[Computer-go] issues on 19x19

Ken Friedenbach kenliz at cruzio.com
Wed Apr 27 10:57:10 PDT 2011

It was some time ago when I worked on this with Arthur Molin in 1988. But I recall we evaluated some ladder incorrectly as "escapes" when the first capture of a surrounding stone occurred, not realizing that there was a shortage of liberties involved, and we had to continue the analysis until three or more liberties gave the owner of the attacked string time to counter attack.


On Apr 27, 2011, at 10:17 AM, Daniel Shawul wrote:

> Oh thanks a lot!  I was letting it search both of the moves but I agree one should definitely be enough.
> I will implement the "helping stone" concept. 
> Captures are always tried in qsearch that help stop the search when "standing pat". Will that 
> be enough to avoid the 'snapback' problem ? Thanks again.
> On Wed, Apr 27, 2011 at 12:58 PM, Ken Friedenbach <kenliz at cruzio.com> wrote:
> 1.) Ladder analysis can be reduced to almost linear search with a few branching points by doing an analysis of the two liberties available. One liberty has a "helping stone" adjacent, and the other has no helping stone adjacent, so would usually lead to three liberties for the running string. A ladder analysis can usually be limited to just the liberty without a helping stone. The only branching occurs if both liberties have a helping stone. And of course, you must read through captures to see if there is a snapback, etc.
> 2). Don't know how to help there...
> Ken Friedenbach
> On Apr 27, 2011, at 8:23 AM, Daniel Shawul wrote:
> > I am having problems making my engine play a decent game on 19x19 board. Some KGS games
> > exposed the following problems.
> >
> > 1) Ladder problem. It often mis-evaluates ladders despite my best efforts to detect them
> > in the playouts. So I resorted to doing qsearch at the root to weed out moves which
> > loose tactically, and make winning moves immediately. That helped but sometimes the qsearch explodes and engine looses on time.
> > I tried putting a limit to the depth of qsearch but it still runs into problems. Using a small depth limit may
> > help in 9x9 but on 19x19, atleast 40 plies is required to detect one ladder along the diagonal... so what to do ?

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