Clarification on Documentation

[maourynd]

Hello - Im confused about the documentation on the site:

https://docs.timefold.ai/timefold-solver/latest/using-timefold-solver/modeling-planning-problems#problemFact

This states that: "A problem fact is any object with getters that does not change during planning. In school timetabling, the timeslots and rooms are problem facts:"

But, in the example application provided (Spring) - Room and Timeslot on the PlanningEntity Lesson are denoted as PlanningVariables. I thought PlanningVariables whole thing is that their properties can be changed and should be optimized.

If that's the case, then the statement "... In school timetabling, the timeslots and rooms are problem facts.." is incorrect. Right? I'm just trying to wrap my head around this.

Thanks!

[triceo]

Hello @maourynd!

Think about it like this:

• Lesson is a class, annotated as a planning entity. Therefore it must have at least one planning variable.
• A planning variable is a field on a planning entity. This field will be changed by the solver. In this case, there are two variable fields - room and timeslot.
• The values of that field are Room and Timeslot classes respectively. These classes do not change - they are not entities, and therefore they themselves have no variables. This makes them problem facts.

The value of the variable fields changes, you change one the lesson from Monday to Tuesday.
But the Timeslot itself is still the same - it is immutable. Monday is still Monday.