guesswork

defmodule Guesswork.Constraint.Arithmetic.InternalVariable.Base36 do
  @moduledoc """
  A simple module for building the base36 string version of an integer.
  """

  defp get_char(x) do
    if x < 26, do: x + 65, else: x + 48 - 25
  end

  defp get_char_list(x) when x < 0, do: []
  defp get_char_list(x), do: [get_char(Integer.mod(x, 36)) | get_char_list(div(x, 36) - 1)]

  @doc """
  Returns the base 36 string of the the integer prefixed with `_`.

    ## Examples

    iex> Guesswork.Constraint.Arithmetic.InternalVariable.Base36.calculate(0)
    "_A"
    iex> Guesswork.Constraint.Arithmetic.InternalVariable.Base36.calculate(15)
    "_P"
    iex> Guesswork.Constraint.Arithmetic.InternalVariable.Base36.calculate(27)
    "_2"
    iex> Guesswork.Constraint.Arithmetic.InternalVariable.Base36.calculate(51)
    "_AP"
    iex> Guesswork.Constraint.Arithmetic.InternalVariable.Base36.calculate(1118)
    "_5C"
  """
  @spec calculate(integer()) :: String.t()
  def calculate(x) do
    "_#{x |> get_char_list() |> Enum.reverse() |> to_string()}"
  end
end