defmodule YouTubeID do
  @youtube_chars "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_"
  @base length(String.graphemes(@youtube_chars))

  def to_integer(<<video_id::binary-size(11)>>) do
    video_id
    |> String.graphemes()
    |> Enum.reverse()
    |> Enum.with_index()
    |> Enum.reduce(0, fn {char, index}, acc ->
      char_value = contains_at(@youtube_chars, char)
      acc + char_value * trunc(:math.pow(@base, index))
    end)
  end

  defp contains_at(string, char) do
    String.split(string, "", trim: true)
    |> Enum.find_index(&(&1 == char))
  end
end