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Subject: Re: Singular extensions

Author: Robert Hyatt

Date: 13:18:29 01/08/98

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On January 08, 1998 at 11:48:22, Bas Hamstra wrote:

>On January 07, 1998 at 22:51:59, Robert Hyatt wrote:
>
>Bob, a while ago we had a short conversation about singular extensions.
>
>You said something like if a move is way better than the rest research
>that move till Depth+1.
>
>But what is the basic idea of singular extensions? Is it something like
>q-search but then not for captures but forcing moves? How could it lead
>to searching at greater depth *average* as was said of Deep Blue?
>
>Could you tell something more about this, about the general idea, not
>the implementation? (I'll be back for that later hopefully).
>
>
>Regards,
>Bas Hamstra.

The general idea is this:  If one move is clearly better than all the
other moves, by a significant amount, it *might* be a good move, or it
*might* be a delaying move that just barely holds off some significant
threat by the opponent.

There are two cases:  PV singular and fail-high singular.  I'll explain
the first as it is easy to understand.

You are searching at a node P, with window (alpha,beta).  You search the
first move P.1 with this window.  you search the remainder of the moves
with (alpha-X, beta-X) where X=a constant you choose by experimentation.
If all the remaining moves fail low, with this lowered window, you know
that all of these moves are much worse (at least X worse) than the best
move that didn't fail low with the alpha/beta window.  This move is
called
a singular move and is then re-searched with 1 more ply added on.

It gets messy if your move ordering isn't perfect, because when you
lower the
window, and a move fails high, now you have to wonder which of the
following
is true:  move is better than the rest, but not better than the best so
far.  if this is true, this is not a singular node since there are two
good moves to try.  but this move might be way better than the best so
far, and *it* might in fact be singular, so you re-search it with the
normal
window, and then re-search the old best move with the offset window to
see
if one is better than the other by a significant amount.

The idea is, if one move is better than all the rest, it might be the
only
move that prevents your opponent from zapping you (ie a spite move but
not
a check since a check would be extended anyway probably).  This simply
follows long forcing lines where at each ply you only have one saving
move.

but it is computationally expensive...



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