defmodule Cldr.DateTime do
  @moduledoc """
  Provides an API for the localization and formatting of a `DateTime`
  struct or any map with the keys `:year`, `:month`,
  `:day`, `:calendar`, `:hour`, `:minute`, `:second` and optionally `:microsecond`.

  `Cldr.DateTime` provides support for the built-in calendar
  `Calendar.ISO`.  Use of other calendars may not produce
  the expected results.

  CLDR provides standard format strings for `DateTime` which
  are reresented by the names `:short`, `:medium`, `:long`
  and `:full`.  This allows for locale-independent
  formatting since each locale may define the underlying
  format string as appropriate.
  """

  require Cldr
  alias Cldr.DateTime.{Format, Formatter}
  alias Cldr.LanguageTag

  @format_types [:short, :medium, :long, :full]

  @doc """
  Formats a DateTime according to a format string
  as defined in CLDR and described in [TR35](http://unicode.org/reports/tr35/tr35-dates.html)

  Returns `{:ok, formatted_datetime}` or `{:error, reason}`.

  * `datetime` is a `%DateTime{}` `or %NaiveDateTime{}`struct or any map that contains the keys
    `:year`, `:month`, `:day`, `:calendar`. `:hour`, `:minute` and `:second` with optional
    `:microsecond`.

  * `options` is a keyword list of options for formatting.  The valid options are:
    * `format:` `:short` | `:medium` | `:long` | `:full` or a format string or
      any of the keys returned by `Cldr.DateTime.available_format_names` or a format string.
      The default is `:medium`
    * `locale:` any locale returned by `Cldr.known_locale_names()`.  The default is `
      Cldr.get_current_locale()`
    * `number_system:` a number system into which the formatted date digits should
      be transliterated
    * `era: :variant` will use a variant for the era is one is available in the locale.
      In the "en" for example, the locale `era: :variant` will return "BCE" instead of "BC".
    * `period: :variant` will use a variant for the time period and flexible time period if
      one is available in the locale.  For example, in the "en" locale `period: :variant` will
      return "pm" instead of "PM"

  ## Examples

      iex> {:ok, datetime} = DateTime.from_naive(~N[2000-01-01 23:59:59.0], "Etc/UTC")
      iex> Cldr.DateTime.to_string datetime, locale: Cldr.Locale.new!("en")
      {:ok, "Jan 1, 2000, 11:59:59 PM"}
      iex> Cldr.DateTime.to_string datetime, format: :long, locale: Cldr.Locale.new!("en")
      {:ok, "January 1, 2000 at 11:59:59 PM UTC"}
      iex> Cldr.DateTime.to_string datetime, format: :full, locale: Cldr.Locale.new!("en")
      {:ok, "Saturday, January 1, 2000 at 11:59:59 PM GMT"}
      iex> Cldr.DateTime.to_string datetime, format: :full, locale: Cldr.Locale.new!("fr")
      {:ok, "samedi 1 janvier 2000 à 23:59:59 UTC"}

  """
  def to_string(date, options \\ [])
  def to_string(%{year: _year, month: _month, day: _day, hour: _hour, minute: _minute,
      second: _second, calendar: calendar} = datetime, options) do
    options = Keyword.merge(default_options(), options)

    with \
      {:ok, locale} <- Cldr.validate_locale(options[:locale]),
      {:ok, cldr_calendar} <- Formatter.type_from_calendar(calendar),
      {:ok, format_string} <- format_string_from_format(options[:format], locale, cldr_calendar),
      {:ok, formatted} <- Formatter.format(datetime, format_string, locale, options)
    do
      {:ok, formatted}
    else
      {:error, reason} -> {:error, reason}
    end
  end

  def to_string(datetime, _options) do
    error_return(datetime, [:year, :month, :day, :hour, :minute, :second, :calendar])
  end

  defp default_options do
    [format: :medium, locale: Cldr.get_current_locale()]
  end

  @doc """
  Formats a DateTime according to a format string
  as defined in CLDR and described in [TR35](http://unicode.org/reports/tr35/tr35-dates.html)

  Returns `formatted_datetime` or raises an exception.

  * `datetime` is a `%DateTime{}` `or %NaiveDateTime{}`struct or any map that contains the keys
    `:year`, `:month`, `:day`, `:calendar`. `:hour`, `:minute` and `:second` with optional
    `:microsecond`.

  * `options` is a keyword list of options for formatting.  The valid options are:
    * `format:` `:short` | `:medium` | `:long` | `:full` or a format string or
      any of the keys returned by `Cldr.DateTime.available_format_names` or a format string.
      The default is `:medium`
    * `locale:` any locale returned by `Cldr.known_locale_names()`.  The default is `
      Cldr.get_current_locale()`
    * `number_system:` a number system into which the formatted date digits should
      be transliterated
    * `era: :variant` will use a variant for the era is one is available in the locale.
      In the "en" for example, the locale `era: :variant` will return "BCE" instead of "BC".
    * `period: :variant` will use a variant for the time period and flexible time period if
      one is available in the locale.  For example, in the "en" locale `period: :variant` will
      return "pm" instead of "PM"

  ## Examples

      iex> {:ok, datetime} = DateTime.from_naive(~N[2000-01-01 23:59:59.0], "Etc/UTC")
      iex> Cldr.DateTime.to_string! datetime, locale: Cldr.Locale.new!("en")
      "Jan 1, 2000, 11:59:59 PM"
      iex> Cldr.DateTime.to_string! datetime, format: :long, locale: Cldr.Locale.new!("en")
      "January 1, 2000 at 11:59:59 PM UTC"
      iex> Cldr.DateTime.to_string! datetime, format: :full, locale: Cldr.Locale.new!("en")
      "Saturday, January 1, 2000 at 11:59:59 PM GMT"
      iex> Cldr.DateTime.to_string! datetime, format: :full, locale: Cldr.Locale.new!("fr")
      "samedi 1 janvier 2000 à 23:59:59 UTC"

  """
  def to_string!(date_time, options \\ [])
  def to_string!(date_time, options) do
    case to_string(date_time, options) do
      {:ok, string} -> string
      {:error, {exception, message}} -> raise exception, message
    end
  end

  # Standard format
  defp format_string_from_format(format, %LanguageTag{cldr_locale_name: locale_name}, calendar)
  when format in @format_types do
    format_string =
      locale_name
      |> Format.date_time_formats(calendar)
      |> Map.get(format)

    {:ok, format_string}
  end

  # Look up for the format in :available_formats
  defp format_string_from_format(format, %LanguageTag{cldr_locale_name: locale_name}, calendar) when is_atom(format) do
    format_string =
      locale_name
      |> Format.date_time_available_formats(calendar)
      |> Map.get(format)


      if format_string do
        {:ok, format_string}
      else
        {:error, {Cldr.InvalidDateTimeFormatType, "Invalid datetime format type. " <>
                  "The valid types are #{inspect @format_types}."}}
      end
  end

  # Format with a number system
  defp format_string_from_format(%{number_system: number_system, format: format}, locale, calendar) do
    {:ok, format_string} = format_string_from_format(format, locale, calendar)
    {:ok, %{number_system: number_system, format: format_string}}
  end

  # Straight up format string
  defp format_string_from_format(format_string, _locale, _calendar) when is_binary(format_string) do
    {:ok, format_string}
  end

  defp error_return(map, requirements) do
    {:error, {ArgumentError, "Invalid date_time. Date_time is a map that requires at least #{inspect requirements} fields. " <>
             "Found: #{inspect map}"}}
  end
end