Discovered while running the backfill on prod: certain histogram
blocks contain an undocumented extension byte format whose naive
uint16 LE interpretation yields physically impossible peak values
(150+ in/s when the device max is 10). Concrete example from
K558LKSG.3I0H block at body+7424:
bytes [6:10] = 05 79 69 00
current code: T_peak = uint16 LE = 0x7905 = 30981 → 154.9 in/s
reality: T_peak = byte[6] = 5 → 0.025 in/s (matches BW display)
The high byte (0x79 here) appears to be an extension field — possibly
"time of peak within interval" or a Histogram+Continuous sub-mode
marker. Observed across BE9558 and BE18003 units in prod data; never
appeared in the BE12844 fixture corpus the codec was originally
verified against.
Effect on prod: 26 out of 1433 blocks in this one event had inflated
peaks, plus dozens of similar events across the fleet → sum(PVS)
inflated from baseline 988 to 34501 (35x). Rolled back via the
pre-backfill snapshot before any UI exposure.
Defensive fix: bounds-check peak counts in `_decode_block`. Any
field exceeding `_MAX_PEAK_COUNT` (4096 = ~20 in/s, well past the
device's 10 in/s Normal-range FS) causes the block to be skipped
entirely. Other valid blocks in the same event still decode
correctly.
Trade-off: those skipped blocks lose their per-interval data
(peaks + frequencies). Acceptable until the extension format is
reverse-engineered — better than propagating bogus values into PVS
computations downstream.
The 24 existing tests all still pass — the fixtures used during the
original codec development don't exercise the extension-byte case.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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