Guido Schimmels

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Guido Schimmels,

a German software developer contributing to FSF GNU and similar projects, such as GNU TeXmacs [1], and the ROX Desktop [2]. He further is contributor of the Ultimate++ framework [3]. In the late 90s and early 2000s, Guido Schimmels was engaged in computer chess programming, and made a couple of posts in CCC.

PVS vs. NegaScout

Guido Schimmels in a CCC post on the difference of PVS vs. NegaScout [4]:

The difference is how they handle re-searches: PVS passes alpha/beta while NegaScout passes the value returned by the null window search instead of alpha. But then you can get a fail-low on the research due to [search anonomalies](Search_Instability "Search Instability"). If that happens NegaScout returns the value from the first search. That means you will have a crippled [PV](Principal_Variation "Principal Variation"). Then there is a refinement [Reinefeld](Alexander_Reinefeld "Alexander Reinefeld") suggests which is to ommit the re-search at the last two plies (depth > 1) - but that won't work in a real program because of [search extensions](Extensions "Extensions"). NegaScout is slightly an [ivory tower](https://en.wikipedia.org/wiki/Ivory_Tower) variant of PVS (IMHO).

PVS


value = PVS(-(alpha+1),-alpha)
if(value > alpha && value < beta) {
  value = PVS(-beta,-alpha);
}

NegaScout


value = NegaScout(-(alpha+1),-alpha)
if(value > alpha && value < beta && depth > 1) {
  value2 = NegaScout(-beta,-value)
  value = max(value,value2);
}