Day 2: Gift Shop

Mix.install([:kino])

Section

input = Kino.Input.text("input", monospaced: true)

# n = 211

# digits = floor(:math.log10(n)) + 1
#   mid = div(digits, 2)
#   pow = :math.pow(10, digits - mid) |> round()

#   {div(n, pow), rem(n, pow)}

defmodule RepeatingNumbers do
  def generate(lower, upper) do
    Enum.flat_map(2..(trunc(:math.log10(upper)) + 1), fn total_len ->
      1..div(total_len, 2)
      |> Enum.filter(&(rem(total_len, &1) == 0))
      |> Enum.flat_map(fn sub_len ->
        reps = div(total_len, sub_len)
        start_min = Integer.pow(10, sub_len - 1)
        start_max = Integer.pow(10, sub_len) - 1

        for start <- start_min..start_max do
          num =
            start
            |> Integer.to_string()
            |> String.duplicate(reps)
            |> String.to_integer()

          if num >= lower and num <= upper do
            num
          end
        end
      end)
    end)
    |> Enum.reject(&amp;is_nil/1)
    |> Enum.uniq()
  end
end

input
|> Kino.Input.read()
|> String.split(["-", ",", " "], trim: true)
|> Enum.map(&amp;String.to_integer/1)
|> Enum.chunk_every(2)
|> Enum.map(fn [l, r] ->
  RepeatingNumbers.generate(l, r)
  |> Enum.sum()
end)
|> Enum.sum()

input
|> Kino.Input.read()
|> String.split(["-", ",", " "], trim: true)
|> Enum.map(&String.to_integer/1)
|> Enum.chunk_every(2)
|> Enum.sum_by(fn [l, r] -> 
  Stream.resource(
    fn -> 1 end,
    fn n -> {[n * Integer.pow(10, 1 + floor(:math.log10(n))) + n], n + 1} end,
    fn _ -> :ok end
  )
  |> Stream.drop_while(&amp; &amp;1 < l)
  |> Enum.take_while(&amp; &amp;1 <= r)
  |> Enum.sum()
end)

# |> Enum.take(10)

# n = 3

# n * Integer.pow(10, 1 + round(:math.log(n) / :math.log(10))) + n

11
22
33
44
55

99
111
222
333
444
555
666
777
888
999

1010
1111
1212
...

11111
22222
33333
44444
55555
...
99999

100100