chore: create ExamRoomCapacity-type

_examRoomCapacityIso for convenience

src/Model/Types/Exam.hs

@@ -19,6 +19,8 @@ module Model.Types.Exam
   , _examOccurrenceMappingRule
   , _examOccurrenceMappingMapping
   , traverseExamOccurrenceMapping
+  , ExamOccurrenceCapacity(..)
+  , _examOccurrenceCapacityIso
   , ExamGrade(..)
   , numberGrade
   , ExamGradeDefCenter(..)
@@ -225,6 +227,33 @@ traverseExamOccurrenceMapping :: Ord roomId'
                               => Traversal (ExamOccurrenceMapping roomId) (ExamOccurrenceMapping roomId') roomId roomId'
 traverseExamOccurrenceMapping = _examOccurrenceMappingMapping . iso Map.toList (Map.fromListWith Set.union) . traverse . _1
 
+-- | Natural extended by representation for Infinity.
+-- 
+-- Maybe doesn't work, because the 'Ord' instance puts 'Nothing' below 0
+-- instead of above every other number.
+data ExamOccurrenceCapacity = Unrestricted | Restricted Natural
+  deriving (Show, Eq)
+
+-- | Unrestricted is bigger then everything else, otherwise use instance from 'Natural'.
+instance Ord ExamOccurrenceCapacity where
+  compare Unrestricted Unrestricted     = EQ
+  compare Unrestricted (Restricted _n)  = GT
+  compare (Restricted _n) Unrestricted  = LT
+  compare (Restricted a) (Restricted b) = compare a b
+
+-- | Addition monoid with 'Unrestricted' interpreted as infinity.
+instance Semigroup ExamOccurrenceCapacity where
+  (<>) Unrestricted _b = Unrestricted
+  (<>) _a Unrestricted = Unrestricted
+  (<>) (Restricted a) (Restricted b) = Restricted $ a + b
+
+-- | Addition monoid with 'Unrestricted' interpreted as infinity.
+instance Monoid ExamOccurrenceCapacity where
+  mempty = Restricted 0
+
+_examOccurrenceCapacityIso :: Iso' ExamOccurrenceCapacity (Maybe Natural)
+_examOccurrenceCapacityIso = iso (\case {Unrestricted -> Nothing; Restricted n -> Just n})
+                                  (\case {Nothing -> Unrestricted; Just n -> Restricted n})
 
 data ExamGrade
   = Grade50