function parse(header) {
  const protocols = new Set();
  let start = -1;
  let end = -1;
  let i = 0;

  for (i; i < header.length; i++) {
    const code = header.charCodeAt(i);

    if (end === -1 && tokenChars[code] === 1) {
      if (start === -1) start = i;
    } else if (
      i !== 0 &&
      (code === 0x20 /* ' ' */ || code === 0x09) /* '\t' */
    ) {
      if (end === -1 && start !== -1) end = i;
    } else if (code === 0x2c /* ',' */) {
      if (start === -1) {
        throw new SyntaxError(`Unexpected character at index ${i}`);
      }

      if (end === -1) end = i;

      const protocol = header.slice(start, end);

      if (protocols.has(protocol)) {
        throw new SyntaxError(`The "${protocol}" subprotocol is duplicated`);
      }

      protocols.add(protocol);
      start = end = -1;
    } else {
      throw new SyntaxError(`Unexpected character at index ${i}`);
    }
  }

  if (start === -1 || end !== -1) {
    throw new SyntaxError('Unexpected end of input');
  }

  const protocol = header.slice(start, i);

  if (protocols.has(protocol)) {
    throw new SyntaxError(`The "${protocol}" subprotocol is duplicated`);
  }

  protocols.add(protocol);
  return protocols;
}
Parses the `Sec-WebSocket-Protocol` header into a set of subprotocol names.

@param {String} header The field value of the header
@return {Set} The subprotocol names
@public