aoc2020day9.ex

defmodule Day9 do
  @preamble_length 25

  def read_input do
    File.read!("input/day9.txt")
    |> String.split("\n", trim: true)
    |> Enum.map(&String.to_integer/1)
  end

  def valid?(window, item) do
    for i <- window, j <- window, i != j do
      i + j == item
    end
    |> Enum.any?
  end

  def partial_sum_edges(sequence, target) do
    first = Enum.at(sequence, 0)
    Enum.reduce_while(sequence, { 0, first, 0 }, fn val, { total, smallest, largest } ->
      cond do
        total == target -> { :halt, { total, smallest, largest } }
        total > target ->  { :halt, { :notfound, 0, 0 } }
        true -> { :cont, { total + val, min(smallest, val), max(largest, val) } }
      end
    end)
  end

  def find_partial_sum(data, target) do
    case partial_sum_edges(data, target) do
      { :notfound, _, _ } -> find_partial_sum(Enum.drop(data, 1), target)
      { ^target, first, last } -> first + last
    end
  end

  def find_first_invalid(window, [item | rest]) do
    if valid?(window, item) do
      find_first_invalid(Enum.drop(window, 1) ++ [item], rest)
    else
      item
    end
  end

  def part1 do
    [window, data] = [Enum.take(read_input(), @preamble_length), Enum.drop(read_input(), @preamble_length)]
    find_first_invalid(window, data)
    |> IO.inspect
  end

  def part2 do
    [window, data] = [Enum.take(read_input(), @preamble_length), Enum.drop(read_input(), @preamble_length)]
    invalid_number = find_first_invalid(window, data)
    find_partial_sum(window ++ data, invalid_number)
    |> IO.inspect
  end
end