defmodule AOC.Day3 do
  @line_length 31

  def count_trees(lines, jump), do: count_trees(lines, jump, 0, 0)
  def count_trees([], _, _, acc), do: acc

  def count_trees([hd | tl], jump, x, acc) do
    acc =
      acc +
        if String.at(hd, x) == "#" do
          1
        else
          0
        end

    x = rem(x + jump, @line_length)
    count_trees(tl, jump, x, acc)
  end

  def part1 do
    AOC.Util.input_lines(3, 1)
    |> count_trees(3)
  end

  def part2 do
    lines = AOC.Util.input_lines(3, 1)

    count_down2 = Enum.take_every(lines, 2) |> count_trees(1)
    tree_counts = Enum.map([1, 3, 5, 7], &count_trees(lines, &1))

    Enum.reduce([count_down2 | tree_counts], &*/2)
  end
end