[gecode-users] Symbolic Constraints - fd_relation contraint in Gecode?

[Mikael Zayenz Lagerkvist]

Hi,

On Wed, May 20, 2009 at 8:36 PM, Denys Duchier
<denys.duchier at univ-orleans.fr> wrote:
> I could be wrong about this, but I just had a quick glance at the
> implementation and its seems to allocate datastructures of a size
> proportial to the width of the domain.  So if you have small values and
> very large values, the width is going to be very large.
>
> If I am right, then you might be better off encoding your N values into
> the interval [0,N-1] and then possibly using an element constraint to
> decode them where necessary.
>
> Then again, I could be completely off base :-/

Actually, you are completely right, and it also says so in the
documentation [1]:
    Warning:
        If the domains for the $x_i$ are not dense and have similar
        bounds, lots of memory will be wasted (memory consumption
        is in $
O\left(|x|\cdot|\min_i(\underline{x_i})-\max_i(\overline{x_i})|\right)$
        for the basic algorithm (epk = EPK_MEMORY) and additionally
        $ O\left(|x|^2\cdot|\min_i(\underline{x_i})|\max_i(\overline{x_i})|\right)$
        for the incremental algorithm (epk = EPK_SPEED).

The memory-consumption in the size of the domain is inevitable for
reasonably efficient extensional propagators. The translation into a
dense domain from a sparse domain needs to happen at one point or
another anyway for efficient data-structure management, and I choose
not to do it automagically.

Cheers,
Mikael

[1] https://www.gecode.org/doc-latest/reference/group__TaskModelIntExt.html#gf885611633b7b2bfb4d9512071dee9a3

-- 
Mikael Zayenz Lagerkvist, http://www.ict.kth.se/~zayenz/