AoC 2020 Day 15 Part 2 - brute force without Enum.reduce

day15part2.ex

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

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

    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)

    run(start, stop, Enum.at(already_spoken, -1), start - 1, spoken)
  end

  defp run(turn, stop, _last, before, spoken) when turn <= stop do
    if before == turn - 1 do
      before = :atomics.exchange(spoken, 0 + 1, turn)
      run(turn + 1, stop, 0, (before == 0 && turn) || before, spoken)
    else
      n = turn - 1 - before
      before = :atomics.exchange(spoken, n + 1, turn)
      run(turn + 1, stop, n, (before == 0 && turn) || before, spoken)
    end
  end

  defp run(turn, stop, last, _before, _spoken) when turn > stop do
    last
  end
end