fromPgn: implement DECIMAL field type (fixes PGN 129808 DSC MMSI decode)

[fakehec]

Implements the DECIMAL field type in readValue(), which was previously unhandled.

Problem

FieldType: "DECIMAL" had no branch in readValue(), so a 40-bit DECIMAL field fell through to the generic binary reader (bs.readBits) and was decoded as a little-endian integer. Per the canboat spec, DECIMAL means each byte holds two decimal digits (00–99).

This is used by PGN 129808 (DSC Call Information) for dscMessageAddress and mmsiOfShipInDistress, so DSC consumers such as signalk-dsc logged incorrect MMSIs for every call. Verified against a Raymarine MFD, which decodes the same on-wire PGN correctly.

DECIMAL is used only by PGN 129808 in the current spec, so blast radius is minimal.

Fix

• Decode each byte as two decimal digits and concatenate.
• Return a digit string to preserve leading zeros (e.g. coast-station identities 00MIDxxxx).
• All-0xFF (or any byte > 99) → null (not available).
• Bitstream advance is identical to the previous binary read, so downstream field alignment is unchanged.

Verified end-to-end with a real DSC position-response frame: the MMSI now decodes correctly (matching the MFD) instead of the previous bogus integer.

Closes #451

Summary by CodeRabbit

• Bug Fixes
  • Improved decoding of decimal values, preserving leading zeros and correctly interpreting packed decimal digits.
  • Decimal values are now reported as unavailable when data is truncated, undefined, contains invalid bytes, or exceeds the supported range.
  • Improved handling of incomplete bit-aligned fields to prevent incorrect readings.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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• lib/fromPgn.ts

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📝 Walkthrough

Walkthrough

readValue now validates DECIMAL fields, detects invalid bytes across the full field, consumes non-byte-aligned remainder bits, and returns padded digit strings or null.

Changes

DECIMAL decoding

Layer / File(s)	Summary
Packed decimal readValue path lib/fromPgn.ts	The decoder rejects truncated fields, tracks invalid bytes above 99, consumes remaining bits, and returns null for invalid fields. Valid bytes remain zero-padded digit strings.

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

Merge Risk: ⚪ Minimal · up to abfee

The PR makes a localized DECIMAL decoding correction; no actionable merge-blocking risk remains after normal checks and review.

Suggested reviewers: sbender9

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly identifies the DECIMAL implementation and its PGN 129808 MMSI decoding fix.
Linked Issues check	✅ Passed	The changes satisfy the DECIMAL decoding, validation, alignment, and PGN 129808 correction requirements in [#451].
Out of Scope Changes check	✅ Passed	The changes are limited to DECIMAL decoding behavior and related bitstream handling described in [#451].

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

Warning

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lib/fromPgn.ts

ESLint skipped: missing config or dependency (missing-dependency). The ESLint configuration references a package that is not available in the sandbox.

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[coderabbitai]

Actionable comments posted: 2

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@lib/fromPgn.ts`:
- Around line 1278-1285: Update the byte-validation loop in the surrounding
parser to use a boolean invalid/unavailable flag that is set when any byte
exceeds 99 and never reset by later valid bytes. Return [null, undefined]
whenever that flag is triggered, including for isolated 0xFF bytes, while
preserving normal string construction for valid bytes.
- Around line 1274-1284: Update the DECIMAL handling block around nbytes and its
byte-reading loop to resolve bitLength safely, detect insufficient bs.bitsLeft
before reading, and handle truncated input without allowing an uncaught
bitstream exception. After processing the full-byte portion, consume any
remaining bitLength remainder so downstream fields stay aligned, and update the
return logic below the loop to use the existing isValid state.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 5cc521ab-af6d-4272-8bbc-236be3dcd5e0

📥 Commits

Reviewing files that changed from the base of the PR and between c584d70 and 94fbae1.

📒 Files selected for processing (1)

• lib/fromPgn.ts

[fakehec]

Addressed the CodeRabbit review:

• Invalid-byte detection now latches — a valid byte following an 0xFF/>99 byte can no longer unmask the field (e.g. [0xFF, 0x12] yields null instead of "25518").
• Bitstream underflow on truncated packets is guarded before reading (this block runs outside the surrounding try/catch).
• Non-byte-aligned remainder bits are now consumed so downstream fields stay aligned when BitLength isn't a multiple of 8.

Thanks for the review.