Day 3

Helpers

defmodule Helpers do
  use Bitwise

  def data() do
    "data/day03.txt"
    |> File.stream!()
    |> Stream.map(&String.trim/1)
  end

  def string_to_intlist(s) do
    s
    |> String.graphemes()
    |> Enum.map(&String.to_integer/1)
  end

  def add_lines(l1, l2) when length(l1) == length(l2) do
    l1
    |> Enum.zip(l2)
    |> Enum.map(fn {n1, n2} -> n1 + n2 end)
  end

  def list_to_dec(l) do
    l
    |> Enum.join("")
    |> String.to_integer(2)
  end

  def string_to_int2(s) do
    s
    |> String.to_integer(2)
  end

  def filter_common_bit([result], _bit, _most_common), do: result

  def filter_common_bit(data, bit, most_common) when bit >= 0 and most_common in [true, false] do
    data
    |> Enum.group_by(fn n -> (n &amp;&amp;&amp; 1 <<< bit) > 0 end)
    |> then(fn result ->
      if length(result[false]) > length(result[true]) do
        filter_common_bit(result[!most_common], bit - 1, most_common)
      else
        filter_common_bit(result[most_common], bit - 1, most_common)
      end
    end)
  end
end

Part 1

import Helpers

data_list = data() |> Enum.to_list()

cutoff = length(data_list) / 2

totals =
  data_list
  |> Stream.map(&string_to_intlist/1)
  |> Enum.reduce(&add_lines/2)

gamma =
  totals
  |> Enum.map(&if &1 >= cutoff, do: 1, else: 0)
  |> list_to_dec()

epsilon =
  totals
  |> Enum.map(&amp;if &amp;1 < cutoff, do: 1, else: 0)
  |> list_to_dec()

gamma * epsilon

Part 2

import Helpers
use Bitwise

data_list =
  data()
  |> Enum.map(&string_to_int2/1)

bit_count = 12

oxygen = filter_common_bit(data_list, bit_count - 1, true)
co2 = filter_common_bit(data_list, bit_count - 1, false)

oxygen * co2