udpscope/docs/claude/invariants-measurements.md

# Invariants: the measurements pane

Why the pane is refreshed on a cadence, why neither pass may read its window raw, and how a crossing is counted.

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

- **The measurements pane is refreshed on a cadence, and its cost is the *window's*, not the
  frame's.** A measurement is a statistic of the visible window and a live view moves that window
  every frame, so "recompute when the window moves" is "recompute every frame" at O(visible
  samples) *per measurement*. With the pane holding four measurements over a 4.7M-sample window
  that measured **13.66 ms of tick mean** (baseline 0.41 ms) and cost the run a third of its
  frames. Three things fixed it, and all three matter: `update_measures()` recomputes at most
  every `kMeasureRefreshUs` (100 ms; 400 ms when a pass costs more than `kMeasureHeavyUs`), once
  when the window settles and once per mode change -- and reading *that* backwards is what made
  it worse first, 457 recomputes in one run, because a pinned window then recomputed every
  frame; the window is materialized in bounded chunks (never two whole-window f32 temporaries)
  and the loop holds it **by reference** (copying both vectors per measurement was 300 MB of
  memcpy a frame); and `measure_stats()` is one shared pass carrying both extremes with their
  first positions, the sum and the count, so average/min/max/peak-to-peak/amplitude need no
  pass of their own. **Adding a type must not add a pass** -- that is what `compute_measure()`'s
  `stats` argument is for.
  The marks' *ink* is set where they are created (`install()` in `plot_host.cpp`): 0.75 px and
  dashed 4/4 through `Plot::set_cursor_stroke()`, which is per cursor because the trigger marker
  (also a cursor) keeps the solid `line_width_px` stroke. A new measure mark must take that
  stroke -- it is created in the same loop, so it does; a new *instrument* should not.
  The **marks** the pane draws (`set_measure_marks()` -- full-width `h_ruler`s and full-height
  cursors, i.e. the "lines across the plot") are re-placed every frame from the result, so a
  throttled pass is a *stepped* ruler: acceptable, because the ruler and the number beside it
  come from the same pass and agree. What is *not* acceptable is a ruler that appears and
  disappears, so an invalid pass falls back to the last valid result for its **level** marks and
  only drops its **time** marks: a level is a property of the signal (a mean, a top, an
  amplitude) and outlives the window it was measured in, while a time mark is a position *inside*
  that window and may not. Both are emitted in **one pass after the numbers, never inside the
  loop that computes them**: that loop has three `continue`s (an unresolvable signal, a channel
  with no window) and each one was a way for the rulers to blink off and back. Fixing one and not
  the others is how that bug survived a round of "fixes". `clamp fires: N` in the dump is the other half of this: `clamp_to_data()`
  compares with a relative tolerance, because clamping to exactly `window_s_` can leave the
  group's span an ULP above it and a clamp that fires every frame is a re-frame loop, not a
  clamp.
- **A measurement must not read its window raw.** Both passes go through
  `plot::data::MinMaxPyramid`, which the producer already maintains at amortised O(1) -- so the
  pane inherits the summary of the new samples rather than computing one, which is what the
  "evaluate only on new samples" ask comes to here. Throttling the cadence (above) bounded *how
  often*; this bounds *how much*, and it is the larger factor: 19.7 ms per recompute to 1.2--1.6
  ms, reading **0.1 %** of a 2 M-sample window. Pass one (`measure_scan_buckets()`) adds whole
  level-0 buckets -- 64 samples each, carrying min, max an **exact** sum and a finite count -- and
  locates each extreme to a *bucket*; one raw `measure_scan()` of that bucket alone then pins it
  to a sample, two buckets at most whatever the window's size. Pass two
  (`measure_accumulate_bucket()`) folds a bucket's **sums** only when it lies **strictly** on one
  side of its level (`k.lo > level` or `k.hi < level`): then every sample falls in the same bin and
  none is *at* the level, so the fold is exact and not an approximation. It asks **separately**
  whether the bucket enters a side of the hysteresis band (see the invariant below), because a
  single test against `level ± h` refuses exactly the buckets a crossing passes through. A bucket
  wholly past a threshold holds that crossing at its first sample, exactly -- which is where the raw
  walk would have found it. It refuses a bucket that straddles the level, one that enters a side
  somewhere inside itself, and one holding a non-finite sample (the raw walk takes the first
  *finite* sample of the new side and an aggregate cannot say which that is); the caller then opens
  that bucket. **Nothing may be folded before a refusal** -- a refused bucket goes to
  `measure_accumulate()`, which adds the same samples again, and sums added above the `return false`
  double-counted every opened bucket, reading `top=0.614304` on a unit sine against the 2/pi it must
  be. Four things this rests on, each of which breaks it silently:
  * **Exactness is the point, so nothing may stride or decimate.** `peak to peak` must still read
    exactly `2` on a unit sine and `top` exactly `0.636620` (2/pi). A bucket's sum is a sum.
  * **The bucket index space must be the window's.** `ChannelBuffer::acquire()` calls
    `pyramid_.drop_before(values_->base())`, so a channel's pyramid is in the value ring's
    absolute index space -- the same space `live_window_locked()` answers in, since a channel's
    stride is always 1. The snapshot's pyramid is built from sample 0 and never dropped from, so
    its space is the snapshot's own. `MeasureWindowReader::origin_` is what turns one into a
    window offset, and a mismatch there measures the wrong samples rather than failing.
  * **The raw path stays, and stays exact**, for the window's partial edge buckets, for the
    samples produced since this frame's `acquire_all()` (one frame of production, so bounded), and
    for the straddling buckets. A window shorter than a bucket degrades to O(window) and must
    still be right.
  * **Both passes walk the same fetched bucket list** (`App::measure_bucket_scratch`, reused
    across recomputes -- a 2 M-sample window is 32 k buckets). Fetching twice is a second walk of
    the pyramid for nothing.
  `measure cost:`'s `covered`/`buckets`/`raw` is the guard, and it needs the fast mock to say
  anything.
- **`MeasureOptions::noise_pct` is a percentage, clamped in `measure_set_level()` to half the room to the
  nearer extreme**, so a band can never invert its own thresholds. It moves the **edge decision only**: the
  sums still split at the bare level, because `Top` on a unit sine must read 2/pi (0.636620).
- **`measure_accumulate` loops samples outer, levels inner, and that is not backwards.** The filter holds one
  state for the whole walk, so levels-outer would step it once per level *per sample* and hand every level but
  the last a different filter state.
- **A non-zero `hf_fc` disables *both* bucket fast paths**, with the single
  `if (!filtered && reader.bucket_range(...))`. An empty `buckets` makes `bulk_from == bulk_to == n` *and*
  leaves `lo_search` / `hi_search` at 0, so `pin()` becomes a no-op -- and that is the right answer, because a
  bucket is an aggregate of unfiltered samples and cannot be mixed into a filtered walk in either direction.
- **A crossing is counted with hysteresis, and the band moves the *edge decision* only.**
  `MeasureLevel::hysteresis` is a Schmitt trigger about `level`: an edge is counted once the signal
  has passed `level + h` going up or `level - h` going down, and a sample inside the band decides
  nothing. Without it `period`, `frequency` and `pulse duration` are unusable on anything real --
  the mid level is exactly where a waveform moves *fastest*, so an interferer faster than the
  fundamental recrosses a single level several times per true crossing. Measured on a 5 Hz sine with
  a 500 Hz tone at a tenth of the amplitude (`tools/mock_udpstreamer.py --noise 0.1`): 9 true
  crossings counted as **79**, `period=0.025622 freq=39.029` for a true 0.2 s / 5 Hz, against
  `period=0.200188` with the band. Four rules, each of which was wrong first:
  * **`measure_set_level()` is the only place the band is sized**, and `src/ui.cpp` calls it rather
    than assigning `.level` itself: two expressions for the width is how the raw walk and the bucket
    path end up disagreeing about where a crossing was. `h = min(0.10 * (hi - lo), 0.5 * min(hi -
    level, level - lo))`. The 0.10 is measured -- 5 % reads a 0.1 interferer and fails at 0.2
    (`period=0.075`), 10 % holds to 0.3 and to white noise at sigma=0.05.
  * **The clamp is load-bearing.** `PulseDuration` lets the user put the split near an extreme, and
    a fixed tenth of the span there puts the upper threshold *above every sample*: the walk never
    decides that side and the measurement reads nothing at all, which is worse than reading
    something noisy. On a ringing 3.3 V logic signal at a 90 % split: `enters=0` with a fixed band,
    `enters=4 period=0.200000` with the clamp.
  * **The `sum`/`count` split stays at the bare `level`.** That split *is* `Top`/`Bottom` -- the mean
    above and below the threshold -- so a band there changes a documented exact reading. `top` on a
    unit sine must still be `0.636620`.
  * **`h = 0` must reduce exactly to the single-level walk**, including the `v == level` skip, so the
    band is a parameter of the engine rather than a mode of it.
  The cost is the noise's, not the band's: the clean fast mock reads `raw=119412 (0.1%)` of 215 M at
  1.41 ms a recompute and with the 0.1 interferer `raw=2.0%` at 1.96 ms. A
  `/tmp/check_measure.cpp`-style check is how the walk itself was verified (`g++ -std=c++20 -O2 -I
  src ... src/measure.cpp`): over a clean sine, sines with 0.1 and 0.3 interferers, 10 %/50 % duty
  squares and a 90 % threshold, the **bucket walk must equal the raw walk exactly** -- same
  `enters`, same `count[]`, identical `first_enter`/`last_enter` to 0.0 -- which is the assertion
  that the fast path is a fast path and not an approximation. The app's reading is
  `[measures-probe]` against `--noise 0.1`.
- **A statistic's observation is one *recompute*, not one sample.** The pane already refreshes on
  a cadence (at most one pass per 100 ms while the window scrolls, exactly once when it
  settles), and the accumulator is fed from that pass's result — so `n` counts windows measured.
  Feeding it per sample would make `n` a sample count and the mean a mean of *samples*, which is
  not what any of these measurements measure: a frequency over a window is not a property of a
  sample. The reading that names this defect is `[stats-probe]`'s `n` in the thousands rather
  than the tens.
- **The accumulator is Welford's, and the reason is the same one the time ring is `f64` for.** A
  measurement's values are routinely large with a small spread — an absolute stream time is the
  obvious case — and `Σx²  - (Σx)²/n` cancels to noise there. The standalone check is that the
  spread of `{2,4,4,4,5,5,7,9}` survives a `+1e9` offset unchanged.
- **What resets the statistics is "what is being measured changed", not "what is in view
  changed".** An explicit Reset, an add/edit/delete (all three funnel through
  `rebuild_measures_pane()`), a CONFIG, and a change of display window each change the
  *question*, so the history is cleared. A pause, a resume, a trigger latch, a pan and a zoom do
  not: they change which samples answer it, which is precisely what averaging over many windows
  is for. The pane-wide toggle does not reset either — it only shows and hides labels that are
  always created and always fed, so reading the plot for a moment does not cost the history.