Harmony and sequencing

Scales, chords, arps, and Euclidean rhythms, all allocation-free.

The harmony toolkit ships in @msh/device-sdk/device/assembly. Everything here is real-time safe with no allocation, so it runs in process.

Scales

60+ scales in the ScaleType catalog (the first 21 match the host's mesh scales). SCALE_NAMES and ROOT_NAMES give labels for selects.

Function	Does
`foldToScale(note, root, scaleId)`	Snap a note to the nearest scale member (ties up).
`foldToMask(note, mask)`	Same, against a raw 12-bit mask.
`transposeInScale(note, steps, root, scaleId)`	Move by scale degrees, wrapping octaves.
`degreeOf(note, root, scaleId)`	Scale-degree index of a note.
`scaleDegreeToNote(degree, octave, root, scaleId)`	MIDI note for a degree.
`scaleMask(scaleId, root)`	12-bit in-scale mask.

Or hold a Scale and re-root it to the project key per block (allocation-free):

let scale: Scale;
prepare(): void { scale = new Scale(ScaleType.Dorian, 0); }
// per block:
scale.setMask(ctx.meshScaleMask(), ctx.meshKey());
const snapped = scale.fold(note);

Scale methods: setScale, setRoot, setMask, mask, fold, transpose, degree, contains, note, intervalAt, degreeCount. Build one from the project scale with ctx.meshScale() / ctx.globalScale().

Chords

60+ qualities in ChordQuality (CHORD_QUALITY_NAMES for labels). Chords write into a StaticArray<i32> you own (size 8 covers the largest).

Function	Does
`buildChord(rootNote, quality, out)`	Write a chord's notes into `out`; returns the count.
`invert(out, count, n)`	Raise the lowest `n` voices an octave, in place.
`dropVoicing(out, count, n)`	Drop the `n`-th voice from the top an octave, in place.
`diatonicTriad(degree, octave, root, scaleId, out)`	3-note stack of thirds within a scale.
`diatonicSeventh(...)`	4-note version.

Interval constants (MAJOR_THIRD, PERFECT_FIFTH, MINOR_SEVENTH, ...) read better than raw semitones.

Rhythm and sequencing

Tool	Does
`euclid(pulses, steps, rotation, out)`	Fill a `StaticArray<bool>` with a Euclidean pattern. Re-bake only on a param change.
`Clock(beatsPerStep)`	Tempo-synced step clock. `update(ctx.ppqPosition())` is `true` on each new step; `step()`, `phase()`.
`Arpeggiator(maxNotes)`	`noteOn`/`noteOff` to collect held notes, `setPattern(ArpPattern.UpDown)`, `setOctaves`, `next()` per step.
`MonoNoteStack(maxNotes)`	Mono voice priority: `current(NotePriority.Highest \| Lowest \| Last)`.

ArpPattern: Up, Down, UpDown, DownUp, Converge, Diverge, Random, AsPlayed.

Tempo sync

Function	Returns
`noteToFreq(note)` / `freqToNote(hz)`	Pitch conversion.
`syncToHz(beats, bpm)`	A note division as a rate in Hz (tempo-synced LFO).
`syncToSamples(beats, bpm, sampleRate)`	The same in samples.
`beatsToSeconds(beats, bpm)`	Quarter notes to seconds.

Beat constants: BEATS_WHOLE, BEATS_HALF, BEATS_QUARTER, BEATS_EIGHTH, BEATS_SIXTEENTH, BEATS_THIRTYSECOND, times DOTTED or TRIPLET.

Param factories

ParamDef.scaleSelect(id, name, short), ParamDef.rootSelect(...), ParamDef.chordSelect(...) build selects wired to these catalogs. The value is the catalog index, so read it straight into ScaleType / ChordQuality. Pair with m.addSetting(...).

How-to: Play in key, Build chords, Sequence and arp.

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