From: jhc@ivan.uucp (James H. Coombs)
Subject: Zarkov 2.6 Playing Style and Strength
Reply-To: jhc@ivan.brown.edu.uucp (James H. Coombs)
Organization: IRIS - Brown University
Date: Thu, 13 Aug 1992 03:29:57 GMT

I received e-mail asking me....

	why you selected Zarkov 2.6 over the other chess programs. Say,
	m-chess, ect.

The review in *Chess Horizons* that I summarized recently had a lot to
do with it.  Basically, I was looking for a database, and ChessBase
seemed to be too expensive for my needs.  So, I chose Bookup.  I just
started playing a correspondence game, and I saw an ad that mentioned
correspondence games, so that motivated me to actually place my order.
And, Zarkov works with Bookup, so I decided to get Zarkov at the same
time.  I am using Bookup for the correspondence game, but Zarkov turns
out to be my choice for storing tournament games---not in a formal
database but in a separate file for each game.

I have never seen MChess, so I can't make a comparison.  I was hoping
that my posting on the specifics of Zarkov's analysis would motivate
someone to provide a comparison with another program, but no responses
so far....

CCR says that MChess is the strongest of the category, with means of:

	MChess		2401
	KnightStalker	2334
	Rexchess 2.3	2319
	Zarkov 2.50	2276

These are for a 33 MHz 80486.  If Zarkov 2.6 is actually 50 points
stronger than 2.5, then that leaves just MChess ahead by more than a
standard deviation.

But my choice was feature based.  Even on an 8088 with 80286 Turbo
(effectively about 8.84 MHz), Zarkov is strong enough for me (rated
1136).  And, for analysis, I can let it run long enough to complete 6
ply per position, which may take 15 minutes for some positions, but I
have other things to do.

	You also stated that it has three modes of play, Agressive,
	positional, and cautious. This seems to be of a big interest,
	does this feature yeild three different opponets as it

Ok, first the Playing Strength feature: beginner, novice, intermediate,
master.  John Stanhope, author of Zarkov, tells me:

	The intent is for Zarkov to play at about the same strength as
	humans with ratings of 850, 1200, 1600, ~2000, ~2400.  On a
	machine slower than about a 16Mhz 286, the expert level may
	drop below 2000.  The master level will vary from about 2000 on
	a 9Mhz XT to about 2400 on a 486-33.  Note that these ratings
	are independent of time control -- it is assumed that the
	opponent will take about the same time to move as the computer,
	so that both will play stronger at long time controls, just as
	in human games.  If you take much longer to move than the
	computer, then the rating estimate will be inaccurate.  To
	create the lower strength levels, I put delays into the search
	routine, add randomness to the evaluator, and allow the
	computer to blunder.  I think this provides a better simulation
	of an 'intermediate (1600 rated)' player than just cranking the
	time control way down for the computer.

In part, John is reacting to my comparison with my use of a Fidelity
machine, in which I set the time control to 30 or 60 seconds per move
and give myself 40/2 so that I can compete with it.  Also, I was
asking about Zarkov's rating of my performance.

Now on the Playing Styles: cautious, balanced, aggressive.  John says:

	The balanced style of play uses an evaluation function which is
	identical for the computer and the opponent.  The Aggressive
	and Cautious styles have asymmetric evaluation functions as
	shown below.  A '+' indicates that the computer values this
	heuristic more for itself than for the opponent, while a '-'
	means the opposite.

			       Balanced    Aggressive    Cautious
	Piece Material           =            +             -
	Pawn Material            =            -             +
	Pawn Structure           =            -             +
	Piece Mobility           =            +             =
	King Safety              =            -             =
	Piece Defense            =            -             +

	I think that the Aggressive and Balanced styles play at about the same
	strength while the Cautious style is a bit weaker.

As for the rest of the question...

	Can you change modes during the game as a person might. That is
	attact in the opening only to find yourself over extended and
	playing cautious not to loose.

Yes, you can change styles, but remember that you are changing the
computer's style with the setting, not your own.

To get a sense of the difference in the Playing Styles, I took one of my
recent tournament games and ran it through Zarkov's analysis.  Below, I
give the analysis where the styles started to deviate.  All moves before
the deviation were in Zarkov's book.  I didn't try to determine any
effect of Style on the book because I do not know enough about how Zarkov
selects openings and don't have time for exhaustive testing.  In fact, I
decided to have it analyze one of my games so that I could quickly find a
point of deviation.  In John's description, however, I see mention of
heuristics but not openings, so I suspect that Style does not come into
play until Zarkov is out of book.

By the way, the game was written out by Zarkov, which also produced the
diagrams (where I "marked" the game with Alt-M)---but I typed the
comments into this document instead of using Zarkov's annotate command.
I ran the analysis on a 25 MHz 80486, giving 60 seconds per move, which
was enough to reach 7 ply in these positions.

1.d4 d5 2.c4 c6 3.Nf3 Nf6 4.Nc3 dxc4 5.e4 b5

Coombs plays the Geller Gambit instead of the typical Slav 5. a4.

6.e5 Nd5 7.a4 Nxc3 8.bxc3

r n b q k b - r
p - - - p p p p
- - p - - - - -
- p - - P - - -
P - p P - - - -
- - P - - N - -
- - - - - P P P
R - B Q K B - R


Style      Eval  Ply Suggested Line
AGGRESSIVE -53   7   c8f5  f1e2  e7e6  a4b5  c6b5  c1g5  f8e7
BALANCED   -56   7   g7g6  a4b5  c6b5  f3g5  c8f5  d1f3  b8d7
CAUTIOUS   -53   7   h7h6  d1c2  d8d7  c2e4  d7b7  a4b5  c6b5

In all three styles, Zarkov rates Black ahead by about 1/2 pawn.

Without a doubt, 8....  Bf5 would be more aggressive than 8.... h6, and
8.... g6 followed by Bg7 seems like a reasonable middle strategy.

[The fact that the line ends with Bd7 instead of Bg7 is meaningless, as
are most of moves at the end of the suggested lines---in my brief
experience---and that is generally not what Zarkov will play as the game
proceeds and Z can start its search from a deeper point in the game.  As
I said more fully in another posting, people can benefit from examining
the analysis for transition points, not from attempting to play like a
computer.  Of course, our goal here is to see what Zarkov would play in
the different styles.]

My opponent played 8.... Bb7

r n - q k b - r
p b - - p p p p
- - p - - - - -
- p - - P - - -
P - p P - - - -
- - P - - N - -
- - - - - P P P
R - B Q K B - R


AGGRESSIVE  -14   7   e5e6  f7f6  h2h3  d8d5  d1e2  a7a5  c1f4
BALANCED    -39   7   h2h3  e7e6  c1d2  f8e7  f1e2  h7h6  d1c2
CAUTIOUS    -66   7   a1b1  d8d5  f1e2  e7e6  e1g1  h7h6  d1c2

Again, 9. e6 is certainly more aggressive than either 9. Rb1 or 9. h3.
Rb1 does seem more aggressive than h3, which just seems like a bad move
to me, but then I'm an only an intermediate at best.  (I played 9. Ng5,
10. Qh5 and eventually won the game.)

In sum, Playing Style has a significant effect on the way Zarkov plays.
It certainly deserves more study, and I am sure that people can find more
compelling examples than I have given, but this should give people an
idea of what they will find.

Finally, are there no MChess, RexChess, KnightStalker users out there?
Let us hear what these programs have to offer so that people can decide
what might serve them best.

--Jim