Just the part 1 for now.

I did not reconstruct the map, just looked at corners. It’s ugly and verbose but it works.

Now on part 2, but I already find the sea monster, it’s my function that finds corners

  def find_corners(tiles) do
    # Map all edges to corresponding tiles
    Enum.reduce(tiles, %{}, fn {id, tile}, acc ->
      edges = edges(tile)

      Enum.reduce(edges ++ Enum.map(edges, &flip/1), acc, fn edge, acc ->
        {_old, acc} =
          Map.get_and_update(acc, edge, fn old ->
            if old do
              {old, [id | old]}
            else
              {old, [id]}
            end
          end)

        acc
      end)
    end)
    # Filters out singleton
    |> Enum.filter(fn {_edge, ids} -> length(ids) == 1 end)
    # A list of one is just the element inside that list
    |> Enum.map(fn {edge, [id]} -> {edge, id} end)
    # Reverse the reduce to find how many edges maps this tile id
    |> Enum.reduce(%{}, fn {_edge, id}, acc ->
      {_old, acc} =
        Map.get_and_update(acc, id, fn old ->
          if old do
            {old, old + 1}
          else
            {old, 1}
          end
        end)

      acc
    end)
    # filter out candidates
    |> Enum.filter(fn {_id, match} -> match > 2 && match < 5 end)
    |> Enum.map(fn {id, _match} -> id end)
  end

I got is working this afternoon, and I’ve just cleaned the code a little bit.

Not very proud of this solution as it is messy and hard to read. But it completes in 15ms (not counting transforming the raw input into list of tiles (which are lists of lists of chars).

I tried to leverage my corner-finding code from part 1 for part 2, but I didn’t make much progress. After sleeping on it I decided to start over with another approach for part 2. I spent most of the time getting the tile combining to work. After that, it was fairly easy to implement the search for the sea monster.

Here is my solution.

Not sure if it’s too late, but here’s my day 20 solution if anyone’s interested.

advent20/day20.ex at master · h-j-k/advent20 (github.com)

I’ve also documented my walkthrough if you want to understand the logic behind the steps I’ve taken… advent20/README.md at master · h-j-k/advent20 (github.com)