day3.livemd

Day 3

Setup

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

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

Part 1

input_arr =
  input
  |> Kino.Input.read()
  |> String.trim()
  |> String.split("\n")

length = input_arr |> hd() |> String.length()
max_digit = length - 1

most_and_least_common_at = fn bin_list, pos ->
  digits_at_pos = Enum.map(bin_list, &String.at(&1, pos))

  count_ones = Enum.count(digits_at_pos, fn x -> x == "1" end)
  count_zeros = Enum.count(digits_at_pos, fn x -> x == "0" end)

  if count_ones >= count_zeros, do: {1, 0}, else: {0, 1}
end

gamma =
  0..max_digit
  |> Enum.map(fn pos ->
    {gamma, _eps} = most_and_least_common_at.(input_arr, pos)
    gamma
  end)
  |> Integer.undigits(2)

eps =
  0..max_digit
  |> Enum.map(fn pos ->
    {_gamma, eps} = most_and_least_common_at.(input_arr, pos)
    eps
  end)
  |> Integer.undigits(2)

gamma * eps

Part 2

select_most_at_pos = fn pos, bin_list ->
  {most, _} = most_and_least_common_at.(bin_list, pos)

  bin_list
  |> Enum.reject(fn bin -> String.at(bin, pos) != Integer.to_string(most) end)
end

select_least_at_pos = fn pos, bin_list ->
  {_, least} = most_and_least_common_at.(bin_list, pos)

  bin_list
  |> Enum.reject(fn bin -> String.at(bin, pos) != Integer.to_string(least) end)
end

oxygen =
  Enum.reduce_while(0..max_digit, input_arr, fn pos, acc ->
    if length(acc) == 1 do
      {:halt, acc}
    else
      acc = select_most_at_pos.(pos, acc)
      {:cont, acc}
    end
  end)
  |> hd()
  |> Integer.parse(2)
  |> elem(0)

co2 =
  Enum.reduce_while(0..max_digit, input_arr, fn pos, acc ->
    if length(acc) == 1 do
      {:halt, acc}
    else
      acc = select_least_at_pos.(pos, acc)
      {:cont, acc}
    end
  end)
  |> hd()
  |> Integer.parse(2)
  |> elem(0)

oxygen * co2