fix: correct Ruuvi Air history VOC/NOx decoding

[TheSomeMan]

Use the flags bit as bit 0 of the 9-bit value, matching the DF6/E1 decoders and Ruuvi reference implementation. Update regression tests accordingly.

Fixes #295

Fix Ruuvi Air history VOC/NOx 9-bit decoding

Summary

This PR fixes VOC and NOx decoding in AirHistoryDecoder.

The Air history decoder was treating Flags bits 6 and 7 as the most significant bit of the reconstructed 9-bit VOC/NOx values:

value = value_byte | (flag_bit << 8)

However, the Ruuvi encoding stores:

• value bits [8:1] in the data byte
• value bit [0] (LSB) in the corresponding Flags bit

The correct decoding is therefore:

value = (value_byte << 1) | flag_lsb

Changes

• Fix AirHistoryDecoder._get_9bit_value() to reconstruct the 9-bit value using the Flags bit as the LSB.
• Rename the local variable from value_bit9 to value_lsb to make the bit significance explicit.
• Update the Air history VOC/NOx test vectors to use the correct encoding.
• Add regression coverage where the Flags bit is set, so the old MSB-style decoder would fail.

Correct decoding

VOC:

voc = (voc_byte << 1) | ((flags >> 6) & 0x01)

NOx:

nox = (nox_byte << 1) | ((flags >> 7) & 0x01)

Example regression case

VOC byte = 0x01
Flags b6 = 1

Correct:
(0x01 << 1) | 1 = 3

Previous behavior:
0x01 | (1 << 8) = 257

and:

NOx byte = 0x02
Flags b7 = 1

Correct:
(0x02 << 1) | 1 = 5

Previous behavior:
0x02 | (1 << 8) = 258

Consistency with existing decoders

This also makes AirHistoryDecoder consistent with the existing DF6 and E1 advertisement decoders in this project, which already treat the Flags bit as value bit [0].

It also matches the Ruuvi reference implementations:

• Ruuvi Air history reference decoder:
  https://github.com/ruuvi/ruuvi.air.ble_nus/blob/master/scripts/ruuvi_ble_nus_read_hist.py

• Generic Ruuvi 9-bit endpoint decoder:
  https://github.com/ruuvi/ruuvi.endpoints.c/blob/master/src/ruuvi_endpoints_internal.h

Invalid value handling

The invalid value remains 0x1FF:

(0xFF << 1) | 1 = 0x1FF

so existing invalid-value handling remains valid.

Related documentation

The Ruuvi documentation currently uses wording such as "bit 9" alongside "least significant bit in Flags byte", which can be interpreted ambiguously.

The intended representation is:

value byte: bits [8:1]
Flags bit:  bit [0] (LSB)

The Ruuvi documentation is being clarified separately to make this explicit.

Summary by CodeRabbit

• Bug Fixes

  • Corrected decoding of 9-bit VOC and NOx measurements in Air history data.
  • Updated interpretation of encoded measurement bits, including values stored across data and flag fields.
  • Corrected decoded values for records using VOC/NOx flag bits.
  • Preserved existing handling of invalid or sentinel measurement values.

• Documentation

  • Updated Air history protocol documentation to reflect the corrected VOC/NOx encoding.

[coderabbitai]

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• tests/integration/test_integration_bleak_air_history.py

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📝 Walkthrough

Walkthrough

The Air history decoder now interprets VOC and NOx data bytes as bits 8–1 and flag bits as bit 0. Documentation, unit tests, and integration packet generation use the corrected 9-bit encoding. Sentinel handling remains unchanged.

Changes

Air history decoding

Layer / File(s)	Summary
Correct 9-bit decoding and regression coverage ruuvitag_sensor/decoders/air_history_decoder.py, tests/decoders/test_air_history_decoder.py	_get_9bit_value now shifts the data byte left and combines the selected flag bit as the least-significant bit. Protocol documentation, test vectors, expected values, and invalid-value coverage reflect the corrected encoding.
Correct integration packet encoding tests/integration/test_integration_bleak_air_history.py	The integration helper packs valid VOC and NOx values using data-byte bits 8–1 and flag bits 0. Values at or above 0x1FF use the invalid sentinel representation.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to 3baad

Air-history VOC and NOx values now decode according to the corrected 9-bit wire format, including flag-bit LSBs and invalid-value handling. The corresponding unit and integration packet coverage has been updated, with no remaining merge-blocking risk identified.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely identifies the correction to Ruuvi Air history VOC/NOx decoding.
Linked Issues check	✅ Passed	The changes satisfy issue #295 by reconstructing VOC and NOx with the Flags bit as the least-significant bit, updating regression tests, preserving 0x1FF invalid handling, and correcting test record g…
Out of Scope Changes check	✅ Passed	All modified files directly support issue #295. No unrelated code or test changes are present.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 7 functions across 3 files.

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Actionable comments posted: 1

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Inline comments:
In `@ruuvitag_sensor/decoders/air_history_decoder.py`:
- Line 88: Update create_air_history_record to encode 9-bit values using value
>> 1 for bytes 21 and 22 and value & 0x01 for flag bits 6 and 7, matching
AirHistoryDecoder._get_9bit_value. Reject values greater than or equal to 0x1FF
because it is the decoder’s invalid marker.

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ℹ️ Review info

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Configuration used: defaults

Review profile: CHILL

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• ruuvitag_sensor/decoders/air_history_decoder.py
• tests/decoders/test_air_history_decoder.py

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