ex_money

defmodule Money.Arithmetic do
  @moduledoc false

  defmacro __using__(_opts) do
    quote location: :keep do
      import Kernel, except: [div: 2, round: 1]
      alias Cldr.Currency

      @doc """
      Add two `Money` values.

      ## Example

          iex> Money.add Money.new(:USD, 200), Money.new(:USD, 100)
          {:ok, Money.new(:USD, 300)}

          iex> Money.add Money.new(:USD, 200), Money.new(:AUD, 100)
          {:error, {ArgumentError, "Cannot add monies with different currencies. " <>
            "Received :USD and :AUD."}}
      """
      @spec add(Money.t, Money.t) :: Money.t
      def add(%Money{currency: same_currency, amount: amount_a}, %Money{currency: same_currency, amount: amount_b}) do
        {:ok, %Money{currency: same_currency, amount: Decimal.add(amount_a, amount_b)}}
      end

      def add(%Money{currency: code_a}, %Money{currency: code_b}) do
        {:error, {ArgumentError, "Cannot add monies with different currencies. " <>
          "Received #{inspect code_a} and #{inspect code_b}."}}
      end

      @doc """
      Add two `Money` values and raise on error.

      ## Examples

          iex> Money.add! Money.new(:USD, 200), Money.new(:USD, 100)
          #Money<:USD, 300>

          Money.add! Money.new(:USD, 200), Money.new(:CAD, 500)
          ** (ArgumentError) Cannot add two %Money{} with different currencies. Received :USD and :CAD.
      """
      def add!(%Money{} = a, %Money{} = b) do
        case add(a, b) do
          {:ok, result} -> result
          {:error, {exception, message}} -> raise exception, message
        end
      end

      @doc """
      Subtract one `Money` value struct from another.

      Returns either `{:ok, money}` or `{:error, reason}`.

      ## Example

          iex> Money.sub Money.new(:USD, 200), Money.new(:USD, 100)
          {:ok, Money.new(:USD, 100)}
      """
      def sub(%Money{currency: same_currency, amount: amount_a}, %Money{currency: same_currency, amount: amount_b}) do
        {:ok, %Money{currency: same_currency, amount: Decimal.sub(amount_a, amount_b)}}
      end

      def sub(%Money{currency: code_a, amount: amount_a}, %Money{currency: code_b, amount: amount_b}) do
        {:error, {ArgumentError, "Cannot subtract two %Money{} with different currencies. " <>
          "Received #{inspect code_a} and #{inspect code_b}."}}
      end

      @doc """
      Subtract one `Money` value struct from another and raise on error.

      Returns either `{:ok, money}` or `{:error, reason}`.

      ## Examaples

          iex> Money.sub! Money.new(:USD, 200), Money.new(:USD, 100)
          #Money<:USD, 100>

          Money.sub! Money.new(:USD, 200), Money.new(:CAD, 500)
          ** (ArgumentError) Cannot subtract monies with different currencies. Received :USD and :CAD.
      """
      def sub!(%Money{} = a, %Money{} = b) do
        case sub(a, b) do
          {:ok, result} -> result
          {:error, {exception, message}} -> raise exception, message
        end
      end

      @doc """
      Multiply a `Money` value by a number.

      * `money` is a %Money{} struct

      * `number` is an integer or float

      > Note that multipling one %Money{} by another is not supported.

      Returns either `{:ok, money}` or `{:error, reason}`.

      ## Example

          iex> Money.mult(Money.new(:USD, 200), 2)
          {:ok, Money.new(:USD, 400)}

          iex> Money.mult(Money.new(:USD, 200), "xx")
          {:error, {ArgumentError, "Cannot multiply money by \\"xx\\""}}
      """
      @spec mult(Money.t, number) :: Money.t
      def mult(%Money{currency: code, amount: amount}, number) when is_number(number) do
        {:ok, %Money{currency: code, amount: Decimal.mult(amount, Decimal.new(number))}}
      end

      def mult(%Money{} = money, other) do
        {:error, {ArgumentError, "Cannot multiply money by #{inspect other}"}}
      end

      @doc """
      Multiply a `Money` value by a number and raise on error.

      ## Examples

          iex> Money.mult!(Money.new(:USD, 200), 2)
          #Money<:USD, 400>

          Money.mult!(Money.new(:USD, 200), :invalid)
          ** (ArgumentError) Cannot multiply money by :invalid
      """
      def mult!(%Money{} = money, number) do
        case mult(money, number) do
          {:ok, result} -> result
          {:error, {exception, message}} -> raise exception, message
        end
      end

      @doc """
      Divide a `Money` value by a number.

      * `money` is a %Money{} struct

      * `number` is an integer or float

      > Note that dividing one %Money{} by another is not supported.

      ## Example

          iex> Money.div Money.new(:USD, 200), 2
          {:ok, Money.new(:USD, 100)}

          iex> Money.div(Money.new(:USD, 200), "xx")
          {:error, {ArgumentError, "Cannot divide money by \\"xx\\""}}
      """
      @spec div(Money.t, number) :: Money.t
      def div(%Money{currency: code, amount: amount}, number) when is_number(number) do
        {:ok, %Money{currency: code, amount: Decimal.div(amount, Decimal.new(number))}}
      end

      def div(%Money{} = money, other) do
        {:error, {ArgumentError, "Cannot divide money by #{inspect other}"}}
      end

      @doc """
      Divide a `Money` value by a number and raise on error.

      ## Examples

          iex> Money.div Money.new(:USD, 200), 2
          {:ok, Money.new(:USD, 100)}

          Money.div(Money.new(:USD, 200), "xx")
          ** (ArgumentError) "Cannot divide money by \\"xx\\""]}}
      """
      def div!(%Money{} = money, number) do
        case Money.div(money, number) do
          {:ok, result} -> result
          {:error, {exception, message}} -> raise exception, message
        end
      end

      @doc """
      Returns a boolean indicating if two `Money` values are equal

      ## Example

          iex> Money.equal? Money.new(:USD, 200), Money.new(:USD, 200)
          true

          iex> Money.equal? Money.new(:USD, 200), Money.new(:USD, 100)
          false
      """
      @spec equal?(Money.t, Money.t) :: boolean
      def equal?(%Money{currency: same_currency, amount: amount_a}, %Money{currency: same_currency, amount: amount_b}) do
        Decimal.equal?(amount_a, amount_b)
      end

      def equal?(_, _) do
        false
      end

      @doc """
      Compares two `Money` values numerically. If the first number is greater
      than the second :gt is returned, if less than :lt is returned, if both
      numbers are equal :eq is returned.

      ## Examples

          iex> Money.cmp Money.new(:USD, 200), Money.new(:USD, 100)
          :gt

          iex> Money.cmp Money.new(:USD, 200), Money.new(:USD, 200)
          :eq

          iex> Money.cmp Money.new(:USD, 200), Money.new(:USD, 500)
          :lt

          iex> Money.cmp Money.new(:USD, 200), Money.new(:CAD, 500)
          {:error,
           {ArgumentError,
            "Cannot compare monies with different currencies. Received :USD and :CAD."}}
      """
      def cmp(%Money{currency: same_currency, amount: amount_a}, %Money{currency: same_currency, amount: amount_b}) do
        Decimal.cmp(amount_a, amount_b)
      end

      def cmp(%Money{currency: code_a}, %Money{currency: code_b}) do
        {:error, {ArgumentError, "Cannot compare monies with different currencies. " <>
          "Received #{inspect code_a} and #{inspect code_b}."}}
      end

      @doc """
      Compares two `Money` values numerically and raises on error.

      ## Examples

          Money.cmp! Money.new(:USD, 200), Money.new(:CAD, 500)
          ** (ArgumentError) Cannot compare monies with different currencies. Received :USD and :CAD.
      """
      def cmp!(%Money{} = money_1, %Money{} = money_2) do
        case cmp(money_1, money_2) do
          {:error, {exception, reason}} -> raise exception, reason
          result -> result
        end
      end

      @doc """
      Compares two `Money` values numerically. If the first number is greater
      than the second #Integer<1> is returned, if less than Integer<-1> is
      returned. Otherwise, if both numbers are equal Integer<0> is returned.

      ## Examples

          iex> Money.compare Money.new(:USD, 200), Money.new(:USD, 100)
          1

          iex> Money.compare Money.new(:USD, 200), Money.new(:USD, 200)
          0

          iex> Money.compare Money.new(:USD, 200), Money.new(:USD, 500)
          -1

          iex> Money.compare Money.new(:USD, 200), Money.new(:CAD, 500)
          {:error,
           {ArgumentError,
            "Cannot compare monies with different currencies. Received :USD and :CAD."}}
      """
      def compare(%Money{currency: same_currency, amount: amount_a}, %Money{currency: same_currency, amount: amount_b}) do
        amount_a
        |> Decimal.compare(amount_b)
        |> Decimal.to_integer
      end

      def compare(%Money{currency: code_a}, %Money{currency: code_b}) do
        {:error, {ArgumentError, "Cannot compare monies with different currencies. " <>
          "Received #{inspect code_a} and #{inspect code_b}."}}
      end

      @doc """
      Compares two `Money` values numerically and raises on error.

      ## Examples

          Money.compare! Money.new(:USD, 200), Money.new(:CAD, 500)
          ** (ArgumentError) Cannot compare monies with different currencies. Received :USD and :CAD.
      """
      def compare!(%Money{} = money_1, %Money{} = money_2) do
        case compare(money_1, money_2) do
          {:error, {exception, reason}} -> raise exception, reason
          result -> result
        end
      end

      @doc """
      Split a `Money` value into a number of parts maintaining the currency's
      precision and rounding and ensuring that the parts sum to the original
      amount.

      * `money` is a `%Money{}` struct

      * `parts` is an integer number of parts into which the `money` is split

      Returns a tuple `{dividend, remainder}` as the function result
      derived as follows:

      1. Round the money amount to the required currency precision using
      `Money.round/1`

      2. Divide the result of step 1 by the integer divisor

      3. Round the result of the division to the precision of the currency
      using `Money.round/1`

      4. Return two numbers: the result of the division and any remainder
      that could not be applied given the precision of the currency.

      ## Examples

          Money.split Money.new(123.5, :JPY), 3
          {¥41, ¥1}

          Money.split Money.new(123.4, :JPY), 3
          {¥41, ¥0}

          Money.split Money.new(123.7, :USD), 9
          {$13.74, $0.04}
      """
      def split(%Money{} = money, parts) when is_integer(parts) do
        rounded_money = Money.round(money)

        div =
          rounded_money
          |> Money.div!(parts)
          |> round

        remainder = sub!(rounded_money, mult!(div, parts))
        {div, remainder}
      end

      @doc """
      Round a `Money` value into the acceptable range for the defined currency.

      * `money` is a `%Money{}` struct

      * `opts` is a keyword list with the following keys:

        * `:rounding_mode` that defines how the number will be rounded.  See
        `Decimal.Context`.  The default is `:half_even` which is also known
        as "banker's rounding"

        * `:cash` which determines whether the rounding is being applied to
        an accounting amount or a cash amount.  Some currencies, such as the
        :AUD and :CHF have a cash unit increment minimum which requires
        a different rounding increment to an arbitrary accounting amount. The
        default is `false`.

      There are two kinds of rounding applied:

      1.  Round to the appropriate number of fractional digits

      2. Apply an appropriate rounding increment.  Most currencies
      round to the same precision as the number of decimal digits, but some
      such as :AUD and :CHF round to a minimum such as 0.05 when its a cash
      amount.

      ## Examples

          iex> Money.round Money.new(123.7456, :CHF), cash: true
          #Money<:CHF, 125>

          iex> Money.round Money.new(123.7456, :CHF)
          #Money<:CHF, 123.75>

          Money.round Money.new(123.7456, :JPY)
          #Money<:JPY, 124>
      """
      def round(%Money{} = money, opts \\ []) do
        money
        |> round_to_decimal_digits(opts)
        |> round_to_nearest(opts)
      end

      defp round_to_decimal_digits(%Money{currency: code, amount: amount}, opts \\ []) do
        rounding_mode = Keyword.get(opts, :rounding_mode, @default_rounding_mode)
        currency = Currency.for_code(code)
        rounding = if opts[:cash], do: currency.cash_digits, else: currency.digits
        rounded_amount = Decimal.round(amount, rounding, rounding_mode)
        %Money{currency: code, amount: rounded_amount}
      end

      defp round_to_nearest(%Money{currency: code, amount: amount} = money, opts \\ []) do
        currency  = Currency.for_code(code)
        increment = if opts[:cash], do: currency.cash_rounding, else: currency.rounding
        do_round_to_nearest(money, increment, opts)
      end

      defp do_round_to_nearest(money, 0, opts) do
        money
      end

      defp do_round_to_nearest(money, increment, opts) do
        rounding_mode = Keyword.get(opts, :rounding_mode, @default_rounding_mode)
        rounding = Decimal.new(increment)

        rounded_amount =
          money.amount
          |> Decimal.div(rounding)
          |> Decimal.round(0, rounding_mode)
          |> Decimal.mult(rounding)

        %Money{currency: money.currency, amount: rounded_amount}
      end
    end
  end
end