Day 8: Resonant Collinearity

Mix.install([:kino])

Section

input = Kino.Input.textarea("Input", monospace: true)

{{width, height}, antennas} =
  input
  |> Kino.Input.read()
  |> String.split()
  |> Enum.with_index()
  |> then(fn rows ->
    {{String.length(elem(hd(rows), 0)), Enum.count(rows)},
     Enum.reduce(rows, %{}, fn {row, y}, antennas ->
       row
       |> String.graphemes()
       |> Enum.with_index()
       |> Enum.reduce(antennas, fn
         {".", _x}, antennas ->
           antennas

         {antenna, x}, antennas ->
           Map.update(antennas, antenna, [{x, y}], &[{x, y} | &1])
       end)
     end)
     |> Map.values()
     |> Enum.flat_map(fn antennas ->
       for {x1, y1} = a1 <- antennas,
           {x2, y2} = a2 when a1 != a2 <- antennas do
         {x1, y1, x2 - x1, y2 - y1}
       end
     end)}
  end)

defmodule Antenna do
  @width width
  @height height

  def antinodes(antenna, dx, dy) do
    Stream.unfold(antenna, fn
      {x, y} when x >= 0 and x < @width and y >= 0 and y < @height ->
        {{x, y}, {x + dx, y + dy}}

      _ ->
        nil
    end)
  end
end

MapSet.size(
  for antinode when not is_nil(antinode) <-
        Enum.map(antennas, fn {x, y, dx, dy} ->
          Enum.at(Antenna.antinodes({x, y}, dx, dy), 2)
        end),
      into: MapSet.new(),
      do: antinode
)

MapSet.size(
  for {x, y, dx, dy} <- antennas,
      antinode <- Antenna.antinodes({x, y}, dx, dy),
      into: MapSet.new(),
      do: antinode
)