[computer-go] Rotate the dog or the tail?

[Chrilly]

In all the pattern-matching articles I know off only 1 pattern orientation 
is stored and the area around the board-point is rotated. I do not 
understand the advantage of this method.
It should be more efficient for matching to store the pattern 8 times.
Lets assume a Hashing-Method like described by Mark Boon.

The size of the Pattern-Hashtable is no argument. Even with 3000 patterns 
one gets at most 24000 mirror-patterns. Assuming 20 Bytes/Pattern makes 500 
KByte. This is even for a mobile-Go nowadays no Problem. Maybe I make a 
logic error, but the number of matching operations should be (in the mean) 
the same, if one looks up 8 mirrors in 3000 Patterns or 1 original in 24000 
mirrors.
But looking up 8 mirrors jumps around in the Hashtable, whereas the 1 
original pattern searches in a linear way through the Hashtable. One needs 
also only 1 Hash-Adress calculation (which is cheap).
There should be only 1 cache-miss at the first pattern, the following 
patterns are already prefetched. A cache-miss is the most expensive 
"operation" on current CPUs. Additionally one does not need to rotate the 
board (which is not so cheap). One starts also only 1 matching loop and not 
8. Every loop-termination causes a branch-misprediction. This is the second 
most expensive operation.

This assumes, that the Hashtable is not organized as n-linked-listst (like 
in the Mark Boon code), but as a single Array. Patterns with Hashcode 0 are 
in position 0...h0, with Hashcode1 from h0+1...h1,,,,
One simply needs for every Hashcode a pointer to the starting region of the 
Array. As the patterns are only updated at startup (or even read in from 
disk), building up such an array is simple. Also the Array-Size is not 
criticial. The number of patterns is known beforehand.

In the paper by P.Drake et al. for every pattern a normalized position with 
a gravity concept is developed. It do not know the details of this method, 
but from intuition is sounds complicated and slow to calculated gravity and 
the the corresponding pattern.

Chrilly