import assert from 'node:assert';

export class Frame {
  static pack (head: Buffer, body: Buffer): Buffer {
    const totalLength = 9 + head.length + body.length + 1;
    const buffer = Buffer.allocUnsafe(totalLength);
    buffer[0] = 0x28;
    buffer.writeUInt32BE(head.length, 1);
    buffer.writeUInt32BE(body.length, 5);
    head.copy(buffer, 9);
    body.copy(buffer, 9 + head.length);
    buffer[totalLength - 1] = 0x29;
    return buffer;
  }

  static unpack (frame: Buffer): [Buffer, Buffer] {
    assert(frame[0] === 0x28 && frame[frame.length - 1] === 0x29, 'Invalid frame!');
    const headLen = frame.readUInt32BE(1);
    const bodyLen = frame.readUInt32BE(5);
    // assert(frame.length === 9 + headLen + bodyLen + 1, `Frame ${frame.toString('hex')} length does not match head and body lengths!`);
    return [frame.subarray(9, 9 + headLen), frame.subarray(9 + headLen, 9 + headLen + bodyLen)];
  }
}