defmodule ApaNumber do
  @moduledoc """
  APA : Arbitrary Precision Arithmetic - internal helper functions - ApaNumber.
  """
  @parse_digit_memory_speed_border Application.get_env(
                                     :apa,
                                     :parse_digit_memory_speed_border,
                                     42
                                   )

  @doc """
  Parses a binary (number string) into an ApaNumber tuple.

  It works with signs, leading and trailing zeros and additional chars will be ignored.
  If successful, returns a tuple in the form of `{integer_value, exponent}`:

  ApaNumber.parse("+0003.00e+00000 Dollar")
  {3, 0}

  When the binary cannot be parsed, the atom `:error` will be returned.

  The limit only depends on the internal integers - because of Elixir "unlimited" integers I would say "arbitrary".

  ## Examples

    iex> ApaNumber.parse("0003")
    {3, 0}

    iex> ApaNumber.parse("+0003")
    {3, 0}

    iex> ApaNumber.parse("-0003")
    {-3, 0}

    iex> ApaNumber.parse("-0000120.1200")
    {-12012, -2}

    iex> ApaNumber.parse("-0000120.1200")
    {-12012, -2}

    iex> ApaNumber.parse("-03 Euro")
    {-3, 0}

    iex> ApaNumber.parse("-0003e-2")
    {-3, -2}

    iex> ApaNumber.parse("-3e-0002")
    {-3, -2}

    iex> ApaNumber.parse("3e-12")
    {3, -12}

    iex> ApaNumber.parse("+0003e+12")
    {3, 12}

    iex> ApaNumber.parse("+0003e+00000")
    {3, 0}

    iex> ApaNumber.parse("+0003.00e+00000 Dollar")
    {3, 0}
  """
  @spec parse(binary) :: {integer(), integer()} | :error
  def parse("+" <> rest) when byte_size(rest) + 1 > @parse_digit_memory_speed_border do
    parse_unsigned_decimal(rest)
  end

  def parse("-" <> rest) when byte_size(rest) + 1 > @parse_digit_memory_speed_border do
    case parse_unsigned_decimal(rest) do
      {int_value, exp} -> {int_value * -1, exp}
      :error -> :error
    end
  end

  def parse(binary)
      when byte_size(binary) > @parse_digit_memory_speed_border and is_binary(binary) do
    parse_unsigned_decimal(binary)
  end

  def parse("+" <> rest) do
    parse_unsigned(rest)
  end

  def parse("-" <> rest) do
    case parse_unsigned(rest) do
      {int_value, exp} -> {int_value * -1, exp}
      :error -> :error
    end
  end

  def parse(binary) when is_binary(binary) do
    parse_unsigned(binary)
  end

  ###################################################################################################
  # Decimal like version for improve speed in case of bigger strings
  # at the cost of more memory consumption - see @parse_digit_memory_speed_border in docs
  ###################################################################################################
  defp parse_unsigned_decimal(bin) do
    {int, rest} = parse_digits_decimal(bin)
    {float, rest} = parse_float_decimal(rest)
    {exp, _rest} = parse_exp_decimal(rest)

    if int == [] and float == [] do
      :error
    else
      int = if int == [], do: '0', else: int
      exp = if exp == [], do: '0', else: exp

      {List.to_integer(int ++ float), List.to_integer(exp) - Kernel.length(float)}
    end
  end

  defp parse_float_decimal("." <> rest), do: parse_digits_decimal(rest)
  defp parse_float_decimal(bin), do: {[], bin}

  defp parse_exp_decimal(<<e, rest::binary>>) when e in [?e, ?E] do
    case rest do
      <<sign, rest::binary>> when sign in [?+, ?-] ->
        {digits, rest} = parse_digits_decimal(rest)
        {[sign | digits], rest}

      _ ->
        parse_digits_decimal(rest)
    end
  end

  defp parse_exp_decimal(bin), do: {[], bin}

  defp parse_digits_decimal(bin), do: parse_digits_decimal(bin, [])

  defp parse_digits_decimal(<<digit, rest::binary>>, acc) when digit in ?0..?9 do
    parse_digits_decimal(rest, [digit | acc])
  end

  defp parse_digits_decimal(rest, acc) do
    {:lists.reverse(acc), rest}
  end

  ###################################################################################################
  # Apa version - with more speed for less then @parse_digit_memory_speed_border digits an much less memory consumption
  # see @parse_digit_memory_speed_border in docs
  ###################################################################################################
  defp parse_unsigned(bin) do
    {int, _int_len, int_trailing_zeros, int_rest} = parse_digits(bin)
    {float, float_len, float_trailing_zeros, float_rest} = parse_float(int_rest)
    {exp, _exp_rest} = parse_exp(float_rest)

    parse_unsigned(int, int_trailing_zeros, float, float_len, float_trailing_zeros, exp)
  end

  defp parse_unsigned(int, _int_trailing_zeros, _float, _float_len, _float_trailing_zeros, _exp)
       when int == :error do
    :error
  end

  defp parse_unsigned(int, int_trailing_zeros, float, _float_len, _float_trailing_zeros, exp)
       when (exp == :error and float == :error) or (exp == :error and float == 0) do
    parse_unsigned_integer(int, int_trailing_zeros)
  end

  defp parse_unsigned(int, _int_trailing_zeros, float, float_len, float_trailing_zeros, exp)
       when exp == :error do
    parse_unsigned_float(int, float, float_len, float_trailing_zeros)
  end

  defp parse_unsigned(int, int_trailing_zeros, float, _float_len, _float_trailing_zeros, exp)
       when float == :error or float == 0 do
    parse_unsigned_integer(int, int_trailing_zeros, exp)
  end

  defp parse_unsigned(int, _int_trailing_zeros, float, float_len, float_trailing_zeros, exp) do
    parse_unsigned_float(int, float, float_len, float_trailing_zeros, exp)
  end

  ###

  defp parse_unsigned_integer(int, int_trailing_zeros) do
    int = div(int, ApaNumber.pow10(int_trailing_zeros))
    {int, int_trailing_zeros}
  end

  defp parse_unsigned_integer(int, int_trailing_zeros, exp) do
    int = div(int, ApaNumber.pow10(int_trailing_zeros))
    {int, int_trailing_zeros + exp}
  end

  ###

  defp parse_unsigned_float(int, float, float_len, float_trailing_zeros) do
    int_value = int * ApaNumber.pow10(float_len - float_trailing_zeros) + float
    float_exp = float_trailing_zeros - float_len
    {int_value, float_exp}
  end

  defp parse_unsigned_float(int, float, float_len, float_trailing_zeros, exp) do
    float = div(float, ApaNumber.pow10(float_trailing_zeros))
    int_value = int * ApaNumber.pow10(float_len - float_trailing_zeros) + float
    float_exp = float_trailing_zeros - float_len
    {int_value, float_exp + exp}
  end

  defp parse_float("." <> rest), do: parse_digits(rest)
  defp parse_float(bin), do: {:error, 0, 0, bin}

  defp parse_exp(<<e, sign, rest::binary>>) when e in [?e, ?E] and sign == ?- do
    {exp, _exp_len, _exp_trailing_zeros, exp_rest} = parse_digits(rest)
    {-1 * exp, exp_rest}
  end

  defp parse_exp(<<e, sign, rest::binary>>) when e in [?e, ?E] and sign == ?+ do
    {exp, _exp_len, _exp_trailing_zeros, exp_rest} = parse_digits(rest)
    {exp, exp_rest}
  end

  defp parse_exp(<<e, rest::binary>>) when e in [?e, ?E] do
    {exp, _exp_len, _exp_trailing_zeros, exp_rest} = parse_digits(rest)
    {exp, exp_rest}
  end

  defp parse_exp(bin) do
    {0, bin}
  end

  defp parse_digits(<<digit, rest::binary>>) when digit in ?0..?9 do
    parse_digits(rest, digit - 48, 0, 1)
  end

  defp parse_digits(rest), do: {:error, 0, 0, rest}

  defp parse_digits(<<digit, rest::binary>>, acc, trailing_zeros, len)
       when digit in ?0..?9 do
    trailing_zeros = if digit == 48 and acc > 0, do: trailing_zeros + 1, else: 0

    # unbelievable but mult by 10 is so time expensive here !!!
    # there is a difference of 280 K in 5 sec between + and *
    # 404.86 K in 5 sec (benchee)
    # parse_digits(rest, acc + 10 + (digit - 48), trailing_zeros, len + 1)

    # 123.90 K in 5 sec (benchee)
    parse_digits(rest, acc * 10 + (digit - 48), trailing_zeros, len + 1)
  end

  defp parse_digits(rest, acc, trailing_zeros, len),
    do: {acc, len, trailing_zeros, rest}

  ###################################################################################################

  @doc """
  Creates a string from an ApaNumber tuple.

  ## Examples

    iex> ApaNumber.to_string({3, 0}, -1, -1)
    "3"
  """
  @spec to_string({integer(), integer()}, integer(), integer()) :: binary | :error
  def to_string({0, _exp}, precision, scale) do
    to_string_integer({0, 0}, precision, scale)
  end

  def to_string({int_value, exp}, precision, scale) when exp >= 0 do
    to_string_integer({int_value, exp}, precision, scale)
  end

  def to_string({int_value, exp}, precision, scale) when exp < 0 and int_value < 0 do
    "-" <> to_string_decimals({int_value * -1, exp}, precision, scale)
  end

  def to_string({int_value, exp}, precision, scale) when exp < 0 do
    to_string_decimals({int_value, exp}, precision, scale)
  end

  ########## to_string_integer  exp >= 0
  defp to_string_integer({int_value, exp}, precision, scale) when precision <= 0 do
    to_string_integer({int_value, exp}, scale)
  end

  defp to_string_integer({int_value, exp}, precision, scale) when precision > 0 do
    len =
      int_value
      |> Kernel.abs()
      |> Integer.to_string()
      |> Kernel.byte_size()

    diff = len - precision

    if diff > 0 do
      int_value = div(int_value, ApaNumber.pow10(diff))
      to_string_integer({int_value, exp + diff}, scale)
    else
      to_string_integer({int_value, exp}, scale)
    end
  end

  defp to_string_integer({int_value, exp}, scale) do
    shifted_int = int_value * ApaNumber.pow10(exp)

    Integer.to_string(shifted_int)
    |> scale_up_integer(scale)
  end

  defp scale_up_integer(int_string, scale) when scale <= 0 do
    int_string
  end

  defp scale_up_integer(int_string, scale) when scale > 0 do
    scale_zeros = String.duplicate("0", scale)
    "#{int_string}.#{scale_zeros}"
  end

  ########## to_string_decimals  exp < 0
  defp to_string_decimals({int_value, exp}, precision, scale) when precision <= 0 do
    to_string_decimals({int_value, exp}, scale)
  end

  defp to_string_decimals({int_value, exp}, precision, scale) when precision > 0 do
    len =
      int_value
      |> Kernel.abs()
      |> Integer.to_string()
      |> Kernel.byte_size()

    diff = len - precision

    if diff > 0 do
      fac = ApaNumber.pow10(diff)
      shifted_int = div(int_value, fac) * fac

      to_string_decimals({shifted_int, exp}, scale)
    else
      to_string_decimals({int_value, exp}, scale)
    end
  end

  defp to_string_decimals({int_value, exp}, scale) when scale == 0 do
    int_value
    |> div(ApaNumber.pow10(Kernel.abs(exp)))
    |> Integer.to_string()

    ## faster and less mem then:
    # |> Integer.to_charlist()
    # |> IO.iodata_to_binary()
  end

  defp to_string_decimals({int_value, exp}, scale) when scale < 0 do
    int_value
    |> Integer.to_charlist()
    |> list_and_length()
    |> to_string_decimals_list(exp)
    |> IO.iodata_to_binary()
  end

  defp to_string_decimals({int_value, exp}, scale) when scale + exp >= 0 do
    zeros = scale + exp

    (int_value * ApaNumber.pow10(zeros))
    |> Integer.to_charlist()
    |> list_and_length()
    |> to_string_decimals_list(-scale)
    |> IO.iodata_to_binary()
  end

  defp to_string_decimals({int_value, exp}, scale) when scale + exp < 0 do
    shrink = Kernel.abs(scale + exp)

    int_value
    |> div(ApaNumber.pow10(shrink))
    |> Integer.to_charlist()
    |> list_and_length()
    |> to_string_decimals_list(-scale)
    |> IO.iodata_to_binary()
  end

  ###
  defp to_string_decimals_list({list, list_len}, exp) when list_len + exp > 0 do
    List.insert_at(list, list_len + exp, ?.)
  end

  defp to_string_decimals_list({list, list_len}, exp) do
    # Todo: check for optimizing here because String.duplicate is 5.49x faster!!!
    '0.' ++ :lists.duplicate(-(list_len + exp), ?0) ++ list
  end

  defp list_and_length(list) do
    {list, Kernel.length(list)}
  end

  @doc """
  This is based on Decimal version of pow10 (cool!!! - is much faster then my version).
  Extended with :error guard for < 0 - in case that ever happen.

  ## Examples

    iex> ApaNumber.pow10(3)
    1000

    iex> ApaNumber.pow10(0)
    1
  """
  @spec pow10(non_neg_integer()) :: non_neg_integer() | :error
  def pow10(num) when num < 0, do: :error

  @spec base10?(non_neg_integer()) :: boolean() | :error
  def base10?(num) when num < 0, do: :error

  pow10_max =
    Enum.reduce(0..104, 1, fn int, acc ->
      def pow10(unquote(int)), do: unquote(acc)
      def base10?(unquote(acc)), do: true
      acc * 10
    end)

  def pow10(num) when num > 104, do: pow10(104) * pow10(num - 104)

  def base10?(num) when num >= unquote(pow10_max) do
    if Kernel.rem(num, unquote(pow10_max)) == 0 do
      base10?(Kernel.div(num, unquote(pow10_max)))
    else
      false
    end
  end

  def base10?(_num), do: false
end