defmodule Day04 do
  import Enum, only: [filter: 2, chunk_by: 2, any?: 2, sort: 1]
  def run(cnd), do: 124_075..580_769 |> filter(&valid?(Integer.digits(&1), cnd)) |> length()
  defp valid?(ds, cnd), do: chunk_by(ds, & &1) |> any?(&cnd.(length(&1))) and sort(ds) == ds
end

IO.inspect(Day04.run(&(&1 >= 2)), label: "part 1")
IO.inspect(Day04.run(&(&1 == 2)), label: "part 2")

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.

My solution is here.
I decided I don’t want to brute-force this, so I made a function which computes the next valid password, on top of which I can build a stream of valid passwords.

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()

My Day4 Solution

Any feedbacks are welcome!