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Discussion on: Advent of Code 2020 Solution Megathread - Day 3: Toboggan Trajectory

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patryk profile image
Patryk Woziński

typical December in Europe

There is my solution, Elixir as always. I'll switch to Golang/PHP when I get stuck.

Day 3, part 1:

defmodule AdventOfCode.Day3Part1 do
  @position_move 3
  def calculate(file_path) do
    {_, trees} = file_path
    |> File.stream!()
    |> Stream.map(&String.replace(&1, "\n", ""))
    |> Enum.to_list()
    |> Enum.reduce({0, 0}, fn line, {position, trees} ->
      meet_tree = String.at(line, position) == "#"
      trees = if meet_tree, do: trees + 1, else: trees
      position = rem(position + @position_move, String.length(line))

      {position, trees}
    end)

    trees
  end
end
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Day 3 part 2:

defmodule AdventOfCode.Day3Part2 do
  def calculate(file_path) do
    forest =
      file_path
      |> File.stream!()
      |> Stream.map(&String.replace(&1, "\n", ""))
      |> Enum.to_list()

      [{1, 1}, {1, 3}, {1, 5}, {1, 7}, {2, 1}]
      |> Enum.map(&(analyse_for_slope(forest, &1)))
      |> Enum.reduce(&(&1 * &2))
  end

  defp analyse_for_slope(forest, {move_down, move_right}) do
    {_, trees} =
      Enum.zip(0..Enum.count(forest), forest)
      |> Enum.filter(fn {line_number, _} -> rem(line_number, move_down) == 0 end)
      |> Enum.reduce({0, 0}, fn {_, line}, {position, trees} ->
        meet_tree = String.at(line, position) == "#"
        trees = if meet_tree, do: trees + 1, else: trees
        position = rem(position + move_right, String.length(line))

        {position, trees}
      end)

    trees
  end
end
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What's your experience guys? Better to keep both parts in one class/module / whatever?

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rpalo profile image
Ryan Palo Author

I typically keep them in the same file in different functions, since work from part 1 is often shared to part 2, (like today), but every so often, he throws a curveball and part 2 ends up being a total rethink, and then it may be nicer to have more separation. It probably doesn’t matter a ton :)