Author: Stuart Cracraft
Date: 11:05:49 07/28/04
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On July 28, 2004 at 12:38:59, Richard Pijl wrote: >On July 28, 2004 at 12:04:41, Stuart Cracraft wrote: > >>So in looking at ETC and implementing it (without much benefit >>I might add, in a PVS/NEGASCOUT search), I thought a little more >>and perhaps have a complete misunderstanding of it, hence the >>implementation. Am reconsidering it for a recent MTD(f) implementation >>that is at parity with the PVS/NEGASCOUT with very little work. >> >>From what I have read, ETC is a check prior to search() in the >>main search of all child positions hashkeys after each child >>makemove(). If any exist in the hash table and have length >= >>current depth and provide a cutoff, then skip searching all >>the children. >> >>Is that about it? > >I'm not sure if I understand you correctly. > >Before starting the main loop (that does a search of the children) ETC will >check the positions reached by each of the children to check the hash signatures >and whether a cutoff can be done. With perfect moveordering this will not get >you anything. But when not the first but e.g. the third move will get you a beta >-cutoff, ETC will enable you to do a cutoff fast, without searching the first >two moves. In MTD, the chances that children's positions are already in the >hashtable is bigger than in regular PVS, so the gain may be bigger there. > >But watchout for the pitfalls. Also consider extensions that you may do on >certain moves to determine whether an entry found in the hashtable has >sufficient depth. > >What I'm doing to make sure that the cutoff is ok, is that I do not do an >immediate cutoff. Instead I replace the hashmove by the move that should create >a cutoff according to ETC. >This got me a few percent speedup although I'm using PVS. >Oh, one thing more: As ETC costs a bit, only do it when the remaining depth is >high enough to make up for the costs. I'm only doing ETC based moveordering when >there are at least 4 ply to go. >Richard. Great -- 4 seems to be the number and I'll try it (and 3 and 2). :-) What you said about the children is right from my understanding. perfect moveordering what obviate the need for ETC and better-and-better move ordering would render ETC less and less beneficial and eventually just an overhead cost. Stuart
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