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feat(histogram): decode multi-interval blocks — recovers 415 files

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Sub-minute histogram intervals are packed several to a block so that
every block still covers exactly one minute of data:

    interval   intervals/block   stride
    1 minute   1                 32     <- the standard big-endian block
    15 s       4                 92
    2 s        30                612

    stride = 12 + n * 20

Block = [00][segment][ctr uint16 LE][0a][00], then n x 20-byte records of
8 x uint16 LITTLE-endian values (T_peak, T_halfp, V_peak, V_halfp,
L_peak, L_halfp, M_peak, M_halfp) plus a 2-word tail whose first word is
0000 on every real interval, then a 6-byte block trailer.

The standard 32-byte block is BIG-endian; this variant is LITTLE-endian.

The tail-word check matters: a session ending mid-block leaves buffer
garbage in the remaining interval slots, which decoded as peaks
thousands of times the real value.  Stride detection also requires at
least 2 records, since a 1-record block would have stride 32 and
collide with the standard block.

Recovers 415 files that decoded to nothing: 216 on BE18193 (2 s
intervals) and 199 on BE9440 (15 s).  Before decoding to nothing they
were being accepted by the WAVEFORM codec, which returned garbage
peaking up to 400x the device-reported PPV.

Ground truth BE9440/K440L3AQ.T70H (5,710 intervals) matches its
Blastware ASCII export exactly: 17,130/17,130 geo peaks, 22,840/22,840
frequencies, 5,710/5,710 mic dB(L).  Across all 455 affected files,
1,354/1,365 channel peaks (99.2%) match the device-reported PPV; the 11
that don't are under-reads on BE9440 where the walk stops early.

Fixture (binary + ASCII) saved under tests/fixtures/, which is
gitignored per repo practice — the ground-truth test skips when absent.

Tests: 258 passed, failure list unchanged from baseline.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HgTe8CamXAHcAmaQ6QNcog
This commit is contained in:
serversdown
2026-08-26 04:59:00 +00:00
co-authored by Claude Opus 5
parent 4c58a532de
commit 306104354b
5 changed files with 333 additions and 3 deletions
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CLAUDE.md
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@@ -284,6 +284,31 @@ rotation and corrupts every channel after it.
Corpus result, end to end through the production path:
**exact 37 → 72, truncated 23 → 3, full-length value errors 15 → 0.**
### Histogram codec — multi-interval blocks (2026-08-26)
Sub-minute histogram intervals are packed several to a block, so every
block still covers exactly one minute:
| interval | intervals/block | stride |
|---|---|---|
| 1 min | 1 | 32 (the standard big-endian block) |
| 15 s | 4 | 92 |
| 2 s | 30 | 612 |
`stride = 12 + n * 20`. Block = `[00][segment][ctr uint16 LE][0a][00]`,
then n x 20-byte records of 8 x uint16 **LITTLE**-endian values
(`T_peak, T_halfp, V_peak, V_halfp, L_peak, L_halfp, M_peak, M_halfp`)
plus a 2-word tail whose first word is `0000` on every real interval,
then a 6-byte block trailer.
⚠ The standard 32-byte block is BIG-endian; this variant is LITTLE-endian.
Recovers **415 files** (216 on BE18193, 199 on BE9440) that decoded to
nothing. Ground truth `BE9440/K440L3AQ.T70H` matches its BW ASCII export
on every one of 17,130 geo peaks, 22,840 frequencies and 5,710 mic dB(L)
values; across all 455 affected files 1,354/1,365 channel peaks (99.2%)
match the device-reported PPV.
### Histogram codec — corrected 2026-08-25
The histogram block is **uniformly big-endian**, and the stream's final