Pawn Structure "Holes" and Move Ordering

[Daniel A. Thies]

I just tried this, and it looks like it works. Your mileage may vary. If
it does, let me know....

For a long time, I've been maintaining a hash table for pawn structure
evaluations, since this is one of the most expensive parts of the
evaluation for me. It's also one of the largest factors in the evaluation
- I almost always need this value to decide on a cutoff.

As a part of the evaluation, a bitboard called "holes" is created for
white and black. This bitboard shows every square from the lead pawn back
to the 3rd rank which can no longer be attacked by a pawn.

What I did this morning: I added the "holes" bitboards to the data stored
in the hash table. Now, when I am generating moves in the search, after
killers/history and captures, if the pawn position is stored in the hash
table, I generate moves to the holes before other moves.

This did some very good things for my move ordering, and it's very cheap.
So far, I've completed the first 20 positions in my test set, and this
little cheapo bought me a 0.6% speed increase. Every little bit counts,
right?

On tomorrow's agenda: add the "open files" bitboards to generate rook
moves to open files. Test whether files or holes should go first.
Instinct says it will be rooks to open files, minor pieces to holes, rooks
to semi-open files, and heavy pieces to holes. We shall see.

Dan