seismo-relay/minimateplus/histogram_codec.py

"""
histogram_codec.py — decoder for MiniMate Plus histogram-mode event bodies.

FULLY DECODED 2026-05-20.  Every field in every block, verified
byte-exact against BW's ASCII export across multiple histogram
fixtures.

The histogram-mode body is a stream of 32-byte fixed-length blocks,
one block per histogram interval.  Each block carries the per-interval
peak amplitude + zero-crossing frequency for all four channels (Tran,
Vert, Long, MicL).

────────────────────────────────────────────────────────────────────────────
Body layout (CONFIRMED 2026-05-20)
────────────────────────────────────────────────────────────────────────────

    [stream of 32-byte blocks]

Body length is approximately ``n_intervals * 32`` bytes plus a small
trailing remnant (1-9 bytes typically) at the very end.  Walker should
iterate 32-stride and stop before the tail.

────────────────────────────────────────────────────────────────────────────
32-byte block layout
────────────────────────────────────────────────────────────────────────────

    [0]    0x00                      always-zero tag
    [1]    segment_id  (uint8)       0x00..0x03 — 256 blocks per segment
    [2:4]  block_ctr  (uint16 LE)    resets each segment (0x0100, 0x0101, …)
    [4:6]  0x000a (uint16 LE)        constant marker (= 10)
    [6:8]  T_peak_count   uint16 LE  Tran peak (count × 0.005 → in/s)
    [8:10] T_halfperiod   uint16 LE  Tran half-period in samples (freq = 512 / halfp Hz)
    [10:12] V_peak_count  uint16 LE
    [12:14] V_halfperiod  uint16 LE
    [14:16] L_peak_count  uint16 LE
    [16:18] L_halfperiod  uint16 LE
    [18:20] M_peak_count  uint16 LE  MicL peak (count → dB via mic_count_to_db)
    [20:22] M_halfperiod  uint16 LE  MicL half-period in samples (freq = 512 / halfp Hz)
    [22:24] 0x00 0x00                constant
    [24:28] 4-byte variable          purpose unknown (possibly CRC or timestamp delta)
    [28:32] 0x1e 0x0a 0x00 0x00      constant block-end signature

Block-identification anchor: ``block[22:24] == b"\\x00\\x00"`` AND
``block[28:32] == b"\\x1e\\x0a\\x00\\x00"``.  This is the reliable
distinguisher from non-block content in the file.

────────────────────────────────────────────────────────────────────────────
Per-channel encoding
────────────────────────────────────────────────────────────────────────────

Geophone channels (Tran, Vert, Long):
  - peak_count × 0.005 = peak amplitude in in/s at Normal range
  - half-period in samples → freq_Hz = 512 / half-period

Microphone channel (MicL):
  - peak_count → dB via the same formula used by the waveform codec:
        dB = sign(c) × (81.94 + 20·log10(|c|))    for |c| ≥ 1
        dB = 0                                    for c == 0
  - half-period → freq_Hz = 512 / half-period (same as geo)

Frequency `>100 Hz` sentinel: the device emits half-period ≤ 5 when the
measured zero-crossing rate exceeds the geophone's measurement range
(since 512/5 = 102 Hz; the BW display rounds anything > 100 to ">100").

────────────────────────────────────────────────────────────────────────────
Output shape
────────────────────────────────────────────────────────────────────────────

``decode_histogram_body`` returns a per-channel dict matching the
waveform codec's shape so the rest of the pipeline (.h5 writer,
sidecar, viewer) consumes it without special-casing:

    {"Tran": [peak_count_i for each interval i],
     "Vert": [peak_count_i ...],
     "Long": [peak_count_i ...],
     "MicL": [peak_count_i ...]}

Values are in **16-count units for geo** (LSB = 0.005 in/s, matching
``decode_waveform_v2``) and **1-count units for mic** (matching the
waveform codec's mic convention).  Run through
``waveform_codec.decoded_to_adc_counts`` to scale geo to 1-count ADC.

Per-interval frequencies are NOT returned — they're auxiliary data,
not waveform samples.  Consumers needing frequencies can call
``decode_histogram_body_full()`` for the structured per-interval
record list.
"""

from __future__ import annotations

import struct
from typing import List, Optional, Tuple

# Block-end signature: constant `1e 0a 00 00` in bytes [28:32] of every
# real data block.  More distinctive than the byte-22 `00 00` (which
# matches many false positives), so we anchor on this.
_BLOCK_TAIL = b"\x1e\x0a\x00\x00"
_BLOCK_SIZE = 32

# Marker byte at block[4:6] of every histogram data block.  Used as
# additional validation that we're looking at a real block.
_BLOCK_MARKER = 10

# Maximum plausible peak-count value.  Normal-range geophone tops out
# at 10 in/s = 2000 counts at the 0.005 in/s per count scale; even
# Sensitive range (1.25 in/s FS) wouldn't exceed ~250.  Mic counts run
# 0..~400 in observed data.  4096 leaves comfortable headroom for any
# legitimate value across all modes.
#
# Some prod blocks have been observed with peak-count fields whose
# HIGH byte is non-zero (block[7] != 0 etc.) — observed across BE9558
# and BE18003 units in Histogram-mode events.  Reading these as
# uint16 LE produces values like 30981 / 41733 / 62469, which scale
# to physically impossible peaks (150+ in/s).  Best guess: an
# undocumented "time-of-peak-within-interval" extension byte the
# device writes in some sub-mode (possibly Histogram+Continuous).
# Until reverse-engineered, blocks exceeding this bound are skipped
# rather than propagating bogus values into PVS computations.
_MAX_PEAK_COUNT = 4096

# Geo peak scaling: stored as "count × 0.005 in/s" where 1 count = one
# 0.005 in/s display quantum.  Equivalent to the waveform codec's
# 16-count-unit output (1 unit = 0.005 in/s = 16 ADC counts).
_GEO_LSB_INS = 0.005

# Frequency formula: freq_Hz = _FREQ_NUMERATOR / half_period_samples.
# Empirically determined to be 512 (= sample_rate / 2, where sample rate
# is 1024 sps for the standard MiniMate Plus configuration).
_FREQ_NUMERATOR = 512


def _is_data_block(block: bytes) -> bool:
    """Tight identification of a histogram data block."""
    if len(block) < _BLOCK_SIZE:
        return False
    if block[28:32] != _BLOCK_TAIL:
        return False
    if block[22:24] != b"\x00\x00":
        return False
    if block[0] != 0x00:
        return False
    marker = block[4] | (block[5] << 8)
    if marker != _BLOCK_MARKER:
        return False
    return True


def _decode_block(block: bytes) -> Optional[dict]:
    """Decode one 32-byte histogram block.  Caller must have validated
    with ``_is_data_block`` first.

    Returns ``None`` if any peak field exceeds ``_MAX_PEAK_COUNT`` —
    those blocks contain an undocumented extension byte format whose
    naive uint16 LE interpretation gives physically impossible peaks.
    Skipping the block is safer than propagating bogus values into
    PVS computations downstream.
    """
    # All 16-bit fields are little-endian unsigned.  Peak counts are
    # always non-negative; half-periods are always positive when valid.
    t_peak, t_halfp, v_peak, v_halfp, l_peak, l_halfp, m_peak, m_halfp = struct.unpack_from(
        "<HHHHHHHH", block, 6
    )
    if (t_peak > _MAX_PEAK_COUNT or v_peak > _MAX_PEAK_COUNT
            or l_peak > _MAX_PEAK_COUNT or m_peak > _MAX_PEAK_COUNT):
        return None
    segment_id = block[1]
    block_ctr  = block[2] | (block[3] << 8)
    var_meta   = bytes(block[24:28])
    return {
        "segment_id":  segment_id,
        "block_ctr":   block_ctr,
        "t_peak":      t_peak,
        "t_halfp":     t_halfp,
        "v_peak":      v_peak,
        "v_halfp":     v_halfp,
        "l_peak":      l_peak,
        "l_halfp":     l_halfp,
        "m_peak":      m_peak,
        "m_halfp":     m_halfp,
        "meta_var":    var_meta,
    }


def walk_body(body: bytes) -> List[dict]:
    """Walk the body and return one dict per histogram interval.

    Iterates 32-byte strides from offset 0.  Yields a decoded record
    for every block that passes ``_is_data_block`` validation AND has
    plausible peak values (``_decode_block`` returns None for blocks
    with out-of-bound peaks).  Stops when the remaining bytes are too
    short to form a complete block.
    """
    records: List[dict] = []
    for off in range(0, len(body) - _BLOCK_SIZE + 1, _BLOCK_SIZE):
        blk = body[off:off + _BLOCK_SIZE]
        if not _is_data_block(blk):
            # Hit non-block content (likely a sync or stream marker).
            # Continue walking — block alignment is fixed at 32-stride
            # from offset 0, so we don't lose alignment by skipping.
            continue
        decoded = _decode_block(blk)
        if decoded is None:
            # Block validated as a histogram block but had peak fields
            # outside the plausible range — undocumented extension.
            # Skip rather than propagating bogus PVS contributions.
            continue
        records.append(decoded)
    return records


def decode_histogram_body(body: bytes) -> Optional[dict]:
    """Decode a histogram-mode body into per-channel peak-sample arrays.

    Returns ``{"Tran": [...], "Vert": [...], "Long": [...], "MicL": [...]}``
    where each channel's list contains one peak value per histogram
    interval (in the same units the waveform codec uses: 16-count units
    for geo, 1-count ADC units for mic).  Returns ``None`` if the body
    doesn't contain any valid histogram blocks.

    To convert to physical units:
      - Geo channels: ``count * 0.005`` = peak in in/s at Normal range
        (or run through ``waveform_codec.decoded_to_adc_counts`` first
         to get 1-count ADC values, then ``count / 32767 * 10.0`` for in/s)
      - Mic channel:  use ``waveform_codec.mic_count_to_db(count)``
    """
    records = walk_body(body)
    if not records:
        return None
    return {
        "Tran": [r["t_peak"] for r in records],
        "Vert": [r["v_peak"] for r in records],
        "Long": [r["l_peak"] for r in records],
        "MicL": [r["m_peak"] for r in records],
    }


def decode_histogram_body_full(body: bytes) -> Optional[List[dict]]:
    """Decode a histogram-mode body into the full per-interval record list.

    Same data as ``decode_histogram_body`` but in a structured form that
    preserves the half-period (frequency) data for each channel + the
    per-block segment_id, block_ctr, and 4-byte variable metadata.
    Useful for diagnostic tools, sidecar enrichment, and future-codec
    work.

    Returns ``None`` if the body has no valid blocks.
    """
    records = walk_body(body)
    return records if records else None


def half_period_to_hz(halfp: int) -> Optional[float]:
    """Convert a half-period in samples to frequency in Hz.

    Returns ``None`` for half-period ≤ 5 — the device emits values in
    that range when the measured zero-crossing rate exceeds 100 Hz
    (the BW display reports `>100 Hz` for such cases).  Callers can
    treat ``None`` as the `>100 Hz` sentinel.
    """
    if halfp <= 5:
        return None
    return _FREQ_NUMERATOR / halfp


def geo_count_to_ins(count: int) -> float:
    """Convert a histogram geo peak count to in/s at Normal range."""
    return count * _GEO_LSB_INS