Day 2

Mix.install([
  {:kino, "~> 0.14.2"}
])

Section

input = Kino.Input.textarea("Puzzle Input")

Part 1

defmodule Solution.Part1 do
  def safe_report?(type, list) when type == :inc or type == :dec do
    Enum.all?(list, &safe?(type, &1))
  end

  defp safe?(:inc, v), do: v >= 1 and v <= 3
  defp safe?(:dec, v), do: v <= -1 and v >= -3

  def report_type([a, b | _] = list) do
    cond do
      a > 0 and b > 0 -> {:inc, list}
      true -> {:dec, list}
    end
  end
end

Kino.Input.read(input)
|> String.split("\n", trim: true)
|> Enum.map(fn v -> String.split(v, " ", trim: true) |> Enum.map(&amp;String.to_integer/1) end)
|> Enum.map(fn v ->
  v |> Enum.chunk_every(2, 1, :discard) |> Enum.map(fn [a, b] -> b - a end)
end)
|> Enum.map(&amp;Solution.Part1.report_type/1)
|> Enum.count(fn {type, list} -> Solution.Part1.safe_report?(type, list) end)

Part 2

defmodule Solution.Part2 do
  def safe?(list) do
    diffs = list |> Enum.chunk_every(2, 1, :discard) |> Enum.map(fn [a, b] -> b - a end) 
    safe = Enum.all?(diffs, &safe?(:inc, &1)) or Enum.all?(diffs, &safe?(:dec, &1))
    safe
  end

  def safe_with_dumpener?(list) do
    safe = safe?(list)

    # Just go and try to remove one number and see if it helps
    safe_removed = 0..(length(list) - 1)
    |> Enum.any?(fn index ->
      list
      |> List.delete_at(index)
      |> safe?()
    end)

    safe or safe_removed
  end

  # Checks if the element is safe based on the fact if the sequence increases or decreses
  defp safe?(:inc, v), do: v >= 1 and v <= 3
  defp safe?(:dec, v), do: v <= -1 and v >= -3
end

Kino.Input.read(input)
|> String.split("\n", trim: true)
|> Enum.map(fn v -> String.split(v, " ", trim: true) |> Enum.map(&amp;String.to_integer/1) end)
|> Enum.count(&amp;Solution.Part2.safe_with_dumpener?/1)

Today I Learned

• Sometimes it’s easier to get the easiest solution out. Like just dumb-way checking by removing each number and see if it helps. I spent too much time trying to make it work with some fancy solution…
• List.delete_at/2