presto-spi/src/main/java/io/prestosql/spi/type/MapType.java/MapType/hash
class MapType:    @Override
    public long hash(Block block, int position)
    {
        Block mapBlock = getObject(block, position);
        long result = 0;

        for (int i = 0; i < mapBlock.getPositionCount(); i += 2) {
            result += hashPosition(keyType, mapBlock, i) ^ hashPosition(valueType, mapBlock, i + 1);
        }
        return result;
    }