AoC 2020 Day 10 Part 2

day10_p2.ex

defmodule Day10P2 do
  def run do
    data = "data/10" |> File.read!() |> String.split() |> Enum.map(&String.to_integer/1)

    [0 | data]
    |> Enum.sort(:desc)
    |> Enum.reduce([{hd(data) + 3, 1}], &[{&1, sum3(&1, &2)} | &2])
    |> hd()
    |> elem(1)
  end

  defp sum3(i, [{_, ac}, {_, bc}, {c, cc} | _]) when c == i + 3, do: ac + bc + cc
  defp sum3(i, [{_, ac}, {b, bc} | _]) when b == i + 2, do: ac + bc
  defp sum3(_, [{_, ac} | _]), do: ac
end