Author: Amir Ban
Date: 23:21:12 08/02/98
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On August 02, 1998 at 23:55:02, Ed Schröder wrote: >>I've always done this. The problem is that on some occasions, the null- >>move search can cause a "false-fail-high". The re-search produces a score >>that was worse than the old search, or, more commonly, actually fails low. > >>I reject such fail highs if the re-search is worse than the old best (I >>catch this by searching with (old-best, +infinity), so that if I fail low, >>I know it is worse than the old best move and it gets rejected outright. > >>I've had versions where this was "broken" for various reasons, and they >>always produced an occasional bizarre-looking move that could be an outright >>blunder, or just a slightly weaker move... > >Hi Bob, > >Glad to hear a "false-fail-high" is common practise -:) > >Like to add that I often have noticed such a "false-fail-high" is rewarded >in the next iteration after all. It has puzzled me for years. A few years >ago I made a test-version that always took a "fail-high" as best move, >false or not. Results were not better but also not worse. > >Similar experiences? > >- Ed - I always accept the fail-high move immediately. There are parameters that you are important that you don't mention: What size window is used during normal search ? If it's zero-width, or very small, then fail-high doesn't mean it's much better than the previous best, and you can take the new move or leave it. If you use a 0.3-0.5 window, as I do, fail-high means it's clearly better than the previous, even it later fails low. Also, what window do you use on the re-search that fails low ? If you use new-alpha+1 to infinity, then maybe the fail-low happened because the value is exactly new-alpha. If you use a window of old-alpha to infinity, then a fail-low indeed makes the move suspicious. Amir
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