitations — read it before
concluding something is a bug.

### Threads

Exactly two: a producer thread in `Acquisition` owns `udps_client_t` and calls
`udps_client_poll()`; the GTK main thread owns every widget and GL context. They share
`ScopeModel` under `ScopeModel::mutex()`, taken once per DATA frame by the producer and
once per frame by the UI (`tick()` in `src/ui.cpp`) around `RowSet::bind()`/`acquire()`.
Samples themselves cross via `plot::data::RingBuffer` (single producer, single
consumer) and never through the lock. Cross-thread scalars are atomics on `ScopeModel`
(`trigger_enabled_`, `trigger_armed_`, `trigger_latches_`, `trigger_time_`,
`event_`, `generation_`). Two of them are a pair: the producer writes `capture_edge_`
before `trigger_pending_` and those two carry release/acquire, so a consumer that sees a
capture open sees which edge it belongs to.

### Data path

`ScopeModel` holds **one `BufferStore`** (`live()`): one shared time-base ring plus one
value ring per channel, sized by `plan_partition()` to cover `--max-window` seconds
within the byte budget. The producer always writes it, whatever the trigger is doing —
that is what guarantees a full pre-trigger history on the *first* latch after arming.
The spec's second buffer (item 1) is the latched `EventSnapshot`, which is already an
immutable copy of the captured window; see README for why two live rings were dropped.

What a trace binds to is `ScopeModel::display_mode()`, one of four states, and
`RowSet::bind()` takes it as an argument rather than recomputing it:

- `Live` — no trigger: the live ring, on the clock of the **newest sample**
  (`display_epoch() == -newest`), so the axis is a fixed `[-window, 0]` and x reads as an age.
- `Capturing` — an edge has been accepted (so the pre window was full) and the post
  window is still filling: the live ring, on the coming event's own clock
  (`display_epoch() == -edge`), inside an axis already pinned to `[-pre, +post]`. The
  post side fills in view and the marker sits at x = 0 throughout.
- `Held` — the frozen `EventSnapshot` of the last event, whose stamps are already edge-relative,
  so the display epoch is 0.
- `Waiting` — armed, nothing captured yet: blank, with the live epoch (there is nothing bound,
  but the axis is still a live one).

**Disarm during a capture publishes what the capture got to**, then disarms: the producer
cuts the window at the newest sample and that partial snapshot becomes the held event, so
the picture on screen at the moment of the Disarm stays on screen. `engine.disarm()` clears
`pending_`, which drops the display out of `Capturing`, and with nothing published the view
fell back to the *previous* event — or to blank on a first capture. No event fired, so
`trigger_latches()` is deliberately not bumped; `ScopeModel::event_seq()` counts published
snapshots instead, and the frame tick keys its event framing on *that*. Keep the two
distinct: the latch count is what the probes assert "how many events fired" with.

`Capturing` is a **deliberate departure from SPECS:37** ("data visualisation should not
be refreshed till next event"), asked for explicitly; that reading is what the old "never
the live ring mid-capture" rule enforced. What survives of the rule is that `Held` binds
the snapshot *only* — a held event is never quietly replaced by live data — and that the
*axis* does not move during a capture: it is the event's window from the edge onwards,
never a sliding live view.

`display_mode()` must be read **once per frame** and passed around (the tick does this,
and hands it to `bind()`). The producer can open or close a capture between any two
reads, and two reads in one frame put the bind, the epoch, the pin and the status line on
different clocks.

**The display clock is not the stream clock, and `ScopeModel::stream_at_x0()` is the only
conversion.** `display_x = stream_t - stream_at_x0()` in **every** mode, and the offset lives in
one of two places: a snapshot carries its own (`EventSnapshot::x_offset`, frozen with the
picture) and a live view carries it as minus `display_epoch()`. Anything crossing between the
two clocks goes through that one accessor rather than picking the branch itself — the export's
`abs_seconds` column, the pause handler putting a display window back on the ring to cut it, the
measurement marks — because there are four modes and picking the branch by hand gets one of them
wrong. It reads atomics only, so it is safe to call inside an existing `mutex()` guard.
Three things about the live epoch specifically:
- **It is written after `bind()` and `acquire_all()`, not before.** It must be minus the newest
  sample the *traces* can report, and asking before the acquire is one frame of production
  stale — at 1 MS/s over a 50 ms window that is a third of the screen to the right of the axis.
- **There is no feedback loop**, even though the newest stamp is read through the epoch that is
  about to change: `RowSet::live_edge()` reports in the epoch currently in force, so
  `display_epoch() - live_edge()` is minus the *raw* stamp and the fixed point is reached in one
  frame rather than converged on.
- **`Waiting` takes the live epoch too.** Nothing is bound, but the axis is still a live one, and
  an epoch of 0 there would make the axis jump on the frame a capture closes.

**A measurement mark is glued to its sample; a cursor stays on screen.** The two instruments move
in opposite directions under a rolling axis and that is deliberate — a bench scope's cursors sit
where the user put them while the waveform rolls under, and a measurement's mark names the sample
it measured. The cursors need no code: their x *is* a display coordinate. The marks are stored in
**stream** seconds and reconverted every frame by `place_measure_marks()`, since a mark left in
display coordinates would crawl forward at one window per window. `App::measure_window` is
stream-clocked for the same reason, and that one is load-bearing: the recompute trigger is "the
window moved by 2 % of its span", and in display coordinates a live window is a fixed
`[-window, 0]` — it never moves, so the measurements would never be recomputed again.

Layers to keep straight:

- `SignalInfo` — one signal as the CONFIG described it. The unit of enable/disable.
- `ChannelInfo` / `ChannelBuffer` — the unit of acquisition and plotting. A
  `ChannelBuffer` is a `plot::data::Series` over a (value, time) ring pair.
- `SlotSeries` (`src/channel.hpp`) — a trace's stable series object, rebound each frame
  to either a `ChannelBuffer` or an `EventSeries`.
- `RowSet` (`src/plot_host.hpp`) — **one `plot::Figure`** of up to `kMaxRows` rows (added
  and removed one at a time by the layout editor), each a
  borrowed `plot::Plot*` with `kSlotsPerRow` fixed trace slots. The figure owns
  everything the rows have to agree on: the x window (one x link group every row joins),
  the live follow (the *group's* — a member's own axis must be `XFollow::Off`, since
  `Figure::link_x()` refuses a `Live` member), the left gutter and the crosshair.

- `ExportTable` (`src/export.hpp`) — the visible window as columns, collected from the
  snapshot or the live rings and handed to the `.npy`/`.csv` writers. Always the stored
  samples, never the LOD pyramid's buckets.

`src/ui.cpp` is the GTK4 window (toolbar, trigger panel, signal panel, plot box) plus
the `tick()` frame function that binds, updates the trigger state and rewrites the
status line. One `plot::gtk4::PlotArea` in figure mode draws the whole figure. It is by
far the largest file.