I guess you’ve found the same solution as mine.

WARNING: Huge Exploits!!

Don’t look at my solution until you find your own.

Part 1

#!/usr/bin/env elixir

"day5.txt"
|> File.stream!()
|> Enum.map(&String.trim/1)
|> Enum.map(fn s ->
    s
    |> String.replace(["F", "L"], "0", global: true)
    |> String.replace(["B", "R"], "1", global: true)
    |> String.to_integer(2)
end)
|> Enum.max()
|> IO.puts()

Part 2

#!/usr/bin/env elixir

"day5.txt"
|> File.stream!()
|> Enum.map(&String.trim/1)
|> Enum.map(fn s ->
    s
    |> String.replace(["F", "L"], "0", global: true)
    |> String.replace(["B", "R"], "1", global: true)
    |> String.to_integer(2)
end)
|> Enum.sort()
|> Enum.chunk_every(2, 1, :discard)
|> Enum.find(&match?([a, b] when a != b - 1, &1))
|> Enum.reduce(&+/2)
|> Kernel.div(2)
|> IO.puts()

I’m still working on the legitimacy of this approach.

How about this for the binary parsing @Aetherus, @aaronnamba @egze (looks like @lud was almost there too)?

def char_to_bit(c) when c in 'FL', do: <<0::1>>
def char_to_bit(c) when c in 'BR', do: <<1::1>>

def seat_id(boarding_pass \\ "FBFBBFFRLR") do
  <<id::integer-10>> = for <<char <- boarding_pass>>, into: <<>>, do: char_to_bit(char)
  id
end

Well… same general idea.

There must be a more efficient way to do this, but the meat of it is:

  def seat_id(boarding_pass) do
    boarding_pass
    |> String.graphemes()
    |> Enum.map(&char_to_digit/1)
    |> Enum.reverse()
    |> Enum.with_index()
    |> Enum.reduce(0, fn {digit, exp}, acc -> acc + Bitwise.bsl(digit, exp) end)
  end

  def char_to_digit(str) do
    case str do
      "B" -> 1
      "F" -> 0
      "R" -> 1
      "L" -> 0
    end
  end

Then for part 2, I checked the upper and lower bounds real quick, then:

Enum.to_list(89..989) -- Enum.map(parse_input(filename), &seat_id/1)