Voice-Aware Music RGA

Status

Experimental — not wired into the production merge path. This module (muse/plugins/midi/_crdt_notes.py) exists to:

1. Demonstrate commutative concurrent note editing.
2. Benchmark voice-aware RGA vs. LSEQ and standard three-way merge.
3. Serve as the implementation foundation for a future live collaboration layer.

Why standard RGA is wrong for music

Standard RGA (Roh et al., 2011) orders concurrent insertions at the same position lexicographically by op_id. Two agents inserting a bass note and a soprano note at the same beat would have their pitches interleaved arbitrarily — soprano might appear before bass, producing voice crossings that are musicologically nonsensical.

Music-RGA position key

NotePosition is a NamedTuple with four fields that are compared in order:

NotePosition = (measure, beat_sub, voice_lane, op_id)

At the same (measure, beat_sub), notes are ordered by voice lane — bass before treble — preventing voice crossings regardless of insertion order.

CRDT laws

The three lattice laws hold:

1. Commutativity: merge(a, b).to_sequence() == merge(b, a).to_sequence()
2. Associativity: merge(merge(a, b), c) == merge(a, merge(b, c))
3. Idempotency: merge(a, a).to_sequence() == a.to_sequence()

Verified by tests/test_crdt.py.

Tombstone semantics

Deleted entries are tombstoned (marked tombstone=True) rather than removed. This is standard RGA: the tombstone ensures that the deleted entry's position remains stable for other replicas that may have concurrent insertions relative to it. In the join operation, tombstone wins: if either replica has deleted an entry, the merged result considers it deleted.

Voice lane assignment

Automatic voice lane assignment uses a coarse tessiture model:

Agents performing explicit voice separation can override voice_lane when calling MidiRGA.insert().

Relationship to the commit DAG

At commit time, MidiRGA.to_domain_ops(base_sequence) translates the CRDT state into canonical InsertOp / DeleteOp entries for storage in the commit record. The CRDT state itself is ephemeral — not stored in the object store.

Related files