Merry Christmas everyone!

Here is my solution.

Thanks a lot, I learned a lot by reading your solutions and all !

Merry Xmas !

defmodule AOC.D25 do
  import AOC.Helper.Input

  def run1(test \\ false) do
    [cpubk, dpubk] =
      get_input("D25", test)
      |> split_input()
      |> Enum.map(&String.to_integer/1)

    transforms(cpubk, 7, 1, 1)
    |> encryption_key(0, dpubk, 1)
  end

  def run2() do
    "Merry Xmas !"
  end

  def encryption_key(lsize, count, _subject, value) when lsize == count, do: value

  def encryption_key(lsize, count, subject, value) do
    encryption_key(lsize, count + 1, subject, rem(value * subject, 20_201_227))
  end

  def transforms(pubkey, _subject, lsize, value) when pubkey == value, do: lsize - 1

  def transforms(pubkey, subject, lsize, value) do
    transforms(pubkey, subject, lsize + 1, rem(value * subject, 20_201_227))
  end
end

Merry Xmas everyone, thanks a lot for all the fun!

Also went back to redo Day20 part 2 from scratch to get the last star, here is todays.

defmodule AdventOfCode.Day25 do
  @subject_number 7
  @magic_number 20_201_227

  def run({card_key, door_key}) do
    card_key
    |> loop_size(@subject_number)
    |> encryption_key(door_key)
  end

  def loop_size(key, subject), do: loop_size(1, key, subject, 0)

  def loop_size(value, key, _subject, loop) when key == value, do: loop

  def loop_size(value, key, subject, loop) do
    value
    |> transform(subject)
    |> loop_size(key, subject, loop + 1)
  end

  def encryption_key(loops, subject), do: encryption_key(1, subject, loops)

  def encryption_key(value, _subject, 0), do: value

  def encryption_key(value, subject, loops) do
    value
    |> transform(subject)
    |> encryption_key(subject, loops - 1)
  end

  def transform(value, subject) do
    rem(value * subject, @magic_number)
  end
end

Here’s mine: https://twitter.com/mafinar/status/1343609407201042436 couldn’t resist fitting it in a tweet.

defmodule Day25 do
  def run(a, b), do: t(1, a, lp(1, b, 1))

  def lp(x, k, s), do: ((r = t1(x, 7)) == k && s) || lp(r, k, s + 1)

  def t(x, p, s), do: (s == 0 && x) || t(t1(x, p), p, s - 1)
  def t1(x, p), do: rem(x * p, 20_201_227)
end

Merry Christmas everyone