🎄 Year 2020 🔔 Day 09

Parse input

filename = "#{__DIR__}/../../../inputs/2020/day09.txt"

parsed_input_stream =
  File.stream!(filename)
  |> Enum.map(&String.trim/1)
  |> Enum.map(&String.to_integer/1)

Part One

part_1_result =
  parsed_input_stream
  |> Enum.chunk_every(26, 1, :discard)
  |> Enum.map(&Enum.split(&1, 25))
  |> Enum.find(fn {preamble, [num]} ->
    combinations = for x <- preamble, y <- preamble, x != y, do: x + y
    !Enum.any?(combinations, &(&1 == num))
  end)
  |> then(fn {_preamble, [num]} -> num end)

Part two

Stream.iterate(0, &(&1 + 1))
|> Stream.map(&Enum.drop(parsed_input_stream, &1))
|> Enum.find_value(fn [first | rest] ->
  {min, max, total} =
    Enum.reduce_while(rest, {first, first, first}, fn num, {min_num, max_num, total} ->
      total = num + total
      min_num = min(num, min_num)
      max_num = max(num, max_num)

      if total >= part_1_result do
        {:halt, {min_num, max_num, total}}
      else
        {:cont, {min_num, max_num, total}}
      end
    end)

  if total == part_1_result, do: min + max, else: false
end)