New constraints suggestion by Vangelis Karafillidis

[Liviu Lalescu]

Vangelis and I had a conversation on Facebook. He proposes:

Constraint two sets of activities share min/max common slots from selection.

They are very rarely used, but they might be useful.

Example for min:

We have the following activities (teacher-group-subject)

John A1 Math 4 hours per week
Mary B2 History 3 hour per week
George C1 Physics 3 hours per week
Vangelis C4 Chemistry 2 hours per week...

We need any of the 2 hours of Vangelis (1 of them) to coincide with ANY of the hours of John, Mary, or George...

[Volker Dirr]

hmm... Question for me is why is it needed? So what is the advanatge if you do that?

[Vangelis Karafillidis]

Hello!
Indeed, these situations are really rare. Actually, this is some kind of trick. Let me explain the problem:
Music schools have group lessons (such as History, Sports, Maths, Physics, Music Theory, e.t.c.) and Musical Instruments lessons. For example, when the A1 group of the school attends the Musical Instruments lessons EACH ONE of the students attends a musical instrument lesson with one teacher. So, the group is split into its "components" (students), and this type of lesson is one student with one teacher. I Greece the Ministry of education hires the teachers not from the beginning of the school year, but partially ("incrementally") in "waves". For example a teacher of a rare musical instrument might be hired in February in order to teach for 5 hours! The problem is that according to the already "fixed" musical instruments timetable the students of this teacher might be available on different days! So, it's likely that the teacher might need to come to school every single day of the (5-day) week! In these situations, sometimes we try to "compromise" with this difficult situation. A solution is to "take" the student from the one (of two hours per week) of the Sports lesson, OR from the one (of the two hour per week) of the Byzantine Music lesson, OR the one (of the n hours per week) of some other lessons, and make him/her attend the Musical Instrument lesson. So, the timetable needs to be modified such as (for example) the (already fixed hour) of Monday for the B1 group for Musical Instruments to coincide with ANY of the hours of A1 (the group of one of the students of this rare Musical Instrument lessons) of either Sports, or Byzantine Music, e.t.c. The student of A1 leaves his (for example) Sports lesson, and attends his/her musical instrument lesson at the same time (hour) with the other students of the B1 group, which of course is not normal, but a compromise we make. So, a constraint of the type "two sets of activities share min common slots from selection" is useful.
Of course this type of problems are really rare, and occur because of this strange approach by the Greek Ministry of Education.
I wouldn't recommend implementing this constraints, since (as Liviu told me) it needs monstrous coding. I just thought that it might be easy... Since, it's not easy, and actually it would be useful in really rare conditions, I think that it would be much better if Liviu invested his coding forces to other features of FET.
Vangelis.

[Volker Dirr]

hmm... if it is used only very rarly an other workaround migth be to place those rare musical lesson in the 1st hour or the last hour of a day an set the student not available at that slot. So the student has got free as long as there is no teacher and as soon as there is a teacher, he/she just will get the musical lesson.