Day 3

Setup

Mix.install([
  {:kino, "~> 0.4.1"}
])

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

input =
  input
  |> Kino.Input.read()
  |> String.split("\n", trim: true)
  |> Enum.map(&String.codepoints/1)

Part 1

defmodule Power do
  def count_bits(input) do
    input |> String.graphemes() |> Enum.frequencies()
  end

  def rank_bits(%{"0" => zero, "1" => one}) when zero > one, do: [0, 1]
  def rank_bits(%{"0" => _, "1" => _}), do: [1, 0]
  def rank_bits(%{"1" => _}), do: [1, 0]
  def rank_bits(%{"0" => _}), do: [0, 1]
end

input
|> Enum.zip_with(&Enum.join/1)
|> Enum.map(&(&1 |> Power.count_bits() |> Power.rank_bits()))
|> Enum.zip_with(&Enum.join/1)
|> Enum.map(&(&1 |> String.to_integer(2)))
|> Enum.product()

Part 2

defmodule LifeSupport do
  def oxygen_rating([head | []], _, _), do: head |> Enum.join() |> String.to_integer(2)

  def oxygen_rating(input, bit_to_select, column) do
    bits =
      input
      |> Enum.zip_with(&Enum.join/1)
      |> Enum.map(&(&1 |> Power.count_bits() |> Power.rank_bits()))
      |> Enum.map(&Enum.at(&1, bit_to_select))

    input =
      input
      |> Enum.filter(fn x ->
        y = x |> Enum.join()
        String.to_integer(String.at(y, column)) == Enum.at(bits, column)
      end)

    oxygen_rating(input, bit_to_select, column + 1)
  end

  def co2_rating(input, column), do: oxygen_rating(input, 1, column)
end

oxy = input |> LifeSupport.oxygen_rating(0, 0)
co2 = input |> LifeSupport.co2_rating(0)

oxy * co2