Find the numbers that add to 2020 and multiply them.
First, ripped a bunch of infra from last year's code, like Advent.Input and
main application, etc. Then, using the doctest directive and Examples section,
including the example entries from the puzzle there was easy and created a
working bit of documentation along the way.
Advent.Input.read/1 gets us each line of the input file, and
Advent.Input.parse/1 maps those to integers. Having Advent.eval/1 take a
List as its param makes doctesting easy.
This is just a brute force O(n^2) (i think haha) walk through the entries. The
puzzle text seemed to indicate that there would only be one match, so
Enum.reduce_while it is. Once we've found those pairs, multiply them for the
answer. Have Main.start/2 print the result.
mix run
mix test