suaveplan/packages/ui/react/react-midi/src/midi-contract/codec.ts

import type { Channel, MidiMessage } from "./midi-message.types.js";

const ASSERT_CHANNEL = 0xf;
const STATUS_HIGH_NIBBLE = 0xf0;
const STATUS_LOW_NIBBLE = 0x0f;
const SYSEX_START = 0xf0;
const SYSEX_END = 0xf7;

function asChannel(n: number): Channel {
    return (n & ASSERT_CHANNEL) as Channel;
}

/**
 * Decode a single MIDI 1.0 message from a raw `Uint8Array`. Returns
 * the parsed `MidiMessage` plus the byte offset immediately after
 * the consumed message. SysEx without a terminating `0xF7` returns
 * `null` (caller treats as malformed). Status byte must be in the
 * high half of the byte (`>= 0x80`).
 */
export function decodeMessage(
    bytes: Uint8Array,
    offset = 0
): {
    readonly msg: MidiMessage;
    readonly next: number;
} | null {
    if (offset >= bytes.length) {
        return null;
    }
    const status = bytes[offset];
    if (status === undefined || (status & 0x80) === 0) {
        return null;
    }
    const high = status & STATUS_HIGH_NIBBLE;
    const ch = asChannel(status & STATUS_LOW_NIBBLE);

    if (high === 0x80) {
        const d1 = bytes[offset + 1];
        const d2 = bytes[offset + 2];
        if (d1 === undefined || d2 === undefined) return null;
        return {
            msg: { type: "noteOff", channel: ch, note: d1, velocity: d2 },
            next: offset + 3,
        };
    }
    if (high === 0x90) {
        const d1 = bytes[offset + 1];
        const d2 = bytes[offset + 2];
        if (d1 === undefined || d2 === undefined) return null;
        const type = d2 === 0 ? "noteOff" : "noteOn";
        return {
            msg: { type, channel: ch, note: d1, velocity: d2 } as MidiMessage,
            next: offset + 3,
        };
    }
    if (high === 0xa0) {
        const d1 = bytes[offset + 1];
        const d2 = bytes[offset + 2];
        if (d1 === undefined || d2 === undefined) return null;
        return {
            msg: {
                type: "polyAftertouch",
                channel: ch,
                note: d1,
                pressure: d2,
            },
            next: offset + 3,
        };
    }
    if (high === 0xb0) {
        const d1 = bytes[offset + 1];
        const d2 = bytes[offset + 2];
        if (d1 === undefined || d2 === undefined) return null;
        return {
            msg: {
                type: "controlChange",
                channel: ch,
                controller: d1,
                value: d2,
            },
            next: offset + 3,
        };
    }
    if (high === 0xc0) {
        const d1 = bytes[offset + 1];
        if (d1 === undefined) return null;
        return {
            msg: { type: "programChange", channel: ch, program: d1 },
            next: offset + 2,
        };
    }
    if (high === 0xd0) {
        const d1 = bytes[offset + 1];
        if (d1 === undefined) return null;
        return {
            msg: { type: "channelPressure", channel: ch, pressure: d1 },
            next: offset + 2,
        };
    }
    if (high === 0xe0) {
        const d1 = bytes[offset + 1];
        const d2 = bytes[offset + 2];
        if (d1 === undefined || d2 === undefined) return null;
        return {
            msg: { type: "pitchBend", channel: ch, value: (d2 << 7) | d1 },
            next: offset + 3,
        };
    }
    // System common / real-time
    if (status === SYSEX_START) {
        const end = bytes.indexOf(SYSEX_END, offset + 1);
        if (end === -1) return null;
        return {
            msg: {
                type: "sysEx",
                channel: null,
                data: bytes.slice(offset + 1, end),
            },
            next: end + 1,
        };
    }
    if (status === 0xf8) {
        return { msg: { type: "clock", channel: null }, next: offset + 1 };
    }
    if (status === 0xfa) {
        return { msg: { type: "start", channel: null }, next: offset + 1 };
    }
    if (status === 0xfb) {
        return { msg: { type: "continue", channel: null }, next: offset + 1 };
    }
    if (status === 0xfc) {
        return { msg: { type: "stop", channel: null }, next: offset + 1 };
    }
    if (status === 0xfe) {
        return {
            msg: { type: "activeSensing", channel: null },
            next: offset + 1,
        };
    }
    if (status === 0xff) {
        return {
            msg: { type: "systemReset", channel: null },
            next: offset + 1,
        };
    }
    return null;
}

/**
 * Encode a single `MidiMessage` to its on-wire byte sequence.
 */
export function encodeMessage(msg: MidiMessage): Uint8Array {
    switch (msg.type) {
        case "noteOff":
            return new Uint8Array([0x80 | msg.channel, msg.note, msg.velocity]);
        case "noteOn":
            return new Uint8Array([0x90 | msg.channel, msg.note, msg.velocity]);
        case "polyAftertouch":
            return new Uint8Array([0xa0 | msg.channel, msg.note, msg.pressure]);
        case "controlChange":
            return new Uint8Array([
                0xb0 | msg.channel,
                msg.controller,
                msg.value,
            ]);
        case "programChange":
            return new Uint8Array([0xc0 | msg.channel, msg.program]);
        case "channelPressure":
            return new Uint8Array([0xd0 | msg.channel, msg.pressure]);
        case "pitchBend":
            return new Uint8Array([
                0xe0 | msg.channel,
                msg.value & 0x7f,
                (msg.value >> 7) & 0x7f,
            ]);
        case "sysEx": {
            const out = new Uint8Array(msg.data.length + 2);
            out[0] = SYSEX_START;
            out.set(msg.data, 1);
            out[out.length - 1] = SYSEX_END;
            return out;
        }
        case "clock":
            return new Uint8Array([0xf8]);
        case "start":
            return new Uint8Array([0xfa]);
        case "continue":
            return new Uint8Array([0xfb]);
        case "stop":
            return new Uint8Array([0xfc]);
        case "activeSensing":
            return new Uint8Array([0xfe]);
        case "systemReset":
            return new Uint8Array([0xff]);
    }
}