[Other] Some restructuring

I have taken a look at the different ways I could handle the extensions and it is a complete mess. :)

Currently the search is doing an inCheck at the beginning of each node to determine if we should extend for check. That inCheck is then used by null-moves and futility pruning for the current position.

After that futility pruning does another inCheck to determine if the next position will be in check, this is also done by the late move reduction.

So the worst case scenario does three inCheck lookups for one node. Which is just bad.

I have had a 'history'-array for quite some time that keeps track of passed castling rights, en passant squares and the fifty moves rule. Recently I extended that idea by adding another array that kept track of passed zobrist keys and captures to make the unmakeMove faster.

So basically there already is a stack keeping track of passed positions, only that it lacks information about inCheck and it does not remember what moves was made (only if they were a capture or not).

I am going to complete the stack with the needed information and make the inCheck while making each move so we always have that information.

I do not expect this to speed up the search that much, however the code will be much easier to modify which in turn will help make everything faster.

[Other] More work to be done

After 200 games the two versions finished with about equal score after a winning streak from v0.31 in the end.

I have a 'gut feeling' the v0.32 is be better, I can not see how it would not be. However at this point I am not looking for differences that take thousands of games to determine.

Mediocre is not nearly good enough to start chasing 1-2 elo points here and there, I want atleast 50-70 points (about 60% won games) before I am satisfied.

I think I will leave futility pruning and the evaluation as it is for a while and start looking at how extensions are handled. Currently Mediocre has only a very crude check extension, so I will take a look at extensions for mate threat (if null move returns a mate value the branch is searched more carefully), recapture, single-response and something regarding passed pawns.

I will need a new way of handling the extensions and probably make use of fractional plies for some of them. And while doing this I will take care of the repeated work Mediocre currently is doing for determining if a position is in check (it is done in the check extension, late move reduction and futility pruning, this should only be done once).

Off I go. :)

[Other] Settled for a futility

After some testing I decided which futility pruning I am going to use. These are the highlights:

• Use full evaluation and margins 125 centipawns for frontier nodes and 500 for pre-frontier nodes, if the evaluation + margin does not reach alpha the move is considered for futility pruning
• Consider the type of move, do not prune moves that
  • give check
  • move a pawn/queen/rook to the 7th rank
  • capture a bishop so the opponent loses the bishop pair
  • capture an pawn on the 2nd or 3rd rank
  • capture a queen
  The considerations are taken because these moves are likely to change the score more than average (e.g. moving a pawn to 7th rank will always make it a passed pawn, capturing the queen usually alters the king safety quite a bit etc.), thanks to Mark Lefler for these ideas
• If the move is a capture make sure to add the value of the captured piece to the score and see if it still is below alpha minus the margin
• If the move still is getting pruned after these considerations a final check is made to see if the evaluation is above the best evaluation of the node, if it is that evaluation is recorded so the node does not risk exiting without any evaluation

H.G. Muller pointed out to me that if one non-capture is futile, so is the rest so basically that check could be done before even generating the non-captures. However since Mediocre needs the move to be played on the board in order to determine if it is a checking move I can not do that just yet.

This setup is the best I found, but there does not seem to be a big difference between the implementations I tested. I really like the idea of not pruning moves that are likely to change the score however, and the way it is done here is very simple and effective.

Stumbling upon great things

Once I decided on this matter I went on to some other things, mainly bug fixes and such, but there was one thing that I stumbled upon that really seems to pay off. I wanted a pawn hash table since the pawn structure rarely changes and there is a lot of unnescessary work done in the evaluation.

However the implementation would take quite some work since the zobrist key for the pawns would have to be updated every time a pawn is moving and also the current pawn evaluation uses the king positions as well so that would have to be sorted out.

So I decided to try that later and instead I tried implementing a simple evaluation hash table that keeps track of passed evaluations so if we run into the position again that work does not have to be repeated.

This took about 5 lines of code and speeded up the search a huge amount. It almost feels like I did something wrong since I have read so much about how memory lookups are costly and barely worth it. But Mediocre took another little leap in strength with this change.

I will play a test tournament overnight and if the results are good I think it is worth releasing Mediocre v0.32.

[Other] Tweaking v0.31

I have yet to come up with the perfect implementation of the futility pruning in Mediocre. I ran a 300 game match between four different versions and the only conclusion was it is worth including captures in the futility, and I am already doing that. Using a limited material evaluation versus a full evaluation seems to be pretty much equal in strength.

There are a few more things to try out however, like not pruning pawns advancing to the seventh rank or similar, since they will most likely increase the score quite a bit.

There are also some adjustments to the evaluation that needs to be done. In v0.3 I had only a small bonus for rooks on the seventh rank since I did not check if there were actually pawns there (or opponent king on eight rank). Now in v0.31 I added that check so a rook on seventh should be valued higher than before.

The biggest flaw I found was with the x-ray attacks. A queen running diagonally into an own rook kept getting attack points behind it, this is obviously a bug and will be fixed.

Once I am happy with these things I will give candidate pawns another shot. I will most likely have to run a big test with them turned on and off to see if they are worth the effort in Mediocre. I am not expecting a big improvement from them in any case.

[New Version] v0.31 - Futility pruning and additions to evaluation

Changes:

• Made a few arrays static in the SEE, speeding it up quite a bit
• Futility pruning added
• X-ray attacks in the evaluation (e.g. queen attacking empty squares behind own bishop)
• Eval is adjusted towards 0 if a draw is likely
• Pawns are valued slightly less but becomes a bit more valuable in endings
• Knight outposts added
• King tropism added
• Rook/queen on 7th rank only awarded if enemy king on 8th or enemy pawns still on 7th
• Fixed bug with bishop positioning and also one with pawn evaluation

Note: One particular implementation of futility pruning (and no additions to evaluation) performed slightly better against v0.3, however I believe this setup works better in general. Only testing will show so I am releasing this now since it is quite an improvement and make any needed adjustments in future releases.

Also I commented out the candidate pawns code. It follows the definition used in Fruit, but it got a bit messy and I did not notice any improvement from it. I will have to keep trying in that area.

mediocre_v0.31

[Other] Silly bug with bishops

I am currently reshaping the evaluation a bit and while doing that I noticed a very subtle bug. Instead of using the piece table for black bishops I used the table for black knights. This resulted in black thinking fiancetto was a bad thing since the knights get a penalty for standing on the G7/B7 squares, and also a smaller penalty for the D7/E7 squares (which are quite natural for the bishops).

I had been thinking about adding even more incentive for developing the bishops since it looked like they were rarely developed properly. This bug was probably the problem though.

[Other] New archive site address

Thanks to Wang Zhen the Mediocre archives now have a new ad-free home!

The new address is http://mediocrechess.varten.org/.

[Other] Finally futility

After hours of despair and a bunch of helpful tips that I could not get to work, I decided to post one of my implementations of futility on the Talkchess.com forum. And finally I got the mystery solved. :)

I had overlooked the possibility of all moves in a node fitting the requirements for futility pruning. In some of my implementations this caused a very faulty value being recorded for the node, and in other the mate detection thought no legal moves existed and returned the stalemate score (0).

After having sorted this out, the new implementation of fuility pruning in Mediocre is working like a charm.

I started a little tournament to test it out and the results so far looks quite promising:

   Engine            Score                              Me
1: Mediocre dev 0.31 20,5/30 ..............................
2: Mediocre 0.3      9,5/30  00=0=010101=110=0=00=0000=0001 

And this is with futility pruning for only non-captures, so there is still some in there up for grabs.

[Other] Struggling with futility pruning

About two months ago I tried implementing futility pruning. I failed miserably and decided to blame it on the general structure of Mediocre at the time. :)

Once again I am trying to get it to work and obviously I am doing something terribly wrong. I have tried three different setups and run 100 games matches against version 0.3, all three setups resulted in weaker play.

The theory is so simple that I must be missing something.

Well I will keep trying this time.

[Other] Mediocre website is up

I uploaded the website. For now you can find it at http://hem.passagen.se/maragor/, sorry for the horrible ad covering half the page, it was the easiest solution. When a get a tad better cash flow I will probably get a better location for it.

It is very simple and only intented as a bit more structured way of reading the guides and downloading older versions of Mediocre, atleast for now.

As I said earlier I will be adding content to the site from the blog now and then.

One nice thing is the links to old Jim Ablett executables of Mediocre which I have not had up anywhere before.

Well enjoy. :)