presto-ml/src/main/java/io/prestosql/plugin/ml/LearnStateFactory.java/SingleLearnState/enumerateLabel
class SingleLearnState:        @Override
        public int enumerateLabel(String label)
        {
            if (!labelEnumeration.containsKey(label)) {
                labelEnumeration.put(label, nextLabel);
                nextLabel++;
            }
            return labelEnumeration.get(label);
        }