Author: Robert Hyatt
Date: 11:42:27 09/20/99
Go up one level in this thread
OK... I now have some working code. It is not perfected, by any stretch yet,
but it works in two pieces, and one of the two pieces is really good, from an
accuracy point of view. Here is what I have done:
1. I have written a piece of code that is very good at enumerating candidate
passers in a possition. Nothing fancy except that bitboards help a lot in
discovering which pawns are candidate passers. I produce an 8-bit vector for
each side with a 1 on a file for a candidate passer being present, a 0 if no
candidate is on that file...
2. I am using this in a way that is conservative at present, but which works
pretty well. If white has a candidate, I compare the 'outsideness' of that
candidate against real passers and candidate passers for the other side, and
give a bonus (that is bigger as material comes off) if it has an outside
candidate passer.
Here are a couple of examples:
+---+---+---+---+---+---+---+---+
8 | | | | | | | *K| |
+---+---+---+---+---+---+---+---+
7 | *P| *P| | | | *P| *P| *P|
+---+---+---+---+---+---+---+---+
6 | | | | | | | | |
+---+---+---+---+---+---+---+---+
5 | | | | | | | | |
+---+---+---+---+---+---+---+---+
4 | | | | | | | | |
+---+---+---+---+---+---+---+---+
3 | | | | | | | P | |
+---+---+---+---+---+---+---+---+
2 | P | P | P | | | | P | |
+---+---+---+---+---+---+---+---+
1 | | | | | | | K | |
+---+---+---+---+---+---+---+---+
a b c d e f g h
(fen 6k1/pp3ppp/8/8/8/6P1/PPP3P1/6K1/ w)
old (16.18 crafty) evaluates this as -36 (black is better) because of
those two doubled kingside pawns. new crafty says +.36 because white has
a potential outside passer on the c file, black can't force a passer without
the king helping. Better...
Here is a test of that code, as it is not very well known that two doubled
pawns can stop a majority, when a duo can't, as in the previous example.
+---+---+---+---+---+---+---+---+
8 | | | | | | | *K| |
+---+---+---+---+---+---+---+---+
7 | *P| *P| | | | *P| *P| *P|
+---+---+---+---+---+---+---+---+
6 | | | | | | | | |
+---+---+---+---+---+---+---+---+
5 | | | | | | | | |
+---+---+---+---+---+---+---+---+
4 | | | | | | *B| | |
+---+---+---+---+---+---+---+---+
3 | | | | | | | N | |
+---+---+---+---+---+---+---+---+
2 | P | P | P | | | P | P | K |
+---+---+---+---+---+---+---+---+
1 | | | | | | | | |
+---+---+---+---+---+---+---+---+
a b c d e f g h
This is the position prior to the last one. I (as black) played Bxg3. Old
crafty promptly plays Kxg3 and now it has to watch out for the passed black
pawn on the h file (eventually). new crafty plays fxg3 and black isn't going
to make any passer.
How this is going to work in games is anybody's guess... the outside passed
pawn code has pretty well stood the test of time, this is just taking in one
level more abstract.
More after I run it a while to see how it behaves. BTW, as I mentioned, this
has not slowed me down one bit. In fact, in adding this, I consolidated several
flags into one pawn hash entry to make room for the two new bitmaps (8 bits
each) and actually the new code is a bit faster than the old (without this
majority stuff)...
If you have a position you'd like to see it evaluate, let me know. Best kinds
are those that don't need a search, so we can see what it understands or doesn't
understand with pure evaluation.
Maybe its endgame strength will increase a little more with this. It certainly
seems reasonable at present to assume so...
Bob
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