Similarity between the top engines during 2006-2014 and now, anno 2019
With the release of the open source code of Fruit 2.1 many clones / derivatives appeared, later followed by many clones / derivatives of the hacked Rybka 3. We will give a short overview of both periods, the Fruit-family and the Rybka-family and then look into the situation of the top engines today.
Procedure - Somewhere in 2010/11 programmer Don Daily of Komodo released a tool (called Similarity Tester) that measures the similarity between engines.
Legenda - before viewing the results of the Fruit-family and the Rybka-family consider the meaning of the percentages first.
1. A similarity percentage of over 70% means that engine is a clone with only a few changes.
2. A similarity percentage between 65-69% is considered a clone with considerable changes.
3. A similarity percentage between 60-64% is called a derivative work.
4. A similarity percentage between 55-59% is the grey area, likely a derivative work but impossible to conclude for sure.
5. A similarity percentage between 50-54% Still a bit high, impossible to draw a conclusion.
My recollection the general consensus on these percentages was about 80-90% among the chess programmers, a few wanted to shift the percentages one place up or one place down.
Limitations - Similarity Tester can only proof a clone or a derivative work, it can not conclude the opposite. An engine might score a similarity percentage of 40% (an indication of a very original engine) it still can be a derivative work. There are some tricks to fool Similarity Tester without much elo loss.
And now for anno 2019 and notice the current healthy situation, computer chess healed itself.
And out of curiosity I also put Rebelfish to the test and measure the similarity between Stockfish and ProDeo with its flexible score margins.