Day 9

Mix.install([
  {:nx, "~> 0.5"},
  {:kino, "~> 0.11.3"}
])

Section

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

input_data = Kino.Input.read(input)
test_cases = String.split(input_data, "\n")

defmodule Sol9 do
  def read_case(text) do
    Nx.tensor(Enum.map(String.split(text), &String.to_integer(&1)))
  end

  def get_diffs([first_seq | rest_seqs]) do
    if Nx.sum(first_seq) == Nx.tensor(0) do
      [first_seq | rest_seqs]
    else
      get_diffs([Nx.diff(first_seq), first_seq | rest_seqs])
    end
  end

  def sum_last_element(seqs) do
    Enum.reduce(seqs, 0, fn x, acc -> acc + Nx.to_number(x[-1]) end)
  end

  def get_first_elements(seqs) do
    Enum.map(seqs, &Nx.to_number(&1[0])) |> Enum.reverse()
  end

  def solve_part1(test_cases) do
    Enum.reduce(test_cases, 0, fn test_case, acc ->
      acc +
        Sol9.sum_last_element(Sol9.get_diffs([Sol9.read_case(test_case)]))
    end)
  end

  def solve_part2(test_case) do
    first_elements = Sol9.get_first_elements(Sol9.get_diffs([Sol9.read_case(test_case)]))

    even_elements = Enum.drop_every(first_elements, 2)
    IO.inspect({first_elements, even_elements})
    odd_elements = first_elements -- even_elements
    Enum.sum(odd_elements) - Enum.sum(even_elements)
  end
end

Sol9.solve_part1(test_cases)

Enum.map(test_cases, &Sol9.solve_part2(&1))
|> Enum.sum()

Enum.drop_every([1, 2, 3, 4, 5], 2)