defmodule Day10 do
  def run do
    map =
      File.read!("data.txt")
      |> String.trim()
      |> String.split("\n")
      |> Enum.map(fn el -> String.graphemes(el) |> Enum.map(&String.to_integer/1) end)

    height = length(map)
    width = length(hd(map))

    trailheads =
      Enum.concat(map)
      |> Enum.with_index()
      |> Enum.filter(fn {el, _} -> el == 0 end)
      |> Enum.map(fn {_, idx} -> {div(idx, height), rem(idx, width)} end)

    paths1 =
      Enum.reduce(trailheads, 0, fn th, acc ->
        (walk(th, map) |> List.flatten() |> Enum.uniq() |> length) + acc
      end)

    dbg(paths1)

    # same as part one, just don't uniq() them
    paths2 =
      Enum.reduce(trailheads, 0, fn th, acc ->
        (walk(th, map) |> List.flatten() |> length) + acc
      end)

    dbg(paths2)
  end

  defp walk(coord, map) do
    value = element_at(coord, map)

    if value == 9 do
      coord
    else
      # find the coords of all neighbors
      neighbors(coord)
      # filter by whether the element is 1 greater
      |> Enum.filter(fn c -> element_at(c, map) == value + 1 end)
      # add this path to the list. it'll be empty and then flattened away if it doesn't reach the top
      |> Enum.reduce([], fn el, acc -> [walk(el, map) | acc] end)
    end
  end

  defp neighbors({x, y}) do
    [{-1, 0}, {0, 1}, {1, 0}, {0, -1}]
    |> Enum.map(fn {dx, dy} -> {x + dx, y + dy} end)
    |> Enum.reject(fn {x2, y2} -> x2 < 0 or y2 < 0 end)
  end

  defp element_at({x, y}, map) do
    row = Enum.at(map, x)
    if is_nil(row), do: nil, else: Enum.at(row, y)
  end
end

Day10.run()