Author: Robert Hyatt
Date: 09:50:34 01/17/03
Go up one level in this thread
On January 17, 2003 at 01:20:23, Omid David Tabibi wrote: >On January 17, 2003 at 00:48:48, Robert Hyatt wrote: > >>On January 16, 2003 at 20:29:54, Omid David Tabibi wrote: >> >>>[D] 4k3/8/8/p1p1p1p1/PpPpPpPp/1P1P1P1P/8/4K3 w - - 0 1 >>> >>>At the above position, some programs (e.g. Fritz, Hiarcs, Crafty) reach their >>>maximum depth in a second or two (see the Crafty analysis below), while others >>>(e.g. Tiger, Shredder) need more time as they search deeper (Tiger for example >>>quickly reaches depth 20, and then slowly goes deeper and deeper, see the Tiger >>>analysis below. For Genesis it takes about 4 minutes to reach depth 60). What is >>>different in the former class which enables them to quickly reach the max depth? >>> >> >>This is a hashing problem. There are not many unique positions, so hashing >>should cause the search to be very efficient... nothing but king moves helps >>this even more. >> > >There are only 16 (white) x 24 (black) x 2 (sides) = 768 unique positions. But >still normally you cannot retrieve everything from hash table: Let's say you >have a position stored in hash table with depth 30, but now you need a depth 31 >search, so you cannot use the hash result; after the next iteration you will >need a result for depth 32 and so on and so on... > >Genesis gets about 30% hash hits from the above position. > > >> >>>Another interesting point is that Crafty (and many others) gradually reduce the >>>score as they searches deeper (in the below log you can see that at depth 47 >>>Crafty gives the score 0), while some others like Tiger, remain with their fixed >>>score (-0.24 in the log below). Based on what factors is the score reduced in >>>Crafty? >>> >> >>Probably draw by repetition. The side that is behind won't move his king very >>far from the original square. The other side will try every possible square >>but after 40+ plies he runs out of new squares and the other side can force a >>repetition. > >If so, then why doesn't that happen with all the engines? Tiger never changes >the score; Genesis reached the depth 80 and never changed the score (and to the >best of my knowledge Genesis doesn't have any repetition detection bugs). >Besides, the score drop is usually gradual, not a sudden change to 0. Even on the next iteration many positions are still useful. Because transpositions let you hit the positions with enough draft so that they are useful... The score drop was pretty gradual. It dropped where it discovered it had to play sub-optimally to avoid the repetition, it dropped again the next iteration, and finally it saw "repetition is forced." Any time the king is trapped like that, the depth will skyrocket quickly.
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