Compare A/B/C/D and undo

tools/compare.mjs holds four unnamed sets of a processor's control values, and an undo ring, as pure functions: a document and the running values in, a new document and the parameter moves out. A host reads its values, calls the function, applies the moves through its own parameter door, and stores the document with its session. It works for any plugin: it only ever moves parameters. Presets are not touched.

JavaScript imports @openmixer/omx-panel/compare. Any other host runs the command, one JSON object in, one out:

omx-panel compare switch --to B     { "doc": ..., "values": { ... } }
omx-panel compare copy --to C       { "doc": ..., "values": { ... } }
omx-panel compare undo              { "ring": ..., "captures": [ { ... } ], "values": { ... } }
omx-panel compare redo              { "ring": ..., "values": { ... } }
                                 -> { "doc": ..., "ring": ..., "moves": [ { "symbol": "ratio", "value": 3 } ], "unknown": [ ] }

Leave out doc or ring for a new one. captures is a queue of value sets to take before the step, in order, so a host starts no process while a knob turns. A refusal exits 1 and names its code on stderr; a usage error exits 2.

The document

{ "format": "org.openmixer.compare/1", "active": "A",
  "slots": { "A": { "threshold": -18, "ratio": 3 }, "C": { "threshold": -30, "ratio": 8 } } }

slots holds the slots used. Values are finite numbers, under whatever key the host names a control by.

The operations

Function	Does	Moves
`switchTo(doc, values, to)`	stores the running values in the slot left and makes `to` the running one	one per control that differs from `to`, sorted by symbol; none when `to` is empty, which takes a copy
`copyTo(doc, values, to)`	puts the running values in `to`	none
`capture(ring, values)`	keeps the values as a step; the same values twice are one step, and a new step ends what could be redone; at most 64	none
`undo(ring, values)`, `redo(ring, values)`	steps back or forward, keeping the running values on the other side	to the step's values; none at either end

A control a slot holds that the running plugin does not is named in unknown, not moved, and kept in the slot. A control the slot lacks keeps its value.

Refusal	When
`COMPARE_NO_SLOT`	a slot that is not `A`, `B`, `C` or `D`
`COMPARE_FORMAT`	a document of another format
`COMPARE_VALUE`	a value that is no finite number
`COMPARE_COPY_TO_RUNNING`	a copy onto the slot running

Only parameters move: a plugin's non-parameter state stays where it is, so a plugin whose sound lives in its state compares only what its parameters reach.