docs: private static double coverRadius(double[][] matrix, int[] idx, int i) {
      final int idx_i = idx[i];
      final double[] row_i = matrix[i];
      double m = 0;
      for(int j = 0; j < row_i.length; j++) {
        if(i != j && idx_i == idx[j]) {
          final double d = row_i[j];
          m = d > m ? d : m;
        }
      }
      return m;
    }