presto-ml/src/main/java/io/prestosql/plugin/ml/EvaluateClassifierPredictionsStateSerializer.java/EvaluateClassifierPredictionsStateSerializer/serialize
class EvaluateClassifierPredictionsStateSerializer:    @Override
    public void serialize(EvaluateClassifierPredictionsState state, BlockBuilder out)
    {
        Map<String, Map<String, Integer>> jsonState = new HashMap<>();
        jsonState.put(TRUE_POSITIVES, state.getTruePositives());
        jsonState.put(FALSE_POSITIVES, state.getFalsePositives());
        jsonState.put(FALSE_NEGATIVES, state.getFalseNegatives());
        try {
            VARCHAR.writeSlice(out, Slices.utf8Slice(OBJECT_MAPPER.writeValueAsString(jsonState)));
        }
        catch (JsonProcessingException e) {
            throw new RuntimeException(e);
        }
    }