Day 4 was an exercise in interval relationships, something I've used before in the day job.
This followed the problem. An
Assignment is a new data type; I represent pairs of
Assignments as, well, pairs: 2-tuples. Reading and parsing the datafile is straightforward.
data Assignment = Assignment Int Int deriving (Show, Eq) type Pair = (Assignment, Assignment) pairsP = pairP `sepBy` endOfLine pairP = (,) <$> assignmentP <* "," <*> assignmentP assignmentP = Assignment <$> decimal <* "-" <*> decimal
We can say a few things about relations between intervals (assignments).
Inverval p contains interval q if p starts no later than q and p finishes no earlier than q.
Interval p is wholly before interval q if p finishes before q starts.
contains (Assignment lower1 upper1) (Assignment lower2 upper2) = (lower1 <= lower2) && (upper1 >= upper2) before (Assignment _lower1 upper1) (Assignment lower2 _upper2) = (upper1 < lower2)
A pair of intervals (p, q) has a containment if p contains q or q contains p.
A pair of intervals (p, q) is disjoint if p is before q or q is before p. p and q overlap if they are not disjoint.
hasContainment (assignment1, assignment2) = (assignment1 `contains` assignment2) || (assignment2 `contains` assignment1) disjoint (assignment1, assignment2) = (assignment1 `before` assignment2) || (assignment2 `before` assignment1) overlaps = not . disjoint
With these definitions in place, the solutions are just filtering the list of assignment pairs depending on the relations we want.
part1 = length . (filter hasContainment) part2 = length . (filter overlaps)
The intervals library includes all these relations built-in.