Playing strength first, I collected a number of changes during the last 4 years resulting in a ~35 elo improvement. ~20 elo came from a new idea replacing IID with what I called IIR. A number of other engines (among them Stockfish) adopted the idea.
Polyglot opening books, improved the standard ProDeo opening book with +46 elo, see elsewhere. Created additional books with new options, an overview:
Second book ProDeo will search if first book fails.
[Book Depth = 80]
Maximum plies ProDeo will search books.
[Book Use = vary]
User priority preference, there are 3 options -
vary - Play varied based on the weight, see one of the 4 screenshots.
score - Play the move with the highest Stockfish 12 score.
learn - Play the move with the highest learn value.
[WDL Factor = 250]
Randomize the book, in a nutshell, a low value makes a book very random, a high value narrows the book. The following example makes the effect visible and note what happens to the weights of the big-four 1. e4 | 1. d4 | 1. Nf3 | 1. c4
USB.bin is a follow up of the Ultimate Short Book project and only contains opening positions less than the first 11 moves preventing out-booking. The size of the book has been increased from 14 to 31 million positions.
External use of the 4 Polyglot books including the ProDeo score and learn options is possible with the POLY tool using the 2 recalculate weights options.
Baretail is a tool that immediately updates changed text files on the screen and is automatically loaded at program start. As such one can follow the book moves and its alternatives on the fly. To remove it close it or delete the executable for a permanent solution.