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Subject: Re: what does "fail high" mean? In the context of iterative deepening/

Author: Russell Reagan

Date: 08:55:34 11/30/02

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On November 30, 2002 at 10:39:45, Robert Hyatt wrote:

>Simple.  When you start an iteration, you should use some sort of aspiration
>window, rather than setting alpha=-infinity and beta=+infinity.  In the case
>of Crafty, if the score for iteration N-1 was +.30, then I might start the
>search at iteration N with alpha=0, beta=+60 to try to bracket the expected
>score.  This makes the search tree smaller as you can prune lines that lose
>or win material quickly.  However, if you really do suddenly see a path to
>win a pawn, that score will be >= +.60, which means the search returns beta
>rather than a true score.  You have to re-set beta to something larger and
>search again to get the true score.  That is a "fail high".

Let me see if I understand. Using this output from Monsoon:

>10.   +3.02  00:00:26.78  5824368      PV= Nf6 1. Bd6 Bc4 2. Rf2 e4 3. Bc2
>                                        > e3 4. Rg2 Qxf3 5. Re1 <+1.00>
>11+   +3.77  00:02:16.62  28726569     Nf4! ++
>11.   +5.19  00:04:27.40  58248289     PV= Nf4 1. gxf4 Rd2 2. Bf2 [threat]
>                                        > Qxf3 3. Qxa5 Bd5 4. Qxd5 [threat]
>                                        > Qxd5 5. Bxf5 Rf8 6. c4 Qxc4 [Q]
>                                        > <+3.00>

What happened was Monsoon used +3.02 from the search to depth 10, then it did an
aspiration search using something like (say) +2.27 to +3.77 (which is the score
of ply-1, +3.02, -/+0.75), and it got a fail-high as you describe it, and
immediately reported that score, without having a real PV. Then it re-searches
with a larger window and gets the real PV, and displays it with the real score
of +5.19. Do I understand this correctly?



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