Day 2

Utils

defmodule U do
  def numbers(l) do 
    String.split(l) |> Enum.map(&String.to_integer/1)
  end
end

input = File.stream!("2/example.txt") |> Stream.map(&U.numbers/1)

defmodule Day2 do
  def is_safe([n1, n2 | _] = l) do 
    is_increasing = 0 < n1 - n2
    is_safe(is_increasing, l)
  end

  defp is_safe(_, [_]), do: true

  defp is_safe(is_increasing, [p, n | tail]) do
    diff = p - n
    ((0 < diff) == is_increasing) &amp;&amp; (0 < abs(diff)) &amp;&amp; (abs(diff) < 4) &amp;&amp; is_safe(is_increasing, [n | tail])
  end
end

input |> Stream.filter(&Day2.is_safe/1) |> Enum.count()

defmodule Day2_2 do
  def is_safe(l), do: is_safe(nil, l, true)

  defp is_safe(_, [_], _), do: true
  
  defp is_safe(is_increasing, [n1, n2 | tail], can_fail) do 
    IO.inspect({is_increasing, n1, n2, can_fail})
    if (n1 == n2) || 3 < abs(n1 - n2) do 
      if can_fail do
        is_safe(is_increasing, [n2 | tail], false)
      else
        false
      end
    else
      is_increasing = if is_increasing == nil, do: 0 < n2 - n1, else: is_increasing
      if 0 < (n2 - n1) != is_increasing do
        if can_fail do
          is_safe(is_increasing, [n2 | tail], false)
        else
          false
        end
      else
        is_safe(is_increasing, [n2 | tail], can_fail)
      end
    end
  end
end

input |> Stream.filter(&Day2_2.is_safe/1) |> Enum.count()