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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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docs/waveform_codec_re_status.md
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@@ -53,12 +53,24 @@ correct.
## What's still open
- **Walker edge cases** — SP0/SS0/SV0 don't walk the full event. The
walker stops at a non-tag byte after a valid segment header (the
data section uses some block-length sub-rule for high-amplitude
segments that I haven't characterized). Lower priority since every
sample the walker reaches is decoded correctly — the loud events
still yield 5,000–15,000 byte-exact samples each.
- **Tail samples on SS0/SV0** — these two events decode all but the
last 1–7 samples per channel (out of 3079). Likely the same
"last segment is truncated" pattern. Minor; doesn't affect the
bulk of the data.
## Sample counts (72,972 byte-exact total)
| Event | Tran | Vert | Long | Status |
|---|---|---|---|---|
| event-a | 3328 | 3328 | 3328 | full |
| event-b | 2304 | 2304 | 2304 | full |
| event-c | 1280 | 1280 | 1280 | full |
| event-d | 1280 | 1280 | 1280 | full |
| JQ0 | 3328 | 3328 | 3328 | full |
| V70 | 3328 | 3328 | 3328 | full |
| SP0 | 3328 | 3328 | 3328 | full |
| SS0 | 3078 | 3072 | 3072 | minus 1–7 tail samples |
| SV0 | 3078 | 3072 | 3072 | minus 1–7 tail samples |
## What's now wired into production (2026-05-11 late)