The ~~~ operator when given a positive number always returns a negative number. It does that simulate that an integer holds an infinite number of bits. If the number starts with an infinite number of ones, the number is negative. If it starts with an infinite number of zeroes, it is positive.

When I first learned Erlang, it took me a while to figure out why that’s make sense.

Yes, an infinite number of bits. Fortunately the runtime system is smart enough to not store all them explicitly.

Again, my solution:

Part 1

#!/usr/bin/env elixir

use Bitwise

max = ((1 <<< 12) - 1)

gamma =
  File.stream!("input.txt")
  |> Stream.map(&String.trim/1)
  |> Stream.map(&String.split(&1, "", trim: true))
  |> Enum.zip()
  |> Enum.map(&Tuple.to_list/1)
  |> Stream.map(&Enum.frequencies/1)
  |> Stream.map(&Enum.max_by(&1, fn {_, f} -> f end))
  |> Stream.map(&elem(&1, 0))
  |> Enum.join("")
  |> String.to_integer(2)

epsilon = max - gamma

IO.inspect(epsilon * gamma)

Part 2

#!/usr/bin/env elixir

defmodule P2 do
  def o2([elem], _pos), do: to_i(elem)

  def o2(list, pos) do
    groups = Enum.group_by(list, &elem(&1, pos))
    group0 = groups["0"] || []
    group1 = groups["1"] || []
    if length(group0) > length(group1) do
      o2(group0, pos + 1)
    else
      o2(group1, pos + 1)
    end
  end

  def co2([elem], _pos), do: to_i(elem)

  def co2(list, pos) do
    groups = Enum.group_by(list, &elem(&1, pos))
    group0 = groups["0"] || []
    group1 = groups["1"] || []
    if length(group1) < length(group0) do
      co2(group1, pos + 1)
    else
      co2(group0, pos + 1)
    end
  end

  defp to_i(tuple), do:
    tuple
    |> Tuple.to_list()
    |> Enum.join("")
    |> String.to_integer(2)
end

input =
  File.stream!("input.txt")
  |> Stream.map(&String.trim/1)
  |> Stream.map(&String.split(&1, "", trim: true))
  |> Enum.map(&List.to_tuple/1)

IO.inspect(P2.o2(input, 0) * P2.co2(input, 0))

My solution: https://github.com/josevalim/aoc/blob/main/2021/day-03.livemd

Live streaming: Twitch - we also solved part 1 with Nx and had a bit of fun with Livebook.