Day 5

Mix.install([
  {:kino, "~> 0.13.0"}
])

Part 1

[first, last] =
  Kino.FS.file_path("day5_1.txt")
  |> File.read!()
  |> String.split("\n\n", trim: true) # Splitting sections by empty line
  |> Enum.map(fn section -> String.split(section, "\n", trim: true) end)

defmodule Part1 do
  # Creates a map of the page ordering rules
  # We use the second number as the key
  def precendence(section) do
    Enum.reduce(section, %{}, fn item, acc ->
      [val, key] = String.split(item, "|")
      Map.update(acc, key, [val], fn values -> [val | values] end)
    end)
  end

  def validate_update(numbers, rules) do
    Enum.with_index(numbers)
    |> Enum.reduce_while(:ok, fn {n, idx}, _acc ->
      next_numbers = Enum.slice(numbers, idx + 1..-1//1)

      case Map.get(rules, n) do
        nil ->
          {:cont, :ok}

        forbidden ->
          if Enum.any?(next_numbers, &(&1 in forbidden)) do
            {:halt, :error}
          else
            {:cont, :ok}
          end
      end
    end)
  end
end

rules = Part1.precendence(first)

updates =
  last |> Enum.map(fn line -> 
    numbers = String.split(line, ",")

    case Part1.validate_update(numbers, rules) do
      :ok ->
        Enum.at(numbers, Integer.floor_div(length(numbers), 2)) |> String.to_integer
      :error ->
        0
    end
  end)
|> Enum.sum

Part 2

bad_updates =
  last
  |> Enum.map(fn line ->
    numbers = String.split(line, ",")

    case Part1.validate_update(numbers, rules) do
      :ok ->
        # ignoring correct ones now
        0

      :error ->
        # We just have to sort using the rules
        sorted = Enum.sort(numbers, fn a, b -> b in Map.get(rules, a) end)
        Enum.at(sorted, Integer.floor_div(length(sorted), 2)) |> String.to_integer()
    end
  end)
  |> Enum.sum()