Review

class FieldsWriter {
    final void addDocument(Document doc) throws IOException {
        indexStream.writeLong(fieldsStream.getFilePointer());

        int storedCount = 0;
        Iterator fieldIterator = doc.getFields().iterator();
        while (fieldIterator.hasNext()) {
            Fieldable field = (Fieldable) fieldIterator.next();
            if (field.isStored())
                storedCount++;
        }
        fieldsStream.writeVInt(storedCount);

        fieldIterator = doc.getFields().iterator();
        while (fieldIterator.hasNext()) {
            Fieldable field = (Fieldable) fieldIterator.next();
            // if the field as an instanceof FieldsReader.FieldForMerge, we're in merge mode
            // and field.binaryValue() already returns the compressed value for a field
            // with isCompressed()==true, so we disable compression in that case
            boolean disableCompression = (field instanceof FieldsReader.FieldForMerge);
            if (field.isStored()) {
                fieldsStream.writeVInt(fieldInfos.fieldNumber(field.name()));

                byte bits = 0;
                if (field.isTokenized())
                    bits |= FieldsWriter.FIELD_IS_TOKENIZED;
                if (field.isBinary())
                    bits |= FieldsWriter.FIELD_IS_BINARY;
                if (field.isCompressed())
                    bits |= FieldsWriter.FIELD_IS_COMPRESSED;
                
                fieldsStream.writeByte(bits);
                
                if (field.isCompressed()) {
                  // compression is enabled for the current field
                  byte[] data = null;
                  
                  if (disableCompression) {
                      // optimized case for merging, the data
                      // is already compressed
                      data = field.binaryValue();
                  } else {
                      // check if it is a binary field
                      if (field.isBinary()) {
                        data = compress(field.binaryValue());
                      }
                      else {
                        data = compress(field.stringValue().getBytes("UTF-8"));
                      }
                  }
                  final int len = data.length;
                  fieldsStream.writeVInt(len);
                  fieldsStream.writeBytes(data, len);
                }
                else {
                  // compression is disabled for the current field
                  if (field.isBinary()) {
                    byte[] data = field.binaryValue();
                    final int len = data.length;
                    fieldsStream.writeVInt(len);
                    fieldsStream.writeBytes(data, len);
                  }
                  else {
                    fieldsStream.writeString(field.stringValue());
                  }
                }
            }
        }
    }

}