Advent of Code 2019 - Day 4

Here is my solution.

Very unhygienic code, especially for part 2.

Part 1

359999..799999
|> Stream.map(&Integer.to_string/1)
|> Stream.map(&String.graphemes/1)
|> Stream.reject(fn graphemes -> graphemes |> Stream.chunk_every(2, 1, :discard) |> Enum.any?(fn [a, b] -> a > b end) end)
|> Stream.reject(fn graphemes -> graphemes |> Stream.chunk_every(2, 1, :discard) |> Enum.all?(fn [a, b] -> a != b end) end)
|> Enum.count()
|> IO.inspect()

Part 2

359999..799999
|> Stream.map(&Integer.to_string/1)
|> Stream.map(&String.graphemes/1)
|> Stream.reject(fn graphemes -> graphemes |> Stream.chunk_every(2, 1, :discard) |> Enum.any?(fn [a, b] -> a > b end) end)
|> Stream.map(fn graphemes -> [:x] ++ graphemes ++ [:x] end)  # tricky padding
|> Stream.filter(fn graphemes -> graphemes |> Stream.chunk_every(4, 1, :discard) |> Enum.any?(fn
     [a, b, b, c] when a != b and b != c -> true
     _ -> false
   end) end)
|> Enum.count()
|> IO.inspect()

For AoC, I pick a language I want to get to know better. This year it’s Elixir. I came from the Ruby world (by way of a short detour through Crystal-land). I have already launched a Phoenix-based CMS (closed source for now, unfortunately), but I am still far from fluent.

So I’m sure my solutions won’t be ideal, but I thought I’d post them anyway, since i have enjoyed reading through the various solutions posted here for the first 3 days.

Day 4 Solution

As I was writing this, Aetherus’s post appeared and I realized I forgot about the :discard option on chunk_every… oh well.

Glad to have helped

Here is my solution. I feel it can still be improved.

Edit:
I simplified it after reading @yuchunc 's nice solution. This is always the most enjoyable part during the AOC sessions, because I can learn so much from others here. Thank you guys!

I used quite a few pattern matchiing clauses for the strict double checking
https://github.com/scmx/advent-of-code-2019-elixir/blob/master/lib/day_04_secure_container.ex

One of the first languages I ever learned was Perl (now called Raku?!?), so I tend to reach for regexes a lot, perhaps more than I should. Maybe regexes are a no-no in Elixir world, I don’t know.

This regex needed backreferences, so I was afraid it might turn out super slow, but in this case, the strings involved are so short that it didn’t matter.

  @doc """
      iex> Day4.PasswordFinder2.remove_long_repeats("112233")
      "112233"
      iex> Day4.PasswordFinder2.remove_long_repeats("123444")
      "123"
      iex> Day4.PasswordFinder2.remove_long_repeats("111122")
      "22"
  """
  def remove_long_repeats(str) do
    String.replace(str, ~r/(\d)\1{2,}/, "")
  end

My Day4 Solution

https://github.com/yuchunc/aoc2019/blob/master/lib/day4/day4.ex

Any feedbacks are welcome!

How to prevent someone cheating by using other languages? I personally used Ruby in Day 2.

Your part 2 check is really nice, I wish I’d thought of that.

One small thing I noticed, things like:

  defp increment?([d1 | [d2 | _]]) when d1 > d2, do: false

can be:

  defp increment?([d1, d2 | _]) when d1 > d2, do: false