presto-rcfile/src/main/java/io/prestosql/rcfile/RcFileWriteValidation.java/WriteChecksumBuilder/addPage
class WriteChecksumBuilder:        public void addPage(Page page)
        {
            requireNonNull(page, "page is null");
            checkArgument(page.getChannelCount() == columnHashes.size(), "invalid page");

            totalRowCount += page.getPositionCount();
            for (int channel = 0; channel < columnHashes.size(); channel++) {
                Type type = types.get(channel);
                Block block = page.getBlock(channel);
                XxHash64 xxHash64 = columnHashes.get(channel);
                for (int position = 0; position < block.getPositionCount(); position++) {
                    long hash = hashPositionSkipNullMapKeys(type, block, position);
                    longSlice.setLong(0, hash);
                    xxHash64.update(longBuffer);
                }
            }
        }