I’m happy to finally announce the next minor release of both of my chess programs! You can grab Windows and Linux binaries from the respective Github repos:

Prophet – https://github.com/jswaff/prophet

chess4j – https://github.com/jswaff/chess4j

If you need a Mac build for Prophet, please look here. (Thank you Darius!)

The focus of this release was to continue to improve the evaluation. Prophet 4.3 and chess4j 5.1 have the exact same change log:

• Passed pawn by rank (was a single value)
• Non-linear mobility (was a single value)
• Knight outposts
• Trapped bishop penalty

These changes are worth about +50 ELO in Prophet (which I expect will bring it very close to the 2500 mark on the CCRL Blitz List). I attempted a “supported rook” term, meaning the rook on an open file is connected to another rook, but surprisingly it actually cost a few ELO. Seems that should work though, so I’ve left the code in place but have it commented out.

I had planned on doing some pawn and basic endgame work in this line, and perhaps I still will, but right now I feel the time is right to begin work on neural networks. I’m pausing development for a while to study the literature. Hopefully by spring I’m ready to begin the implementation.

I’m happy to announce updates to both chess engines. Prophet 4.2 is approximately 50 elo stronger than 4.1, and 150 elo stronger than 4.0. (I missed a release announcement or two while this development blog was offline.) The most significant change, and the reason the chess4j major version number has been incremented, is that chess4j now includes an auto tuner! The tuner uses logistic regression with gradient descent to optimize evaluation terms. I’ll write more detail about that in a separate post. Those optimized weights have been added into Prophet, so it benefits from that work as well. Tapered evaluation has been fully implemented as well which added a few elo. I say “fully” because the king evaluation was already tapered, but now both programs fully evaluate the position with a middle game and an endgame score, and weight them based on material on the board. Some concept of mobility has been added as well – a simple count of available squares for both bishops and queens.

Here is how Prophet 4.2 stacks up against its current sparring partners in 1+0.5 games:

Some time back I tried implementing a Principal Variation Search , but as I wrote about in my post PVS – Another Fast Fail , the results were not good. At the time I concluded that if PVS is not a win, it must mean that the cost of the researches is outweighing the nodes saved by doing zero width searches. For that to be the case, it must mean that too often the first move is not the best move, which points to move ordering.

Since then move ordering has certainly improved, as documented in this post on Move Ordering . So, I decided to give PVS another try. In my first attempt, it appeared to be another loss. Then, I decided to not do PVS at the root node, and now it appears to be a very small win.

A win is a win, so I’m merging the changes in, but I think there is more to do here. My suspicion is that, as move ordering improves, the benefits of PVS will increase. The most obvious way to improve move ordering is to add a depth preferred hash table (the current strategy is a very naive “always replace”).

It seems like PVS at the root should work as well, if the program can reliably predict the best move often enough. I know a lot of programs put extra effort into ordering the moves at the root. I remember reading that Bob Hyatt’s Crafty does a quiescence search at the root. So, this is on the backlog as well, and once complete I will revisit the idea of PVS at the root.

For now, it is on to the next thing – Late Move Reductions. I’m hopeful that will yield a significant ELO increase, perhaps finally putting P4 on par with P3.

In my recent post on move ordering, I identified a potential area of improvement to the criteria for deciding if a capture is “good” or “bad.” As I wrote in that post, a capture is good if:

1. It is a promotion (technically even non-capturing promotions are included)
2. The value of the captured piece is greater than the value of the capturing piece
3. The Static Exchange Evaluator (SEE) score is non-negative.

The issue is with knights and bishops. They are roughly the same value (which one is more valuable really depends on the position), but in Prophet the bishop has a slightly higher value. A knight has a material value equal to 3 pawns, but a bishop has 3.2 pawns. The consequence of that is that a simple Bishop x Knight capture would be categorized as “bad” and not tried until all non-captures have been tried.

I don’t have the link handy but I read an older post on talkchess.com where Tord Romstad, the author of Glaurang (pre-cursor to Stockfish), mentioned that he used different piece values for the purposes of move ordering than he did in the evaluation. He said he used 1, 3, 3, 5 and 10. That means Bishop x Knight captures, as well as Knight x Bishop captures would both be categorized as “good.” Also, by giving the queen a value of 10, it means that giving two rooks for a queen would be considered equal by the SEE, where giving a queen + pawn for two rooks would have a negative score.

Sure enough, that simple change seems to be worth about 6 ELO.

As suspected the change to move ordering to separate good captures from bad captures has already paid off. Moving bad captures to the bottom of the move order list made it trivial to skip bad captures in the quiescence search altogether. This is an idea I first read about in a discussion on r.g.c.c. between Bob Hyatt, Feng-Hsiung Hsu and others here https://groups.google.com/g/rec.games.chess.computer/c/H6XjY2L13eQ . Hyatt claimed a small improvement, though Hsu was skeptical. Time has proven Hyatt correct though; I believe this is something most strong programs do. In any event, it seems to be worth about 10 ELO for Prophet4 so it’s a keeper.

Improving move ordering has been on the radar of a while now. I started to suspect that move ordering needed some work when my initial attempts at a PVS search and aspiration windows both failed. I reasoned that, if move ordering is subpar, researches would occur too often causing an overall increase in node counts.

To know, you have to measure, so I added data to the logfiles and wrote a Python script to aggregate (1) time to depth, (2) effective branching factor, and (3) the % of nodes in which we get a fail high on the 1st move and first four moves.

Once I was able to measure, I changed the move ordering scheme FROM:

PV move -> Hash move -> All captures in MVV/LVA order -> Killer 1 -> Killer 2 -> noncaptures in the order they are generated

TO:

PV move -> Hash Move -> “Good captures” in MVV/LVA order -> Killer 1 -> Killer 2 -> noncaptures -> bad captures in SEE order.

A “good capture” is one that is a promotion, a capture in which the value of the captured piece is at least that of the capturing piece, or one with a non-negative SEE value.

Incidentally, while researching this I came across a post I myself had made many years ago: http://talkchess.com/forum3/viewtopic.php?f=7&t=15198&hilit=defer+losing+captures&sid=5543f43760ce939ed16deacd59710e06

The change doesn’t seem to add more than just a couple of ELO directly. However, in non-tactical test suites all the measured metrics were improved: time to solution is down, effective branching factor is lower, the percentage of fail highs on the first move is improved, and the percentage of fail highs in the first four moves is dramatically improved. Additionally, the number of nodes required to reach a solution is cut by about a third, with only a 7% or so decrease in speed (nodes per second). I believe this sets the stage to take another stab at PVS and aspiration windows. First though, I’m going to take a stab at pruning bad captures from the quiescence search.

One potential (probable) area of improvement is that knights and bishops have slightly different values (bishops being the more valuable). For the purposes of determining if a capture is “good” when classifying the captures, they should probably be considered equal.