AoC 2020 Day 15 Part 2

day15part2.ex

defmodule Day15Part2 do
  def go do
    261_214 = solve([1, 2, 3]) |> elem(0)
  end

  defp solve(already_spoken) do
    start = Enum.count(already_spoken) + 1
    stop = 30_000_000

    # Assumes last spoken was spoken for the first time.
    last = {Enum.at(already_spoken, -1), start - 1}

    spoken = :atomics.new(stop, [])
    # Fill already spoken, n is stored with index n + 1 (atomics mem starts at 1).
    Enum.each(Enum.with_index(already_spoken, 1), fn {n, turn} ->
      :atomics.put(spoken, n + 1, turn)
    end)

    Enum.reduce(start..stop, last, fn turn, {_last, before} ->
      if before == turn - 1 do
        before = :atomics.exchange(spoken, 0 + 1, turn)
        {0, (before == 0 && turn) || before}
      else
        n = turn - 1 - before
        before = :atomics.exchange(spoken, n + 1, turn)
        {n, (before == 0 && turn) || before}
      end
    end)
  end
end