AdventOfCode/2020/elixir/day9/day9part2.exs

defmodule Day9Part2 do
  def run do
    input =
      File.read!("input")
      |> String.split("\n", trim: true)
      |> Enum.map(&String.to_integer/1)

    invalid_number = Day9Part1.find_invalid(input)

    input
    |> find_weakness(invalid_number)
    |> IO.puts()
  end

  def find_weakness([current | rest], invalid_number) do
    case find_group(rest, [current], current, invalid_number) do
      {:ok, group} -> Enum.min(group) + Enum.max(group)
      _ -> find_weakness(rest, invalid_number)
    end
  end

  def find_group(_list, group, sum, target) when sum == target, do: {:ok, group}
  def find_group(_list, _group, sum, target) when sum > target, do: :too_big
  def find_group(_list, [], _sum, _target), do: :eol

  def find_group([next | rest], group, sum, target) do
    find_group(rest, [next | group], sum + next, target)
  end
end

defmodule Day9Part1 do
  def run do
    File.read!("input")
    |> String.split("\n", trim: true)
    |> Enum.map(&String.to_integer/1)
    |> find_invalid()
    |> IO.puts()
  end

  def find_invalid([_ | next] = numbers) do
    {preamble, [number | _]} = Enum.split(numbers, 25)
    sorted_preamble = Enum.sort(preamble)
    max = number - hd(sorted_preamble)

    if sum_in_preamble?(sorted_preamble, max, number) do
      find_invalid(next)
    else
      number
    end
  end

  def sum_in_preamble?([], _max, _number), do: false
  def sum_in_preamble?([preamble_head | _], max, _number) when preamble_head >= max, do: false

  def sum_in_preamble?([preamble_head | preamble_rest], max, number) do
    if forms_pair?(preamble_head, preamble_rest, max, number) do
      true
    else
      sum_in_preamble?(preamble_rest, max, number)
    end
  end

  def forms_pair?(_x, [y | _], max, _number) when y > max, do: false
  def forms_pair?(_x, [], _max, _number), do: false
  def forms_pair?(x, [y | _], _max, number) when x + y == number, do: true
  def forms_pair?(x, [_y | rest], max, number), do: forms_pair?(x, rest, max, number)
end

Day9Part2.run()