function parseLinkLabel(state, start) {
  var level, found, marker,
      labelEnd = -1,
      max = state.posMax,
      oldPos = state.pos,
      oldFlag = state.isInLabel;

  if (state.isInLabel) { return -1; }

  if (state.labelUnmatchedScopes) {
    state.labelUnmatchedScopes--;
    return -1;
  }

  state.pos = start + 1;
  state.isInLabel = true;
  level = 1;

  while (state.pos < max) {
    marker = state.src.charCodeAt(state.pos);
    if (marker === 0x5B /* [ */) {
      level++;
    } else if (marker === 0x5D /* ] */) {
      level--;
      if (level === 0) {
        found = true;
        break;
      }
    }

    state.parser.skipToken(state);
  }

  if (found) {
    labelEnd = state.pos;
    state.labelUnmatchedScopes = 0;
  } else {
    state.labelUnmatchedScopes = level - 1;
  }

  // restore old state
  state.pos = oldPos;
  state.isInLabel = oldFlag;

  return labelEnd;
}
Parse link labels

This function assumes that first character (`[`) already matches;
returns the end of the label.

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