code: private ValueFrequency getDistinctValueCount(final ValueCountingAnalyzerResult res) {
          int distinctValueCounter = 0;
          ValueFrequency valueCount = null;
          if (res.getNullCount() > 0) {
              distinctValueCounter++;
              valueCount = new SingleValueFrequency(LabelUtils.NULL_LABEL, res.getNullCount());
          }
          final int uniqueCount = res.getUniqueCount();
          if (uniqueCount > 0) {
              if (uniqueCount > 1) {
                  // more than one distinct value
                  return null;
              }
              distinctValueCounter++;
              final Collection<String> uniqueValues = res.getUniqueValues();
              String label = LabelUtils.UNIQUE_LABEL;
              if (!uniqueValues.isEmpty()) {
                  label = uniqueValues.iterator().next();
              }
              valueCount = new CompositeValueFrequency(label, 1);
          }
  
          if (distinctValueCounter > 1) {
              // more than one distinct value
              return null;
          }
  
          final Collection<ValueFrequency> valueCounts = res.getValueCounts();
          if (valueCounts.size() > 0) {
              distinctValueCounter += valueCounts.size();
              valueCount = valueCounts.iterator().next();
          }
          if (distinctValueCounter > 1) {
              // more than one distinct value
              return null;
          }
          return valueCount;
      }
docs: Determines if a group result has just a single distinct value count. If
  so, this value count is returned, or else null is returned.
  
  @param res
  @return