Ronald de Man says:
This difference has to do with the fact that the "sparse index"
does not point to "k * d->span", but to "k * d->span + d->span / 2".
Since k = idx / d->span, we know that k * d->span <= idx,
so k * d->span is a valid index into the table.
But k * d->span + d->span / 2 might be a value that is bigger than
the largest index for the table (if "idx" happens to be near the
end of the table).
So the last valid entry in the SparseIndex[] array might have
to a point to a block and (sub)index that is not part of the
real table but comes "after" it. To make this work, the generator
adds entries for a few "fake" blocks, each of maximum size 65536,
to the blockLength[] array so that there is something to point to
for the last valid entry in the SparseIndex[] array.
These fake blocks do not correspond to any compressed data.
So the fake blocks avoid the need to detect and handle this special
case in decompress_pairs().
Use indexSpan instead. This could be a teoretical
slwo down becuae we repalce some shifts by divide
and modulo, but this should not be measurable and
instead the code is more understandable now. This
is a big plus becuase this part of code is complex.
Just use the equivalent Ptwist[], renamed in
MapToEdges[] to indicate that gives highest
score to pawns near the edge and then, as
second order, to bottom rank squares.
We currently check onnly first key, while
we shoudl check both, as confirmed by
author on talkchess:
http://www.talkchess.com/forum/viewtopic.php?t=59947&start=30
This bug fix do not change functionality, we simply
find earlier the DTZ entry instead of reload it.
Same initialization logic for both
pawns and pieces.
The advantage of this patch is that we reduce
redundancy and get a single (source) code path
for both cases. This is easier to understand
and to mantain.
Note: This patch makes use of some advanced template
techniques like SFINAE, decltype and the new function
declaration syntax (with trailing return type). This
is not just a show-off, but it is really needed in
this case.
