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codec: crack wide-NN blocks (1X NN / 2X NN); loud events now fully decode

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When NN exceeds 0xFC, the codec extends to 12-bit NN by using the
low nibble of the TYPE byte as the high nibble of NN:

    1X NN  →  nibble-delta block, NN = (X << 8) | NN_byte
    2X NN  →  int8-delta block, same NN encoding

Walker and decode_waveform_v2 now handle both narrow (X=0) and wide
(X != 0) forms uniformly.

Discovered while investigating why SP0/SS0/SV0/event-b walkers stopped
mid-event.  SP0 segment 12 (V continuation, cycle 3) starts with
"11 90" — high nibble of byte 0 = 1 (= nibble-delta block type), low
nibble = 1 plus byte 1 = 0x90 → NN = 0x190 = 400 nibble deltas in
202 bytes.  Walker was rejecting "11" as a non-tag.

Sample count went from 47,364 to 72,972 verified byte-exact:

  event-a:  9984 (full)        was 9984 (full)
  event-b:  6912 (full)        was   738
  event-c:  3840 (full)        was 3840 (full)
  event-d:  3840 (full)        was 3840 (full)
  JQ0:      9984 (full)        was 9984 (full)
  V70:      9984 (full)        was 9984 (full)
  SP0:      9984 (full)        was 5122
  SS0:      9222 (-7 tail)     was 1758
  SV0:      9222 (-7 tail)     was 2114

7 of 9 fixtures now decode end-to-end across all 3 geo channels.
The 2 remaining (SS0, SV0) are missing only 1-7 tail samples per
channel — minor walker edge case at the very end.

74 tests pass (was 71).
This commit is contained in:
Claude
2026-05-16 03:29:13 +00:00
committed by serversdown
parent 85f4bcfe86
commit 0466bb4f44
4 changed files with 77 additions and 28 deletions
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CLAUDE.md
+17 -5
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@@ -114,6 +114,14 @@ custom delta + RLE + variable-width codec.
blocks in the fixture bundle. 12-bit was chosen because ±2047 in
16-count units ≈ ±10 in/s = the geophone's full-scale range at
Normal sensitivity.
- **Wide-NN blocks (`1X NN`, `2X NN`)** — when a `10 NN` or `20 NN`
block's NN would exceed 0xFC, the codec uses a 12-bit NN encoding:
the low nibble of the type byte holds the high nibble of NN (so the
type byte appears as e.g. `0x11` instead of `0x10`). Effective
NN = `((type_byte & 0x0F) << 8) | nn_byte`. Block length follows
the same formula as the narrow form (`NN/2 + 2` for nibble blocks,
`NN + 2` for int8 blocks). Confirmed 2026-05-11 against SP0 cycle
3 V continuation (`11 90` = NN=400 nibble deltas in 202 bytes).
### What's NOT solved
@@ -131,16 +139,20 @@ custom delta + RLE + variable-width codec.
| Event | Tran | Vert | Long | Total |
|---|---|---|---|---|
| event-a | 3328 | 3328 | 3328 | **9984** ← full event |
| event-b | 2304 | 2304 | 2304 | **6912** ← full event |
| event-c | 1280 | 1280 | 1280 | 3840 ← full event |
| event-d | 1280 | 1280 | 1280 | 3840 ← full event |
| JQ0 | 3328 | 3328 | 3328 | **9984** ← full event |
| V70 | 3328 | 3328 | 3328 | **9984** ← full event |
| SP0 | 2048 | 1538 | 1536 | 5122 (walker stops early) |
| SS0 | 734 | 512 | 512 | 1758 (walker stops early) |
| SV0 | 1024 | 578 | 512 | 2114 (walker stops early) |
| event-b | 512 | 226 | 0 | 738 (walker stops early) |
| SP0 | 3328 | 3328 | 3328 | **9984** ← full event |
| SS0 | 3078 | 3072 | 3072 | 9222 (1–7 tail samples missing) |
| SV0 | 3078 | 3072 | 3072 | 9222 (1–7 tail samples missing) |
**Total: 47,364 ADC samples verified byte-exact, zero errors.**
**Total: 72,972 ADC samples verified byte-exact, zero errors.**
7 of 9 fixture events decode end-to-end across all three geo channels.
The remaining two (SS0 / SV0) decode all but the last 1–7 samples per
channel — a minor walker edge case.
### Production-code status (updated 2026-05-11 late)