Enforce no-delay schedule IBM OPL (CPLEX)

Question

I created a schedule in IBM OPL:

dvar sequence schedule in all(j in Jobs) job[j]; 

If the CP-Module generates a solution, the solution is sometimes not a non-delay solution. This is however not allowed and thus I want to enforce a non-delay schedule.

I tried different solutions in the subject to-Section...

 forall(t in Jobs)
   if (t > 1)
   startOf(job[t]) == endOf(job[t-1]);

... but these fail (obviously) when job t-1 is not followed by job t.

Anyone who can give me a hint on how to solve this problem?

Kind regards, Franz

Answer 1

you should try to use endAtStart. (OPL constraint to restrict the relative positions of interval variables.) regards

Answer 2

endAtStart will only work if your Jobs are always ready to start when the preceding Job finishes. If this is not the case, you will receive an error.

A better solution would be to regard the no-delay in the objective. What you are trying to achieve is to start every new job as soon as possible. So you can for example use the staticLex function:

minimize staticLex(startOf(job[1]), startOf(job[2]),...);

However, this expression can become very long and you would need to hardcode the number of intervals. A little workaround would be to assign the intervals decreasing weighting factors :

minimize sum(j in Jobs) startOf(job[j])*100^-ord(Jobs,j);

I hope this works for you!

Regards Jan