presto-main/src/main/java/io/prestosql/operator/aggregation/TypedSet.java/TypedSet/add
class TypedSet:    public void add(Block block, int position)
    {
        requireNonNull(block, "block must not be null");
        checkArgument(position >= 0, "position must be >= 0");

        // containsNullElement flag is maintained so contains() method can have shortcut for null value
        if (block.isNull(position)) {
            containsNullElement = true;
        }

        int hashPosition = getHashPositionOfElement(block, position);
        if (blockPositionByHash.get(hashPosition) == EMPTY_SLOT) {
            addNewElement(hashPosition, block, position);
        }
    }