There is controversy in Iowa and elsewhere on how to effectively keep classrooms as in person as possible. I decided to model the problem in z3py. The model is simple, it minimizes outside contact edges between students in different classrooms.

I hosted it here. Feedback is welcome.

For simplicity (dataset from Yurichevâ€™s book) I used a school of international students, one from every country in Europe. Where borders touch it is assumes students might be exposed to each other. See Network theory and SARS: predicting outbreak diversity if you want to generate your own social networks.

I also wrote a second model that assumes classrooms are a clique and minimizes total edges. It uses the formula for edges within a clique of a given size instead of counting them.

There was a 3x speedup by using fixed length BitVec() values instead of constrained size Int() values to encode the four classroom assignments.

```
if(BIT_COLORS):
student_color=[BitVec('student%d_color' % c,2) for c in range(students_total)]
else:
student_color=[Int('student%d_color' % c) for c in range(students_total)]
for i in range(PERSONS):
s.add(And(student_color[i]>=0, student_color[i] < GROUPS))
```

If you want to learn more about encoding NP hard problems in z3py see the Z3 hamiltonian example, Z3 Playground, and SAT SMT by Example.