docs: public void histogramToStructure(int histogram[] ) {
          col_idx[0] = 0;
          int index = 0;
          for (int i = 1; i <= numCols; i++) {
              col_idx[i] = index += histogram[i-1];
          }
          nz_length = index;
          growMaxLength( nz_length , false);
          if( col_idx[numCols] != nz_length )
              throw new RuntimeException("Egads");
      }