docs: private long getIdxOffsetFor(final int i) {
      checkRowOrder();
      int idx = 0;
      int offset = 0;
      for (final iHistogramRowSeq row : rows) {
        final int sz = row.size();
        if (offset + sz > i) {
          break;
        }
        offset += sz;
        idx++;
      }
      return ((long) idx << 32) | (i - offset);
    }