fix: correct DSC decoder check-bit formula, add de-interleaving and 1…

[blackwellj]

…-bit error tolerance

Summary

The DSC decoder had several critical bugs that prevented correct decoding of DSC (Digital Selective Calling) messages per ITU-R M.493-16:

Bug fixes

1. Wrong check-bit formula — used even parity (total 1s must be even) instead of the correct ITU-R M.493-16 algorithm: 3 check bits encode the count of B-elements (binary 0) among the 7 info bits as a 3-bit number (MSB first). This caused valid symbols to be rejected and invalid symbols to pass.

2. Missing de-interleaving — DSC transmits every symbol twice (DX + RX copies ~5 positions apart). Without de-interleaving, duplicated symbols corrupted field boundaries. Added format-specifier anchor detection that takes every 2nd symbol to recover the original data.

3. Format code mappings reversed — 102 was ALL_SHIPS (should be GEOGRAPHICAL_AREA), 112 was INDIVIDUAL (should be DISTRESS), 114 was INDIVIDUAL_ACK (should be GROUP), 116 was GROUP (should be ALL_SHIPS), 120 was DISTRESS (should be INDIVIDUAL), 123 was ALL_SHIPS_URGENCY_SAFETY (should be AUTOMATIC_SERVICE). Fixed all per ITU-R M.493-16.

4. Missing format specifiers — added 100 (SELECTIVE_CALL), 113 (INDIVIDUAL_ACK), 115 (SHIP_POSITION), 118 (DISTRESS_RELAY), 121 (MEDICAL_TRANSPORT).

5. Wrong position encoding — treated position symbols as 2-digit BCD values instead of single digits 0-9 per ITU-R M.493-11. Quadrant values were also wrong (0-3 instead of 10/11/00/01).

6. Wrong distress nature codes — used value+100 (100-110) instead of the correct 0-31 range (lower 5 bits of symbol value).

7. Wrong phasing symbol stripping — stripped symbols 120-126 unconditionally instead of using de-interleaving.

Improvements

• 1-bit error tolerant symbol decoding: corrects single-bit errors during symbol-to-value conversion, dramatically improving decode rate on noisy signals.
• De-interleaving with format-specifier anchor detection: handles the DX/RX duplication pattern in transmitted DSC messages.
• Reusable symbol encoding/decoding utilities at module level (encode_symbol, bits_to_symbol, decode_mmsi, decode_position).
• Updated all tests to match correct ITU-R M.493-16 specifications.

How to verify