Advent of Code 2020 - Day 6

 This is a running series of posts on the Advent of Code 2020. The introductory post has the details, including the accompanying GitHub repo, in case you reached this directly.

This puzzle builds a little on previous ones. The input parsing requires building groups, separated by blank lines, and we already did that for Day 4. In this case each line will be a set, and the group a list of those sets (the simplest to build the groups). Apply the same caution as before with potential empty groups.

Part 1

We have to count the number of distinct elements in each group, then add the all groups together. Or, in other words, for each group the cardinality of the union of the sets in the group. We just build a quick loop that emulates a reduce operation applying set union, starting with one of the sets in the group.

That gives us the count or each group, we then add them together.

Part 2

Now we are required to count the elements present in all sets in the group instead of all elements present in all sets, or the intersection of the sets. We repeat the procedure above applying the intersection instead of the union for the reduction, and done.