[gecode-users] Unary constraint not-first-not-last propagation does not cancel optional tasks

[Bauke Conijn]

Hi Christian,

I know that optional tasks cannot have their start and end times pruned,
however, their mandatory flag can be marked false.

It seems that the two commented-out lines would discard the optional
tasks when it detects that they cannot be mandatory.

Very similar code can be found in detectable.hpp:
    for (int i=n; i--; )
      if (t[i].mandatory()) {
        GECODE_ME_CHECK(t[i].est(home,est[i]));
      } else if (est[i] > t[i].lst()) {
        GECODE_ME_CHECK(t[i].excluded(home));
        t[i].cancel(home,p,Int::PC_INT_BND); t[i]=t[--n];
      }


Kind regards,

Bauke


On 02/17/2014 03:59 PM, Christian Schulte wrote:
> Hi Bauke,
> 
> No, they can't, in the current setup this would make the propagator
> incorrect. The reasoning is as follows: while the task is still optional,
> one is not allowed to prune anything. The reason why the code still lingers
> on as a comment is that if one introduces an additional wrapper in forms of
> task variables, this kind of reasoing could be used.
> 
> Best
> Christian
> 
> --
> Christian Schulte, Professor of Computer Science, KTH,
> www.ict.kth.se/~cschulte/
> 
> 
> -----Original Message-----
> From: users-bounces at gecode.org [mailto:users-bounces at gecode.org] On Behalf
> Of Conijn, B.J.
> Sent: Friday, February 14, 2014 2:08 PM
> To: users at gecode.org
> Subject: [gecode-users] Unary constraint not-first-not-last propagation does
> not cancel optional tasks
> 
> Hi,
> 
> In not-first-not-last.hpp I found the following piece of code and I was
> wondering why those two lines have been commented out. While running the
> tests with those two lines included, those lines get executed and the test
> still succeeds. Can those two lines be safely included?
> 
> if (t[i].mandatory()) {
>          GECODE_ME_CHECK(t[i].lct(home,lct[i]));
> } else if (lct[i] < t[i].ect()) {
>    //        GECODE_ME_CHECK(t[i].excluded(home));
>    //        t[i].cancel(home,p); t[i]=t[--n];
> }
> 
> Kind regards,
> 
> Bauke
> 
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