defmodule ApaDiv do
  @moduledoc """
  APA : Arbitrary Precision Arithmetic - Division - ApaDiv.
  """

  # only used in division when the loop could be inifinite
  @scale_default Application.get_env(:apa, :scale_default, -1)
  @scale_limit if @scale_default == -1, do: 27, else: @scale_default

  @doc """
  Division - internal function - please call Apa.div(left, right)
  In reference to bcmath I call this function bc_div
  """
  @spec bc_div(String.t(), String.t(), integer(), integer()) :: String.t()
  def bc_div(left, right, precision, scale) when is_binary(left) and is_binary(right) do
    {left_int, left_exp} = ApaNumber.from_string(left)
    {right_int, right_exp} = ApaNumber.from_string(right)

    # Todo: check to use precision/scale to stop recursion
    # bc_div_apa_number({left_int, left_exp}, {right_int, right_exp}, precision, scale)
    ApaNumber.to_string(
      bc_div_apa_number({left_int, left_exp}, {right_int, right_exp}),
      precision,
      scale
    )
  end

  @spec bc_div({integer(), integer()}, {integer(), integer()}, integer(), integer()) ::
          {integer(), integer()}
  def bc_div({left_int, left_exp}, {right_int, right_exp}, _precision, _scale)
      when is_integer(left_int) and is_integer(left_exp) and is_integer(right_int) and
             is_integer(right_exp) do
    bc_div_apa_number({left_int, left_exp}, {right_int, right_exp})
  end

  def bc_div(left, right, precision, scale) do
    raise(ArgumentError, "No string input:
    left: #{inspect(left)}
    right: #{inspect(right)}
    precision: #{inspect(precision)}
    scale: #{inspect(scale)}
    ")
  end

  @spec bc_div_apa_number({integer(), integer()}, {integer(), integer()}) ::
          {integer(), integer()}
  def bc_div_apa_number({left_int, _left_exp}, {right_int, _right_exp})
      when left_int == 0 and right_int == 0 do
    raise(ArgumentError, "Impossible operation - division by zero - 0 / 0 - see doc.")
  end

  def bc_div_apa_number({_left_int, _left_exp}, {right_int, _right_exp})
      when right_int == 0 do
    raise(ArgumentError, "Impossible operation - division by zero - divisor == 0 - see doc.")
  end

  def bc_div_apa_number({left_int, left_exp}, {right_int, right_exp}) do
    bc_div_apa_number(
      {left_int, left_exp},
      {right_int, right_exp},
      rem(left_int, right_int),
      0
    )
  end

  defp bc_div_apa_number(
         {left_int, left_exp},
         {right_int, right_exp},
         rem,
         _acc
       )
       when rem == 0 do
    {div(left_int, right_int), left_exp - right_exp}
  end

  defp bc_div_apa_number({left_int, left_exp}, {right_int, right_exp}, _rem, acc)
       when acc == @scale_limit do
    {div(left_int, right_int), left_exp - right_exp}
  end

  defp bc_div_apa_number({left_int, left_exp}, {right_int, right_exp}, _rem, acc) do
    {new_left_int, new_left_exp} = shift({left_int, left_exp})

    bc_div_apa_number(
      {new_left_int, new_left_exp},
      {right_int, right_exp},
      rem(new_left_int, right_int),
      acc + 1
    )
  end

  defp shift({int_value, exp}) do
    {int_value * 10, exp - 1}
  end
end