defmodule Cldr.Unit.Conversion do
  @moduledoc """
  Unit conversion functions for the units defined
  in `Cldr`.

  """

  @enforce_keys [:factor, :offset, :base_unit]
  defstruct factor: 1,
            offset: 0,
            base_unit: nil

  @type factor :: integer | float | Ratio.t()
  @type offset :: integer | float

  @type t :: %{
          factor: factor(),
          base_unit: [atom(), ...],
          offset: offset()
        }

  alias Cldr.Unit

  import Unit, only: [incompatible_units_error: 2]

  defmodule Options do
    defstruct usage: nil, locale: nil, backend: nil, territory: nil
  end

  @doc """
  Convert one unit into another unit of the same
  unit type (length, volume, mass, ...)

  ## Arguments

  * `unit` is any unit returned by `Cldr.Unit.new/2`

  * `to_unit` is any unit name returned by `Cldr.Unit.units/0`

  ## Returns

  * a `Unit.t` of the unit type `to_unit` or

  * `{:error, {exception, message}}`

  ## Examples

      iex> Cldr.Unit.convert Cldr.Unit.new!(:mile, 1), :foot
      {:ok, Cldr.Unit.new!(:foot, 5280)}

      iex> Cldr.Unit.convert Cldr.Unit.new!(:mile, 1), :gallon
      {:error, {Cldr.Unit.IncompatibleUnitsError,
        "Operations can only be performed between units with the same category and base unit. Received :mile and :gallon"}}

  """
  @spec convert(Unit.t(), Unit.unit()) :: {:ok, Unit.t()} | {:error, {module(), String.t()}}

  def convert(%Unit{} = unit, to_unit) do
    %{unit: from_unit, value: value, base_conversion: from_conversion} = unit

    with {:ok, to_unit, to_conversion} <- Unit.validate_unit(to_unit),
         {:ok, converted} <- convert(value, from_conversion, to_conversion) do
      Unit.new(to_unit, converted, usage: unit.usage, format_options: unit.format_options)
    else
      {:error, {Cldr.Unit.IncompatibleUnitsError, _}} ->
        {:error, incompatible_units_error(from_unit, to_unit)}
    end
  end

  defp convert(value, from, to) when is_number(value) or is_map(value) do
    use Ratio

    with {:ok, from, to} <- compatible(from, to) do
      value
      |> Ratio.new()
      |> convert_to_base(from)
      |> convert_from_base(to)
      |> wrap_ok
    end
  end

  def convert_to_base(value, %__MODULE__{} = from) do
    use Ratio

    %{factor: from_factor, offset: from_offset} = from
    value * from_factor + from_offset
  end

  def convert_to_base(value, [{_, [{_, from}]}]) do
    convert_to_base(value, from)
  end

  # A known translation with a "per" conversion
  def convert_to_base(value, [{_, {_, _} = from}]) do
    convert_to_base(value, from)
  end

  def convert_to_base(value, {_, %__MODULE__{} = from}) do
    convert_to_base(value, from)
  end

  def convert_to_base(value, {numerator, denominator}) do
    use Ratio

    convert_to_base(1.0, numerator) / convert_to_base(1.0, denominator) * value
  end

  def convert_to_base(value, []) do
    value
  end

  def convert_to_base(value, [numerator | rest]) do
    convert_to_base(value, numerator) |> convert_to_base(rest)
  end

  def convert_to_base(_value, conversion) do
    raise ArgumentError, "Conversion not recognised: #{inspect(conversion)}"
  end

  def convert_from_base(value, %__MODULE__{} = to) do
    use Ratio
    %{factor: to_factor, offset: to_offset} = to
    (value - to_offset) / to_factor
  end

  def convert_from_base(value, [{_, [{_, to}]}]) do
    convert_from_base(value, to)
  end

  # A known translation with a "per" conversion
  def convert_from_base(value, [{_, {_, _} = to}]) do
    convert_from_base(value, to)
  end

  def convert_from_base(value, {_, %__MODULE__{} = to}) do
    convert_from_base(value, to)
  end

  def convert_from_base(value, {numerator, denominator}) do
    use Ratio

    convert_from_base(1.0, numerator) / convert_from_base(1.0, denominator) * value
  end

  def convert_from_base(value, []) do
    value
  end

  def convert_from_base(value, [numerator | rest]) do
    convert_from_base(value, numerator) |> convert_from_base(rest)
  end

  defp compatible(from, to) do
    with {:ok, base_unit_from} <- Unit.base_unit(from),
         {:ok, base_unit_to} <- Unit.base_unit(to),
         true <- to_string(base_unit_from) == to_string(base_unit_to) do
      {:ok, from, to}
    else
      _ -> {:error, incompatible_units_error(from, to)}
    end
  end

  defp wrap_ok(unit) do
    {:ok, unit}
  end

  @doc """
  Convert one unit into another unit of the same
  unit type (length, volume, mass, ...) and raises
  on a unit type mismatch

  ## Arguments

  * `unit` is any unit returned by `Cldr.Unit.new/2`

  * `to_unit` is any unit name returned by `Cldr.Unit.units/0`

  ## Returns

  * a `Unit.t` of the unit type `to_unit` or

  * raises an exception

  ## Examples

      iex> Cldr.Unit.Conversion.convert!(Cldr.Unit.new!(:celsius, 0), :fahrenheit)
      ...> |> Cldr.Unit.round
      #Cldr.Unit<:fahrenheit, 32.0>

      iex> Cldr.Unit.Conversion.convert!(Cldr.Unit.new!(:fahrenheit, 32), :celsius)
      ...> |> Cldr.Unit.round
      #Cldr.Unit<:celsius, 0.0>

      Cldr.Unit.Conversion.convert Cldr.Unit.new!(:mile, 1), :gallon
      ** (Cldr.Unit.IncompatibleUnitsError) Operations can only be performed between units of the same type. Received :mile and :gallon

  """
  @spec convert!(Unit.t(), Unit.unit()) :: Unit.t() | no_return()

  def convert!(%Unit{} = unit, to_unit) do
    case convert(unit, to_unit) do
      {:error, {exception, reason}} -> raise exception, reason
      {:ok, unit} -> unit
    end
  end

  @doc """
  Convert a unit into its base unit.

  For example, the base unit for `length`
  is `meter`. The base unit is an
  intermediary unit used in all
  conversions.

  ## Arguments

  * `unit` is any unit returned by `Cldr.Unit.new/2`

  ## Returns

  * `unit` converted to its base unit as a `t:Unit.t()` or

  * `{;error, {exception, reason}}` as an error

  ## Example

      iex> unit = Cldr.Unit.new!(:kilometer, 10)
      iex> Cldr.Unit.Conversion.convert_to_base_unit unit
      {:ok, Cldr.Unit.new!(:meter, 10000)}

  """
  def convert_to_base_unit(%Unit{} = unit) do
    with {:ok, base_unit} <- Unit.base_unit(unit) do
      convert(unit, base_unit)
    end
  end

  def convert_to_base_unit(unit) when is_atom(unit) do
    unit
    |> Unit.new!(1.0)
    |> convert_to_base_unit()
  end

  def convert_to_base_unit([unit | _rest]) when is_atom(unit) do
    convert_to_base_unit(unit)
  end

  @doc """
  Convert a unit into its base unit and
  raises on error

  For example, the base unit for `length`
  is `meter`. The base unit is an
  intermediary unit used in all
  conversions.

  ## Arguments

  * `unit` is any unit returned by `Cldr.Unit.new/2`

  ## Returns

  * `unit` converted to its base unit as a `t:Unit.t()` or

  * raises an exception

  ## Example

      iex> unit = Cldr.Unit.new!(:kilometer, 10)
      iex> Cldr.Unit.Conversion.convert_to_base_unit! unit
      #Cldr.Unit<:meter, 10000>

  """
  def convert_to_base_unit!(%Unit{} = unit) do
    case convert_to_base_unit(unit) do
      {:error, {exception, reason}} -> raise exception, reason
      {:ok, unit} -> unit
    end
  end
end