Day 8

input =
  File.stream!("day8.txt")
  |> Stream.map(fn line ->
    line
    |> String.split(" | ")
    |> Enum.map(fn part ->
      part
      |> String.trim()
      |> String.split(" ")
      |> Enum.map(fn disp ->
        # Sort characters in each entry to simplify later work
        disp
        |> String.to_charlist()
        |> Enum.sort()
        |> List.to_string()
      end)
    end)
    |> List.to_tuple()
  end)

#Stream<[
  enum: %File.Stream{
    line_or_bytes: :line,
    modes: [:raw, :read_ahead, :binary],
    path: "day8.txt",
    raw: true
  },
  funs: [#Function<47.58486609/1 in Stream.map/2>]
]>

Task 1

We simply need to count all occurences of the values that have 2, 3, 4, or 7 highlighted segments.

input
|> Enum.map(fn {_, output} ->
  Enum.count(output, &(byte_size(&1) in [2, 3, 4, 7]))
end)
|> Enum.sum()

390

Task 2

defmodule Day8.Task2 do
  defp a --- b, do: MapSet.difference(a, b)

  defp a +++ b, do: MapSet.union(a, b)

  defp a <~> b, do: MapSet.intersection(a, b)

  #            1.   7.    4. 2|3|5. 2|3|5. 2|3|5.  6|9|0.  6|9|0.  6|9|0.       8.
  def deduce([cf, acf, bcdf, acdeg, acdfg, abdfg, abdefg, abcdfg, abcefg, abcdefg]) do
    eg = abcdefg --- (acf +++ bcdf)
    bd = bcdf --- cf
    adg = acdeg <~> acdfg <~> abdfg
    abfg = abdefg <~> abcdfg <~> abcefg
    a = acf --- cf
    b = abfg <~> bd
    f = abfg <~> cf
    g = adg <~> eg
    d = adg <~> bd
    c = cf --- f
    e = eg --- g

    [a, b, c, d, e, f, g] =
      [a, b, c, d, e, f, g]
      |> Enum.map(&extract/1)

    [
      # 0
      [a, b, c, e, f, g],
      # 1
      [c, f],
      # 2
      [a, c, d, e, g],
      # 3
      [a, c, d, f, g],
      # 4
      [b, c, d, f],
      # 5
      [a, b, d, f, g],
      # 6
      [a, b, d, e, f, g],
      # 7
      [a, c, f],
      # 8
      [a, b, c, d, e, f, g],
      # 9
      [a, b, c, d, f, g]
    ]
    |> Enum.map(&List.to_string(Enum.sort(&1)))
    |> Enum.with_index()
    |> Map.new()
  end

  defp extract(a) do
    # Just additional sanity check
    [v] = MapSet.to_list(a)

    v
  end

  def decode(matches, output) do
    output
    |> Enum.map(&matches[&1])
    |> Integer.undigits()
  end
end

input
|> Enum.map(fn {input, output} ->
  input
  |> Enum.sort_by(&byte_size/1)
  |> Enum.map(&MapSet.new(String.to_charlist(&1)))
  |> Day8.Task2.deduce()
  |> Day8.Task2.decode(output)
end)
|> Enum.sum()

1011785