Day 8

Mix.install([{:utils, path: "#{__DIR__}/utils"}])

Solution

{grid, outside?} = Utils.read_grid("day08")

inside? = &(not outside?.(&1))

fst = fn {a, _} -> a end
snd = fn {_, b} -> b end
add = fn {a, b}, {c, d} -> {a + c, b + d} end
sub = fn {a, b}, {c, d} -> {a - c, b - d} end

part1 = fn a, b ->
  diff = sub.(b, a)
  [sub.(a, diff), add.(b, diff)] |> Enum.filter(inside?)
end

part2 = fn a, b ->
  diff = sub.(b, a)
  left = Stream.unfold(a, &{&1, sub.(&1, diff)})
  right = Stream.unfold(b, &{&1, add.(&1, diff)})
  Enum.map([left, right], &Enum.take_while(&1, inside?))
end

grid
|> Enum.group_by(snd, fst)
|> Map.drop([?.])
|> Map.values()
|> Enum.map(&amp;for a <- &amp;1, b <- &amp;1, a != b, do: part2.(a, b))
|> List.flatten()
|> Enum.uniq()
|> Enum.count()