diff --git a/commities.org b/commities.org index e14d56d..650166a 100644 --- a/commities.org +++ b/commities.org @@ -32,7 +32,7 @@ | E: | 60 | | F: | 30 | - Det er snakk om sensur i inf1300 med 566 besvarelser. D og E kan ikke rette + Det er snakk om sensur i inf1300 med 283 besvarelser. D og E kan ikke rette mot hverandre. De bør helst rette mot B eller C. Har du et bra forslag til meg? Jeg blir GAL. Det var bedre før, da jeg hadde @@ -167,18 +167,21 @@ from z3 import * #+END_SRC - This allows us to generate instances for Z3 to solve with Python. + This allows us to generate instances with Python that Z3 can solve. ** Instances Let's formulate an instance as a four-tuple $(N, C, S, A)$ where - $N$ is the number of exams to correct - $C$ is a list of capacities, where each examiner is identified by - her position of the list - - $S$ is a relation, relating an examiner to one they /should/ form a - committee with - - $A$ is a relation, relating examiners that we should /avoid/ placing in - the same committee + their position of the list + - $S$ is a mapping from a single examiner to a set of examiners they + /should/ form a committee with + - $A$ is a symmetric relation, relating examiners that we should /avoid/ + placing in the same committee + + We define a committee as a set of exactly two examiners (identified by their + index in the list of capacities). The instance, described in the introduction, can be represented with the following Python code: @@ -205,18 +208,20 @@ The end result needs to be a set of committees, where each committee consists of two examiners with a number of exams to correct. An important part of finding a reasonable encoding is to balance what part of the problem - should be solved in pure Python and what should be solved by the SMT-solver. + should be solved with Python and what should be solved by the SMT-solver. My + experience is that a good rule of thumb is to move as much structural + complexity to Python and encode the Z3 instance with simple structures. ** Modeling committees - A natural encoding could be modeling all possible committees as an integer - constant, where the value assigned to a committee corresponds to the number - of exams they correct. + A natural encoding could be modeling a committee as an integer constant, + where the value assigned to a committee corresponds to the number of exams + they correct. It is quite easy to compute all possible committees, and make + one integer constant for each of them. - We let a committee be a set of two exactly two examiners, identified by - their index in the capacity list. Let's write a function that takes a list - of capacities, and return a dictionary, associating (Python) committees to - their corresponding integer constant. + Let's write a function that takes a list of capacities, and return a + dictionary, associating (Python) committees to their corresponding integer + constant. #+BEGIN_SRC python :tangle committees.py def committees(C): @@ -226,11 +231,6 @@ return {c : Int(str(c)) for c in cs} #+END_SRC - # However, we also need to capture that each individual examiner don't have to - # correct more exams than their given capacity. A good rule of thumb in - # SMT-solving is to minimize the number of constants we need to find a - # interpretation for. - Now we must ensure that no examiner receives more exams than their capacity. Given an examiner $i$, where $0 <= i < N$, we let $c_i$ denote the set of all committees $i$ participates in. Then $\sum{c_i} <= C[i]$, i.e. the sum