Day 5

Mix.install([
  {:kino, "~> 0.8.0"}
])

Puzzle Input

input = Kino.Input.textarea("Paste puzzle input")

defmodule Day5 do
  def stack_line?(line), do: String.contains?(line, "[")
  defp empty_string?(char), do: char == ""

  def line_to_chars(line) do
    line
    |> String.split("")
    |> Enum.reject(&empty_string?/1)
  end

  def chars_to_chunk(chars) do
    chars
    |> Enum.chunk_every(4)
  end

  def join_and_index_chunks(chunks) do
    chunks
    |> Enum.map(fn chunk -> Enum.join(chunk, "") |> String.trim() end)
    |> Enum.with_index(1)
  end

  # first `.reduce` iteration
  def reduce_lines({"", _index}, acc), do: acc

  def reduce_lines({value, index}, stacks_map) do
    Map.update(stacks_map, index, [value], fn current_value ->
      Enum.concat(current_value, [value])
    end)
  end

  def map_instruction([_, move, _, from, _, to]) do
    %{
      :move => String.to_integer(move),
      :from => String.to_integer(from),
      :to => String.to_integer(to)
    }
  end

  def apply_instruction_to_stacks(current_instruction, stacks) do
    %{from: from, move: move, to: to} = current_instruction

    existing = Map.get(stacks, to)

    {taken, left_over} =
      Map.get(stacks, from)
      |> Enum.split(move)

    new_to = Enum.concat(Enum.reverse(taken), existing)

    %{stacks | from => left_over, to => new_to}
  end

  def apply_instructions_to_stacks_part_2(current_instruction, stacks) do
    %{from: from, move: move, to: to} = current_instruction

    existing = Map.get(stacks, to)

    {taken, left_over} =
      Map.get(stacks, from)
      |> Enum.split(move)

    new_to = Enum.concat(taken, existing)

    %{stacks | from => left_over, to => new_to}
  end
end

Part 1

[top, bottom] =
  Kino.Input.read(input)
  |> String.split("\n\n")

stacks =
  top
  |> String.split("\n")
  |> Enum.take_while(&Day5.stack_line?/1)
  |> Enum.map(&Day5.line_to_chars/1)
  |> Enum.map(&Day5.chars_to_chunk/1)
  |> Enum.map(&Day5.join_and_index_chunks/1)
  |> Enum.concat()
  |> Enum.reduce(%{}, &Day5.reduce_lines/2)

instructions =
  bottom
  |> String.split("\n")
  |> Enum.map(fn instruction -> String.split(instruction, " ") end)
  |> Enum.map(&Day5.map_instruction/1)
  |> Enum.reduce(stacks, &Day5.apply_instruction_to_stacks/2)
  |> Map.values()
  |> Enum.map(fn v -> Enum.at(v, 0) end)
  |> Enum.map(fn s -> String.replace(s, "[", "") |> String.replace("]", "") end)
  |> Enum.join()

Part 2

[top, bottom] =
  Kino.Input.read(input)
  |> String.split("\n\n")

stacks =
  top
  |> String.split("\n")
  |> Enum.take_while(&Day5.stack_line?/1)
  |> Enum.map(&Day5.line_to_chars/1)
  |> Enum.map(&Day5.chars_to_chunk/1)
  |> Enum.map(&Day5.join_and_index_chunks/1)
  |> Enum.concat()
  |> Enum.reduce(%{}, &Day5.reduce_lines/2)

instructions =
  bottom
  |> String.split("\n")
  |> Enum.map(fn instruction -> String.split(instruction, " ") end)
  |> Enum.map(&Day5.map_instruction/1)
  |> Enum.reduce(stacks, &Day5.apply_instructions_to_stacks_part_2/2)
  |> Map.values()
  |> Enum.map(fn v -> Enum.at(v, 0) end)
  |> Enum.map(fn s -> String.replace(s, "[", "") |> String.replace("]", "") end)
  |> Enum.join()