function parseLinkDestination(state, pos) {
  var code, level, link,
      start = pos,
      max = state.posMax;

  if (state.src.charCodeAt(pos) === 0x3C /* < */) {
    pos++;
    while (pos < max) {
      code = state.src.charCodeAt(pos);
      if (code === 0x0A /* \n */) { return false; }
      if (code === 0x3E /* > */) {
        link = normalizeLink(unescapeMd(state.src.slice(start + 1, pos)));
        if (!state.parser.validateLink(link)) { return false; }
        state.pos = pos + 1;
        state.linkContent = link;
        return true;
      }
      if (code === 0x5C /* \ */ && pos + 1 < max) {
        pos += 2;
        continue;
      }

      pos++;
    }

    // no closing '>'
    return false;
  }

  // this should be ... } else { ... branch

  level = 0;
  while (pos < max) {
    code = state.src.charCodeAt(pos);

    if (code === 0x20) { break; }

    // ascii control chars
    if (code < 0x20 || code === 0x7F) { break; }

    if (code === 0x5C /* \ */ && pos + 1 < max) {
      pos += 2;
      continue;
    }

    if (code === 0x28 /* ( */) {
      level++;
      if (level > 1) { break; }
    }

    if (code === 0x29 /* ) */) {
      level--;
      if (level < 0) { break; }
    }

    pos++;
  }

  if (start === pos) { return false; }

  link = unescapeMd(state.src.slice(start, pos));
  if (!state.parser.validateLink(link)) { return false; }

  state.linkContent = link;
  state.pos = pos;
  return true;
}
Parse link destination

  - on success it returns a string and updates state.pos;
  - on failure it returns null

@param  {Object} state
@param  {Number} pos
@api private