aoc2017_day17.ex

defmodule Day17 do
  def part1(num_steps), do:
    insert(2017, num_steps, [], &List.insert_at/3)
    |> Stream.chunk_every(2, 1)
    |> Enum.find(&match?([2017, _], &1))
    |> Enum.at(1)

  def part2(num_steps), do:
    insert(50_000_000, num_steps, nil,
      fn
        (_current_after_zero, 1, new_after_zero) -> new_after_zero
        (current_after_zero, _pos, _value) -> current_after_zero
      end
    )

  defp insert(count, num_steps, acc, reducer), do:
    insert(count + 1, num_steps, acc, reducer, 0, 0)

  defp insert(0, _num_steps, acc, _reducer, _value, _pos), do: acc
  defp insert(count, num_steps, acc, reducer, value, pos) do
    acc = reducer.(acc, pos, value)
    next_value = value + 1
    next_pos = rem(pos + num_steps, next_value) + 1
    insert(count - 1, num_steps, acc, reducer, next_value, next_pos)
  end
end

System.argv() |> hd() |> String.to_integer() |> Day17.part1() |> IO.inspect
System.argv() |> hd() |> String.to_integer() |> Day17.part2() |> IO.inspect