docs: @Override
      public Object map(Object input) {
          List row = (List) input;
          String[] first = row.get(0).toString().split(delimiter);
          String[] second = row.get(1).toString().split(delimiter);
          String[] stdev = columns.length > 2 ? row.get(2).toString().split(delimiter) : null;
  
          double dist = 0;
          for (int i = 0; i < first.length; i++) {
              double d = Double.parseDouble(first[i]) - Double.parseDouble(second[i]);
              double s = stdev != null ? Double.parseDouble(stdev[i]) : 1;
              dist += (d * d) / (s * s);
          }
          return Math.sqrt(dist);
      }