udpscope/docs/claude/invariants-math-channels.md

Invariants: the math channels and the node editor

A derived channel is an ordinary channel; the engine runs on the producer thread, so its cost is measured in packets.

Part of CLAUDE.md. These are rules that cost a defect to learn; read the ones touching the area you are editing before you edit it.

• A math channel is a channel. ScopeModel::set_math_channels() appends one synthetic SignalInfo (with math = true) per math channel after the stream's own, so the panel's switch, the plot binding, the measurements and the export all handle a derived channel without knowing it is derived -- and another math channel is a valid Source for exactly the same reason. The samples come from the producer: Acquisition::on_data() collects each channel's value for the slot and calls ScopeModel::push_math_sample() once per stored slot, before the deferred latch (publish_event() clamps its snapshot to every ring's produced(), so a derived sample pushed later would be missing from the capture's newest slot). The math therefore runs on the stored (filtered, decimated) samples, which is also why math_prepare() gets slot_dt() * time_stride: a cutoff the user typed in Hz means the rate the filter will actually run at. src/math_channel.cpp is the engine: a graph evaluated as a list walked in order, so a node may only read ones before it and a cycle is impossible by construction rather than by a check (math_channel_valid() states that as a message the dialog shows). /tmp/check_math.cpp-style checks verified the biquads against known responses (LP fc=100 Hz: 50 Hz -> 0.97, 2 kHz -> 0.002; BP fc=1 kHz q=2: 200 Hz -> 0.05, 1 kHz -> 0.95, 4 kHz -> 0.05) -- keep test sines below Nyquist: at dt = 1/10000 a "5 kHz" test tone is the Nyquist point itself and says nothing. The editor is the compact graph: nodes as boxes on a GtkDrawingArea, click to select (its parameters in the form beside it), click an output port then an input port to wire, drag to move; no pan/zoom. Its widgets are created once and only shown, hidden and set by math_dialog_sync(), so no rebuild can invalidate a live callback handle -- the same shape the measurements dialog uses. UDPSCOPE_PROBE_MATH_MS defines two graphs and opens the editor (a construction crash test as much as a state check), and the dump prints each derived channel's extremes beside the source's. Read it on the fast mock, where the two assertions that matter are gaps=0 (the math is on the producer thread, so what it can break is packet capture, never the frame rate) and each math '<name>': ... N samples vs M time slots being equal, with math x2 at y=[-2, 2], math lp at y=[-0.998, 0.998] and math3 at y=[-2, 2]. The 0 visible (order breaks 0, first at -1), y=[0, 0] tail of that line is not a failure: it is visible_samples() reading row 0's axis after the window was destroyed, and UDPSCOPE_EXIT_AFTER_MS arms the trigger first, so the dump can land in Waiting. The live ch N '<name>' ... y=[..] lines are the race-free reading of the same thing. math cost: S slots, N ns/slot (T timed), X ms total est is the per-slot cost; see the math-cost invariant below for why it is sampled, why it is an upper bound, and why a figure anywhere in 78--101 ns/slot is one reading of the same thing. Four rules here were wrong first and are worth keeping:
  • A Source may only name an acquired channel -- ScopeModel::channels(), which is what the dialog's dropdown lists, and both places that create a node must take one from there. signals().front() is the stream's timestamp signal: it is first in every CONFIG, it is not acquired, and a graph sourced from it validates and then reads NaN for ever. The dialog's default was fixed first and the pane's + was not, which is what "[ch] -> [gain x2] produces only nan" was. The dropdown also reconciles: a node whose source is not in the list is moved to what the form is showing, rather than the form showing one channel while the node holds another, and Apply refuses a source that is not acquired (the engine's validator is about the graph -- an output, every input wired above it -- and cannot see the acquired list).
  • The ingest must skip a derived channel. Acquisition::on_data() iterates every channel and reads each one from the packet, so a math channel with no signal read zero and pushed it -- while push_math_sample() pushed the real value on the same slot. Two pushes per slot into a ring whose time ring advances once: the values alternated between the real one and 0, and once both rings wrapped the value ring had drifted a whole window off the time ring, so the derived trace disappeared. ChannelInfo::math is the flag (channel.math) and the loop continues on it. A derived channel's values(c).produced() must equal live().time().produced() -- one push per stored slot -- and the dump prints both as N samples vs M time slots for exactly this: the arithmetic on the plot can look plausible long after the pairing is wrong.
  • gtk_window_destroy() does not emit close-request. The math dialog's pointer is cleared by a close-request handler, so a Cancel that destroyed the window left app.math_dialog pointing at a dead widget: every later open_math_dialog() took the "already open" branch and presented it -- a GTK critical, and nothing on screen, which is "I cannot edit a math channel again". Both paths close (gtk_window_close()), which emits the signal, lets the handler clear the pointer, and then destroys. The same lesson as the other GTK traps here: a programmatic destroy is not a user close, and only one of them runs the handlers.
  • Boxes must not overlap on the canvas. The first auto-place staggered nodes by 24 px for a 132 px box, so every box sat on the one before it, math_node_at()'s topmost-first search always returned the last, and a drag or a click landed on the wrong node -- which reads as the dialog ignoring the pointer. Two columns spaced 160 px apart, rows 58. The probe prints the box the drag began over for exactly this reason. Offering the whole signal list let a graph name something nobody is acquiring (no data for ever) and put the stream's timestamp signal, which has no samples, in it.
  • prepare_math_locked() runs from rebuild_locked(), so every plan change re-prepares the filters: a CONFIG, an enable/disable, and a window change that moves the decimation (that path is set_target_span() -> rebuild_locked()). Coefficients are a function of the stored interval and the filter's state is the old rate's history, so both are rebuilt together. Preparing only on an edit left the cutoff wrong and the state ringing after any window change -- that is what "I tested a low pass and it was a mess" was.
  • The graph's output defaults to the last node, and a node added to a linear graph becomes it. The output began at index 0 -- the source -- so every operation a user built was computed and then thrown away: a gain of 2 and a low pass both produced a channel equal to its input. The output is drawn as the orange dot and settable explicitly, so an earlier node chosen deliberately is kept; this is the default for a graph that has never been given a choice.
  • gtk_spin_button_new_with_range() can leave the value at the range's minimum. The offset field began at -1e6, and on_math_param_changed() reads the widgets, so x*2 saved as x*2 - 2000000. Every numeric field is seeded with its own identity value; the range-minimum default is never a useful starting point for a gain or a corner frequency.
  • The form's rows are grouped per op, not per field: gain and offset are one group because gain + offset is one op. The offset was a row of its own that nothing ever hid, which read as a property of every node.
  • A GtkSpinButton holds zero decimals by default, so every numeric field sets its digits (4 for gain/offset/clamp, 3 for fc/Q); without that a 12.5 Hz cutoff was stored as 12 and a gain of 0.25 as 0. And the canvas drag reports the total offset from where the gesture began, so adding each update's delta to a value the handler then writes back is a runaway box -- it must record the position at drag-begin and ignore a drag that did not begin on a box.
• The math channels' cost is the producer's, and the instrument must not be part of it. push_math_sample() runs once per stored slot -- up to the wire rate -- so what a regression here looks like is gaps, never a frame time. Three things were taken off that path and must not come back: a std::vector local (a heap allocation and free per slot; it is the member math_values_, assign()d), a Source node's channel resolved by name per sample (a linear scan with a std::string compare per candidate, per node, per slot, behind a type-erased call -- math_prepare() resolves it once into MathNodeRt::src, and every rebuild re-prepares), and a fetch_add on the cost counters. That last one was 20-25 % of the measured figure: a lock xadd costs about what a two-node graph's whole arithmetic does, so counting the slots cost more than the work being counted. The counters are producer-written only, so each is a relaxed store() of a value already in hand (math_slot_count_ and friends). Two things about the figure itself:
  • It is sampled, one slot in kMathCostSample, and it is an upper bound. A slot's work is the same order as one clock_gettime, so the measured slot carries a clock read. Timing a run of consecutive slots to amortise that away is wrong and was tried: the producer gets its slots a packet at a time, so a run long enough to be worth timing straddles a packet boundary now and then and charges the measured slots the wait for the next packet. At 2000 samples a packet a 64-slot run crosses one ~3 % of the time, and a ~1.5 ms gap spread over 64 slots reported 799 ns/slot against the true ~80. One slot is always inside one packet.
  • 78--101 ns/slot is one reading. Four identical-config runs spread that far, so the per-slot path is at the noise floor of this harness and a micro-optimisation cannot be shown to work here. Converting MathRuntime from eight parallel std::vector<double> to one array of MathNodeRt was measured and bought nothing -- at six nodes the whole runtime is L1-resident either way. It was kept for the shape, not for a number; do not expect a cache argument to pay at this size.