# `hooks/use-rpn`

<!-- begin:auto-docs -->
_(no submodules)_
<!-- end:auto-docs -->

## Purpose

`useRPN(selector)` wraps the universal RPN reducer and exposes the
consolidated `RPNChange` as React state. RPN ("Registered Parameter
Number") covers the well-known parameters such as pitch-bend range,
fine/coarse tuning, channel pressure sensitivity, and modulation
depth. Like NRPN it arrives over the wire as a four-CC dance — CC
101 (param MSB), CC 100 (param LSB), CC 6 (data MSB), CC 38 (data
LSB) — and the hook folds that into one React event per dance.

## Features

- Folds CC 101/100/6/38 into one `RPNChange` per dance.
- Per-port reducer — fresh state on port identity change.
- Returns `null` until the first completed dance.

## Usage: Basic

```tsx
import { useRPN } from "@suaveplan/react-midi";

function PitchBendRange() {
    const change = useRPN("first");
    // RPN (0,0) is "Pitch Bend Sensitivity".
    if (!change) return <p>(no RPN yet)</p>;
    return (
        <p>
            RPN ({change.paramHigh}.{change.paramLow}) → {change.value}
        </p>
    );
}
```

## Usage: Advanced

```tsx
function useChannelTuning() {
    const change = useRPN("first");
    // RPN (0,1) = fine-tuning; RPN (0,2) = coarse-tuning.
    return change?.paramHigh === 0 ? change : null;
}
```

## API Reference

- `useRPN(selector: string | "first"): RPNChange | null`

## Implementation Notes

The hook mirrors `useNRPN` exactly, replacing only the reducer class
and the parameter-MSB CC numbers (101 instead of 99). The reducer's
data-MSB/LSB CCs (6 and 38) are shared with NRPN — devices follow
the convention of sending a fresh paramHigh selector to switch
between the two address spaces, which the reducer handles via its
"new paramHigh resets partial state" rule. Consumers that need to
listen to *both* NRPN and RPN streams from the same port simply
compose the two hooks side by side; the underlying input adapter
fans the parsed messages out to both subscriptions without
double-binding the Web MIDI `onmidimessage` slot.