public int getOffset(ResultId resultId) {
        return toPosInt(offsetById.get(resultId));
    }
This class contains enough information about how a {@link com.yahoo.search.grouping.request.GroupingOperation} was
transformed into a list {@link com.yahoo.searchlib.aggregation.Grouping} objects, so that the results of those
queries can be transformed into something that corresponds to the original request.

@author Simon Thoresen Hult
container-search/src/main/java/com/yahoo/search/grouping/vespa/GroupingTransform.java