Day 10

Setup

Mix.install([
  {:kino, "~> 0.4.1"}
])

Input

input = Kino.Input.textarea("Please paste your input:")

NavigationSubsystem

defmodule NavigationSubsystem do
  @matching_characters %{
    ?[ => ?],
    ?{ => ?},
    ?( => ?),
    ?< => ?>
  }

  @opening_characters Map.keys(@matching_characters)
  @closing_characters Map.values(@matching_characters)

  @score_part_1 %{?) => 3, ?] => 57, ?} => 1197, ?> => 25137}
  @score_part_2 %{?) => 1, ?] => 2, ?} => 3, ?> => 4}

  defstruct stack: [], invalid_char: nil

  def new(line) do
    %__MODULE__{}
    |> add_line(line)
  end

  def complete?(%{stack: []}), do: true
  def complete?(_), do: false

  def valid?(%{invalid_char: nil}), do: true
  def valid?(_), do: false

  def score_part_1(ns) do
    @score_part_1[ns.invalid_char]
  end

  def score_part_2(ns) do
    ns.stack
    |> Enum.reduce(0, fn char, acc -> acc * 5 + @score_part_2[char] end)
  end

  defp add_line(ns, line) do
    line
    |> String.to_charlist()
    |> Enum.reduce(ns, &amp;add_char(&amp;2, &amp;1))
  end

  defp add_char(%{invalid_char: invalid_char} = ns, _char) when not is_nil(invalid_char), do: ns

  defp add_char(%{stack: [char | rest_stack]} = ns, char) when char in @closing_characters do
    %{ns | stack: rest_stack}
  end

  defp add_char(ns, char) when char in @closing_characters do
    %{ns | invalid_char: char}
  end

  defp add_char(ns, char) when char in @opening_characters do
    %{ns | stack: [Map.get(@matching_characters, char) | ns.stack]}
  end
end

Part 1

input
|> Kino.Input.read()
|> String.split("\n", trim: true)
|> Enum.map(&NavigationSubsystem.new/1)
|> Enum.reject(&NavigationSubsystem.valid?/1)
|> Enum.map(&NavigationSubsystem.score_part_1/1)
|> Enum.sum()

Part 2

input
|> Kino.Input.read()
|> String.split("\n", trim: true)
|> Enum.map(&NavigationSubsystem.new/1)
|> Enum.filter(&NavigationSubsystem.valid?/1)
|> Enum.reject(&NavigationSubsystem.complete?/1)
|> Enum.map(&NavigationSubsystem.score_part_2/1)
|> then(fn scores ->
  scores
  |> Enum.sort()
  |> Enum.at(div(length(scores), 2))
end)