E.10.INT:5.2 - Progress and a stepping stone

In a constructed example, a compressor learns a dictionary entry for a repeated substring in a fixed data block. Its total lossless description of that block, counting the dictionary and encoded data under the same convention, falls from 100 bits before the update to 80 afterwards. “The second version is more interesting” can now be recovered as “this dictionary update reduced the total description by 20 bits for this block.” If the team is choosing more training, it considers the likely further improvement and the work required.

Another construction may deserve attention even though that measure has not improved. In Stanley and Lehman’s Picbreeder example, a participant retained and developed an image resembling an alien face; a car-like image appeared through later changes. The first participant could act on the available image and its variation possibilities before knowing that destination. The practical result of this recovery is to retain the construction and its means of variation for exploration. C.18 governs an archive used for that purpose.

These cases permit different choices: improving a known performance measure and preserving a possible starting point for new work.