localize

defmodule Localize.Currency do
  @moduledoc """
  Defines a currency structure and functions to manage currency
  codes, validate currencies, and retrieve currency metadata.

  Currency data is derived from the Unicode CLDR repository and
  includes all ISO 4217 currency codes and territory-to-currency
  mappings.

  Locale-specific currency data (display names, pluralized names,
  symbols) is loaded on demand from the locale data provider.

  """

  alias Localize.SupplementalData

  @type currency_code :: atom()

  @type currency_status :: :all | :current | :historic | :tender | :unannotated

  @type filter :: list(currency_status() | currency_code()) | currency_status() | currency_code()

  @type territory :: atom() | String.t()

  @type t :: %__MODULE__{
          code: currency_code(),
          alt_code: currency_code(),
          name: String.t(),
          tender: boolean(),
          symbol: String.t(),
          digits: non_neg_integer(),
          rounding: non_neg_integer(),
          narrow_symbol: String.t() | nil,
          cash_digits: non_neg_integer(),
          cash_rounding: non_neg_integer(),
          iso_digits: non_neg_integer(),
          decimal_separator: String.t() | nil,
          grouping_separator: String.t() | nil,
          count: map() | nil,
          from: Date.t() | nil,
          to: Date.t() | nil
        }

  defstruct code: nil,
            alt_code: nil,
            name: "",
            symbol: "",
            narrow_symbol: nil,
            digits: 0,
            rounding: 0,
            cash_digits: 0,
            cash_rounding: 0,
            iso_digits: 0,
            decimal_separator: nil,
            grouping_separator: nil,
            tender: false,
            count: nil,
            from: nil,
            to: nil

  # ── Known currency codes (loaded from ETF at compile time) ───

  @currency_codes SupplementalData.currency_codes()
  @territory_currencies SupplementalData.territory_currencies()

  # ── Currency validation ──────────────────────────────────────

  @doc """
  Validates a currency code and returns its canonical atom form.

  Checks both ISO 4217 codes and registered custom currencies.

  ### Arguments

  * `currency_code` is an atom or string currency code.

  ### Returns

  * `{:ok, currency_code}` where `currency_code` is an atom.

  * `{:error, Localize.UnknownCurrencyError.t()}` if the code
    is not known.

  ### Examples

      iex> Localize.Currency.validate_currency("AUD")
      {:ok, :AUD}

      iex> Localize.Currency.validate_currency(:USD)
      {:ok, :USD}

  """
  @spec validate_currency(atom() | String.t()) ::
          {:ok, currency_code()} | {:error, Exception.t()}
  def validate_currency(currency_code) when is_binary(currency_code) do
    currency_code
    |> String.upcase()
    |> String.to_atom()
    |> validate_currency()
  end

  def validate_currency(currency_code) when is_atom(currency_code) do
    if currency_code in known_currency_codes() do
      {:ok, currency_code}
    else
      {:error, Localize.UnknownCurrencyError.exception(currency: currency_code)}
    end
  end

  # ── Known currencies ─────────────────────────────────────────

  @doc """
  Returns a list of all known currency codes.

  ### Returns

  * A list of atom currency codes.

  ### Examples

      iex> codes = Localize.Currency.known_currency_codes()
      iex> :USD in codes
      true

  """
  @spec known_currency_codes() :: [currency_code(), ...]
  def known_currency_codes do
    @currency_codes
  end

  @doc """
  Returns whether the given currency code is known.

  ### Arguments

  * `currency_code` is an atom or string currency code.

  ### Returns

  * `true` or `false`.

  ### Examples

      iex> Localize.Currency.known_currency_code?(:USD)
      true

      iex> Localize.Currency.known_currency_code?("GGG")
      false

  """
  @spec known_currency_code?(atom() | String.t()) :: boolean()
  def known_currency_code?(currency_code) do
    case validate_currency(currency_code) do
      {:ok, _} -> true
      {:error, _} -> false
    end
  end

  # ── Territory currency functions ─────────────────────────────

  @doc """
  Returns a map of all territory codes to their currency history.

  Each territory maps to a map of currency codes with date ranges
  indicating when each currency was in use.

  ### Returns

  * A map of `%{territory_code => %{currency_code => date_info}}`.

  ### Examples

      iex> currencies = Localize.Currency.territory_currencies()
      iex> us = Map.get(currencies, :US)
      iex> Map.has_key?(us, :USD)
      true

  """
  @dialyzer {:nowarn_function, territory_currencies: 0}
  @spec territory_currencies() :: %{required(atom()) => %{required(atom()) => map()}}
  def territory_currencies do
    @territory_currencies
  end

  @doc """
  Returns the currency history for a specific territory.

  ### Arguments

  * `territory` is a territory code atom or string (e.g., `:US` or `"US"`).

  ### Returns

  * `{:ok, currency_map}` with currency codes and date ranges.

  * `{:error, Localize.UnknownCurrencyError.t()}` if no currencies
    are found for the territory.

  ### Examples

      iex> {:ok, currencies} = Localize.Currency.territory_currencies(:US)
      iex> Map.has_key?(currencies, :USD)
      true

  """
  @spec territory_currencies(territory()) ::
          {:ok, map()} | {:error, Exception.t()}
  def territory_currencies(territory) when is_binary(territory) do
    territory
    |> String.upcase()
    |> String.to_atom()
    |> territory_currencies()
  end

  def territory_currencies(territory) when is_atom(territory) do
    case Map.fetch(territory_currencies(), territory) do
      {:ok, currencies} ->
        {:ok, currencies}

      :error ->
        {:error,
         Localize.UnknownCurrencyError.exception(
           currency: "No currencies for #{inspect(territory)} were found"
         )}
    end
  end

  @doc """
  Returns the current currency for a given territory.

  The current currency is the one with no `:to` end date and
  where `:tender` is not `false`.

  ### Arguments

  * `territory` is a territory code atom or string.

  ### Returns

  * A currency code atom, or `nil` if no current currency exists.

  ### Examples

      iex> Localize.Currency.current_currency_for_territory(:US)
      :USD

      iex> Localize.Currency.current_currency_for_territory(:AU)
      :AUD

  """
  @spec current_currency_for_territory(atom() | String.t()) ::
          currency_code() | nil
  def current_currency_for_territory(territory) when is_binary(territory) do
    territory
    |> String.upcase()
    |> String.to_atom()
    |> current_currency_for_territory()
  end

  def current_currency_for_territory(territory) when is_atom(territory) do
    case territory_currencies(territory) do
      {:ok, history} ->
        history
        |> Enum.find(fn {_currency, info} ->
          Map.has_key?(info, :from) &&
            !Map.has_key?(info, :to) &&
            Map.get(info, :tender, true) != false
        end)
        |> case do
          {currency, _info} -> currency
          nil -> nil
        end

      {:error, _} ->
        nil
    end
  end

  @doc """
  Returns a map of territory codes to their current currency.

  Territories with no current currency are excluded.

  ### Returns

  * A map of `%{territory_code => currency_code}`.

  ### Examples

      iex> map = Localize.Currency.current_territory_currencies()
      iex> Map.get(map, :US)
      :USD

  """
  @spec current_territory_currencies() :: %{atom() => currency_code()}
  def current_territory_currencies do
    territory_currencies()
    |> Enum.reject(fn {territory, _} -> territory == :ZZ end)
    |> Enum.map(fn {territory, _} ->
      {territory, current_currency_for_territory(territory)}
    end)
    |> Enum.reject(fn {_, currency} -> is_nil(currency) end)
    |> Map.new()
  end

  # ── Locale-based currency functions ──────────────────────────

  @doc """
  Returns the effective currency for a given locale.

  If the language tag has a `cu` Unicode extension key set,
  that currency is returned. Otherwise, the current currency
  for the tag's territory is returned.

  ### Arguments

  * `locale` is a locale identifier string, atom, or a
    `t:Localize.LanguageTag.t/0` struct.

  ### Returns

  * `{:ok, currency_code}` where `currency_code` is an atom.

  * `{:error, exception}` if the locale is not valid.

  ### Examples

      iex> {:ok, tag} = Localize.LanguageTag.parse("en-US")
      iex> Localize.Currency.currency_from_locale(tag)
      {:ok, :USD}

      iex> Localize.Currency.currency_from_locale("en-AU")
      {:ok, :AUD}

  """
  @spec currency_from_locale(Localize.LanguageTag.t() | String.t() | atom()) ::
          {:ok, currency_code()} | {:error, Exception.t()}
  def currency_from_locale(locale) when is_binary(locale) or is_atom(locale) do
    with {:ok, language_tag} <- Localize.validate_locale(locale) do
      currency_from_locale(language_tag)
    end
  end

  def currency_from_locale(%Localize.LanguageTag{locale: %{cu: nil}} = locale) do
    current_currency_from_locale(locale)
  end

  def currency_from_locale(%Localize.LanguageTag{locale: %{cu: currency}}) do
    {:ok, currency}
  end

  def currency_from_locale(%Localize.LanguageTag{} = locale) do
    current_currency_from_locale(locale)
  end

  @doc """
  Returns the effective currency format for a given locale.

  If the language tag has a `cf` Unicode extension key set to
  `:account`, returns `:accounting`. Otherwise returns `:currency`.

  ### Arguments

  * `locale` is a locale identifier string, atom, or a
    `t:Localize.LanguageTag.t/0` struct.

  ### Returns

  * `{:ok, format}` where `format` is `:currency` or `:accounting`.

  * `{:error, exception}` if the locale is not valid.

  ### Examples

      iex> {:ok, tag} = Localize.LanguageTag.parse("en-US")
      iex> Localize.Currency.currency_format_from_locale(tag)
      {:ok, :currency}

      iex> Localize.Currency.currency_format_from_locale("en-US")
      {:ok, :currency}

  """
  @spec currency_format_from_locale(Localize.LanguageTag.t() | String.t() | atom()) ::
          {:ok, :currency | :accounting} | {:error, Exception.t()}
  def currency_format_from_locale(locale) when is_binary(locale) or is_atom(locale) do
    with {:ok, language_tag} <- Localize.validate_locale(locale) do
      currency_format_from_locale(language_tag)
    end
  end

  def currency_format_from_locale(%Localize.LanguageTag{locale: %{cf: :account}}) do
    {:ok, :accounting}
  end

  def currency_format_from_locale(%Localize.LanguageTag{}) do
    {:ok, :currency}
  end

  @doc """
  Returns the current currency for a locale's territory.

  This function does not consider the `U` extension parameter `cu`.
  Use `currency_from_locale/1` to get the effective currency
  including overrides.

  ### Arguments

  * `locale` is a locale identifier string, atom, or a
    `t:Localize.LanguageTag.t/0` struct.

  ### Returns

  * `{:ok, currency_code}` where `currency_code` is an atom.

  * `{:ok, nil}` if the locale has no territory or the territory
    has no current currency.

  * `{:error, exception}` if the locale is not valid.

  ### Examples

      iex> {:ok, tag} = Localize.LanguageTag.parse("en-AU")
      iex> Localize.Currency.current_currency_from_locale(tag)
      {:ok, :AUD}

      iex> Localize.Currency.current_currency_from_locale("en-US")
      {:ok, :USD}

  """
  @spec current_currency_from_locale(Localize.LanguageTag.t() | String.t() | atom()) ::
          {:ok, currency_code() | nil} | {:error, Exception.t()}
  def current_currency_from_locale(locale) when is_binary(locale) or is_atom(locale) do
    with {:ok, language_tag} <- Localize.validate_locale(locale) do
      current_currency_from_locale(language_tag)
    end
  end

  def current_currency_from_locale(%Localize.LanguageTag{} = locale) do
    with {:ok, territory} <- Localize.Territory.territory_from_locale(locale) do
      {:ok, current_currency_for_territory(territory)}
    end
  end

  @doc """
  Returns the full currency history for a locale's territory.

  Resolves the territory from the locale using
  `Localize.Territory.territory_from_locale/1` (which considers the `rg` extension,
  the explicit territory, and likely subtags), then returns the
  currency history for that territory.

  ### Arguments

  * `locale` is a locale identifier string, atom, or a
    `t:Localize.LanguageTag.t/0` struct.

  ### Returns

  * `{:ok, currency_map}` where `currency_map` is a map of
    `%{currency_code => %{from: date, to: date, ...}}` entries.

  * `{:error, exception}` if the locale or territory cannot
    be resolved.

  ### Examples

      iex> {:ok, history} = Localize.Currency.currency_history_for_locale("en-US")
      iex> Map.has_key?(history, :USD)
      true

      iex> {:ok, history} = Localize.Currency.currency_history_for_locale("de")
      iex> Map.has_key?(history, :EUR)
      true

      iex> {:ok, history} = Localize.Currency.currency_history_for_locale("ja")
      iex> Map.has_key?(history, :JPY)
      true

  """
  @spec currency_history_for_locale(Localize.LanguageTag.t() | String.t() | atom()) ::
          {:ok, map()} | {:error, Exception.t()}
  def currency_history_for_locale(locale) when is_binary(locale) or is_atom(locale) do
    with {:ok, language_tag} <- Localize.validate_locale(locale) do
      currency_history_for_locale(language_tag)
    end
  end

  def currency_history_for_locale(%Localize.LanguageTag{} = locale) do
    with {:ok, territory} <- Localize.Territory.territory_from_locale(locale) do
      territory_currencies(territory)
    end
  end

  # ── Locale-specific currency functions ────────────────────────

  @doc """
  Returns the currency metadata for the requested currency code
  in the given locale.

  Looks up localized currency data (display name, symbol, plural
  forms) for a specific currency code.

  ### Arguments

  * `currency_code` is an atom or string currency code.

  * `options` is a keyword list of options.

  ### Options

  * `:locale` is a locale identifier atom or a
    `t:Localize.LanguageTag.t/0`. The default is `:en`.

  ### Returns

  * `{:ok, Localize.Currency.t()}` with localized currency metadata.

  * `{:error, exception}` if the currency code is unknown or
    the locale data cannot be loaded.

  """
  @spec currency_for_code(atom() | String.t(), Keyword.t()) ::
          {:ok, t()} | {:error, Exception.t()}
  def currency_for_code(currency_code, options \\ []) do
    locale = Keyword.get(options, :locale, Localize.get_locale())

    with {:ok, code} <- validate_currency(currency_code),
         {:ok, currencies} <- currencies_for_locale(locale) do
      case Map.get(currencies, code) do
        nil -> {:error, Localize.UnknownCurrencyError.exception(currency: currency_code)}
        currency -> {:ok, currency}
      end
    end
  end

  @doc """
  Returns a map of all currencies for a given locale.

  Each key is a currency code atom and each value is a
  `t:Localize.Currency.t/0` struct with localized display names,
  symbols, and plural forms. The result can be filtered by
  currency status.

  ### Arguments

  * `locale` is a locale identifier atom, string, or a
    `t:Localize.LanguageTag.t/0`.

  * `only` is a filter for currency status. The default is `:all`.

  * `except` is a filter for currencies to exclude. The default
    is `nil`.

  ### Returns

  * `{:ok, currencies_map}` where `currencies_map` is a map of
    `%{currency_code => Localize.Currency.t()}`.

  * `{:error, exception}` if the locale data cannot be loaded.

  """
  @spec currencies_for_locale(
          Localize.LanguageTag.t() | atom() | String.t(),
          filter(),
          filter()
        ) ::
          {:ok, map()} | {:error, Exception.t()}
  def currencies_for_locale(locale, only \\ :all, except \\ nil) do
    locale_id = to_locale_id(locale)

    with {:ok, currencies} <- Localize.Locale.get(locale_id, [:currencies]) do
      {:ok, currency_filter(currencies, only, except)}
    end
  end

  @doc """
  Returns a map matching currency strings to currency codes
  for a given locale.

  A currency string is a localized name or symbol representing
  a currency in a locale-specific manner. The map can be used
  to parse user input into currency codes.

  ### Arguments

  * `locale` is a locale identifier atom, string, or a
    `t:Localize.LanguageTag.t/0`.

  * `only` is a filter for currency status. The default is `:all`.

  * `except` is a filter for currencies to exclude. The default
    is `nil`.

  ### Returns

  * `{:ok, string_map}` where `string_map` is a map of
    `%{downcased_string => currency_code}`.

  * `{:error, exception}` if the locale data cannot be loaded.

  """
  @spec currency_strings(
          Localize.LanguageTag.t() | atom() | String.t(),
          filter(),
          filter()
        ) ::
          {:ok, map()} | {:error, Exception.t()}
  def currency_strings(locale, only \\ :all, except \\ nil) do
    with {:ok, currencies} <- currencies_for_locale(locale, only, except) do
      {:ok, build_currency_strings(currencies)}
    end
  end

  @doc """
  Returns the list of strings that map to a given currency
  code in a locale.

  This is the inverse of `currency_strings/3` filtered to a
  single currency. It returns all localized representations
  (name, symbol, plural forms) that identify the currency.

  ### Arguments

  * `currency` is a currency code atom or string.

  * `locale` is a locale identifier atom, string, or a
    `t:Localize.LanguageTag.t/0`.

  ### Returns

  * `{:ok, strings}` where `strings` is a list of downcased
    strings that represent the currency in the locale.

  * `{:error, exception}` if the currency is unknown or the
    locale data cannot be loaded.

  """
  @spec strings_for_currency(
          currency_code() | String.t(),
          Localize.LanguageTag.t() | atom() | String.t()
        ) ::
          {:ok, [String.t()]} | {:error, Exception.t()}
  def strings_for_currency(currency, locale) do
    with {:ok, currency_code} <- validate_currency(currency),
         {:ok, strings} <- currency_strings(locale) do
      result =
        strings
        |> Enum.filter(fn {_string, code} -> code == currency_code end)
        |> Enum.map(fn {string, _code} -> string end)

      {:ok, result}
    end
  end

  @doc """
  Returns the display name for a currency.

  When given a `t:Localize.Currency.t/0` struct, returns its
  `:name` field directly. When given a currency code, looks up
  the localized name from the locale data.

  ### Arguments

  * `currency` is a currency code atom, string, or a
    `t:Localize.Currency.t/0` struct.

  * `options` is a keyword list of options.

  ### Options

  * `:locale` is a locale identifier atom or a
    `t:Localize.LanguageTag.t/0`. The default is `:en`.

  ### Returns

  * `{:ok, display_name}` where `display_name` is a string.

  * `{:error, exception}` if the currency has no display name
    or is unknown.

  """
  @spec display_name(atom() | String.t() | t(), Keyword.t()) ::
          {:ok, String.t()} | {:error, Exception.t()}
  def display_name(currency, options \\ [])

  def display_name(%__MODULE__{name: nil, code: code}, _options) do
    {:error, Localize.CurrencyNoDisplayNameError.exception(currency: code)}
  end

  def display_name(%__MODULE__{name: name}, _options) do
    {:ok, name}
  end

  def display_name(currency_code, options) do
    with {:ok, currency_data} <- currency_for_code(currency_code, options) do
      display_name(currency_data, options)
    end
  end

  @doc """
  Returns the appropriate currency display name based on
  plural rules for the locale.

  Uses the locale's cardinal plural rules to determine which
  plural form of the currency name to use for the given number.

  ### Arguments

  * `number` is an integer, float, or Decimal.

  * `currency` is a currency code atom.

  * `options` is a keyword list of options.

  ### Options

  * `:locale` is a locale identifier atom or a
    `t:Localize.LanguageTag.t/0`. The default is `:en`.

  ### Returns

  * `{:ok, pluralized_name}` where `pluralized_name` is the
    appropriate plural form string.

  * `{:error, exception}` if the currency is unknown or the
    locale data cannot be loaded.

  """
  @spec pluralize(number(), currency_code(), Keyword.t()) ::
          {:ok, String.t()} | {:error, Exception.t()}
  def pluralize(number, currency, options \\ []) do
    locale = Keyword.get(options, :locale, Localize.get_locale())

    with {:ok, currency_code} <- validate_currency(currency),
         {:ok, currency_data} <- currency_for_code(currency_code, locale: locale) do
      counts =
        (currency_data.count || %{})
        |> Map.put_new(:other, currency_data.name)

      plural_category =
        Localize.Number.PluralRule.Cardinal.plural_rule(number, locale)

      {:ok, Map.get(counts, plural_category, counts[:other])}
    end
  end

  # ── Bang versions ──────────────────────────────────────────

  @doc """
  Same as `currency_for_code/2` but raises on error.

  ### Arguments

  * `currency_code` is an atom or string currency code.

  * `options` is a keyword list of options.

  ### Options

  * `:locale` is a locale identifier atom or a
    `t:Localize.LanguageTag.t/0`. The default is `:en`.

  ### Returns

  * A `t:Localize.Currency.t/0` struct.

  ### Raises

  * Raises an exception if the currency code is unknown or the
    locale data cannot be loaded.

  """
  @spec currency_for_code!(atom() | String.t(), Keyword.t()) :: t()
  def currency_for_code!(currency_code, options \\ []) do
    case currency_for_code(currency_code, options) do
      {:ok, currency} -> currency
      {:error, exception} -> raise exception
    end
  end

  @doc """
  Same as `currencies_for_locale/3` but raises on error.

  ### Arguments

  * `locale` is a locale identifier atom, string, or a
    `t:Localize.LanguageTag.t/0`.

  * `only` is a filter for currency status. The default is `:all`.

  * `except` is a filter for currencies to exclude. The default
    is `nil`.

  ### Returns

  * A map of `%{currency_code => Localize.Currency.t()}`.

  ### Raises

  * Raises an exception if the locale data cannot be loaded.

  """
  @spec currencies_for_locale!(
          Localize.LanguageTag.t() | atom() | String.t(),
          filter(),
          filter()
        ) :: map()
  def currencies_for_locale!(locale, only \\ :all, except \\ nil) do
    case currencies_for_locale(locale, only, except) do
      {:ok, currencies} -> currencies
      {:error, exception} -> raise exception
    end
  end

  @doc """
  Same as `currency_strings/3` but raises on error.

  ### Arguments

  * `locale` is a locale identifier atom, string, or a
    `t:Localize.LanguageTag.t/0`.

  * `only` is a filter for currency status. The default is `:all`.

  * `except` is a filter for currencies to exclude. The default
    is `nil`.

  ### Returns

  * A map of `%{downcased_string => currency_code}`.

  ### Raises

  * Raises an exception if the locale data cannot be loaded.

  """
  @spec currency_strings!(
          Localize.LanguageTag.t() | atom() | String.t(),
          filter(),
          filter()
        ) :: map()
  def currency_strings!(locale, only \\ :all, except \\ nil) do
    case currency_strings(locale, only, except) do
      {:ok, strings} -> strings
      {:error, exception} -> raise exception
    end
  end

  @doc """
  Same as `display_name/2` but raises on error.

  ### Arguments

  * `currency` is a currency code atom, string, or a
    `t:Localize.Currency.t/0` struct.

  * `options` is a keyword list of options.

  ### Options

  * `:locale` is a locale identifier atom or a
    `t:Localize.LanguageTag.t/0`. The default is `:en`.

  ### Returns

  * A display name string.

  ### Raises

  * Raises an exception if the currency has no display name
    or is unknown.

  """
  @spec display_name!(atom() | String.t() | t(), Keyword.t()) :: String.t()
  def display_name!(currency, options \\ []) do
    case display_name(currency, options) do
      {:ok, name} -> name
      {:error, exception} -> raise exception
    end
  end

  @doc """
  Same as `strings_for_currency/2` but raises on error.

  ### Arguments

  * `currency` is a currency code atom or string.

  * `locale` is a locale identifier atom, string, or a
    `t:Localize.LanguageTag.t/0`.

  ### Returns

  * A list of downcased strings that represent the currency
    in the locale.

  ### Raises

  * Raises an exception if the currency is unknown or the
    locale data cannot be loaded.

  """
  @spec strings_for_currency!(
          currency_code() | String.t(),
          Localize.LanguageTag.t() | atom() | String.t()
        ) ::
          [String.t()]
  def strings_for_currency!(currency, locale) do
    case strings_for_currency(currency, locale) do
      {:ok, strings} -> strings
      {:error, exception} -> raise exception
    end
  end

  @doc """
  Same as `territory_currencies/1` but raises on error.

  ### Arguments

  * `territory` is a territory code atom or string (e.g., `:US` or `"US"`).

  ### Returns

  * A map of currency codes and date ranges.

  ### Raises

  * Raises an exception if no currencies are found for the territory.

  """
  @spec territory_currencies!(territory()) :: map()
  def territory_currencies!(territory) do
    case territory_currencies(territory) do
      {:ok, currencies} -> currencies
      {:error, exception} -> raise exception
    end
  end

  # ── Currency filtering ─────────────────────────────────────

  @doc """
  Filters a map of currencies by status predicates.

  ### Arguments

  * `currencies` is a map of `%{currency_code => Localize.Currency.t()}`.

  * `only` is a filter or list of filters to include. The default
    is `:all`.

  * `except` is a filter or list of filters to exclude. The default
    is `nil`.

  ### Returns

  * A filtered map of currencies.

  """
  @spec currency_filter(map(), filter(), filter()) :: map()
  def currency_filter(currencies, only \\ :all, except \\ nil)

  def currency_filter(currencies, :all, nil) do
    currencies
  end

  def currency_filter(currencies, only, except) when is_map(currencies) do
    included = expand_filter(currencies, :only, List.wrap(only))
    excluded = expand_filter(currencies, :except, List.wrap(except))

    included
    |> Kernel.--(excluded)
    |> Map.new()
  end

  defp expand_filter(_currencies, :only, [:all]) do
    []
  end

  defp expand_filter(currencies, :only, [:all | _]) do
    Enum.to_list(currencies)
  end

  defp expand_filter(_currencies, :except, [nil]) do
    []
  end

  defp expand_filter(currencies, _, filter_list) do
    currencies_list = Enum.to_list(currencies)

    Enum.flat_map(filter_list, fn
      :all ->
        currencies_list

      :historic ->
        Enum.filter(currencies_list, fn {_, c} -> historic?(c) end)

      :tender ->
        Enum.filter(currencies_list, fn {_, c} -> tender?(c) end)

      :current ->
        Enum.filter(currencies_list, fn {_, c} -> current?(c) end)

      :annotated ->
        Enum.filter(currencies_list, fn {_, c} -> annotated?(c) end)

      :unannotated ->
        Enum.filter(currencies_list, fn {_, c} -> unannotated?(c) end)

      code when is_atom(code) ->
        Enum.filter(currencies_list, fn {k, _} -> k == code end)

      code when is_binary(code) ->
        atom_code = String.to_atom(code)
        Enum.filter(currencies_list, fn {k, _} -> k == atom_code end)
    end)
    |> Enum.uniq()
  end

  # ── Currency status predicates ───────────────────────────────

  @doc """
  Returns whether a currency is historic (no longer in use).

  ### Arguments

  * `currency` is a `t:Localize.Currency.t/0` struct.

  ### Returns

  * `true` or `false`.

  """
  @spec historic?(t()) :: boolean()
  def historic?(%__MODULE__{} = currency) do
    is_nil(currency.iso_digits) ||
      (is_integer(currency.to) && currency.to < Date.utc_today().year)
  end

  @doc """
  Returns whether a currency is legal tender.

  ### Arguments

  * `currency` is a `t:Localize.Currency.t/0` struct.

  ### Returns

  * `true` or `false`.

  """
  @spec tender?(t()) :: boolean()
  def tender?(%__MODULE__{} = currency) do
    !!currency.tender
  end

  @doc """
  Returns whether a currency is currently in use.

  ### Arguments

  * `currency` is a `t:Localize.Currency.t/0` struct.

  ### Returns

  * `true` or `false`.

  """
  @spec current?(t()) :: boolean()
  def current?(%__MODULE__{} = currency) do
    !is_nil(currency.iso_digits) && is_nil(currency.to)
  end

  @doc """
  Returns whether a currency name contains annotations.

  Annotated currencies typically have parenthetical descriptions
  and are often financial instruments rather than legal tender.

  ### Arguments

  * `currency` is a `t:Localize.Currency.t/0` struct.

  ### Returns

  * `true` or `false`.

  """
  @spec annotated?(t()) :: boolean()
  def annotated?(%__MODULE__{} = currency) do
    String.contains?(currency.name, "(")
  end

  @doc """
  Returns whether a currency name does not contain annotations.

  ### Arguments

  * `currency` is a `t:Localize.Currency.t/0` struct.

  ### Returns

  * `true` or `false`.

  """
  @spec unannotated?(t()) :: boolean()
  def unannotated?(%__MODULE__{} = currency) do
    !annotated?(currency)
  end

  # ── Private helpers ──────────────────────────────────────────

  @rtl_mark "\u200F"

  defp to_locale_id(locale), do: Localize.Locale.to_locale_id(locale)

  defp build_currency_strings(currencies) do
    currency_string_pairs =
      Enum.flat_map(currencies, fn {code, currency} ->
        strings =
          [currency.name, currency.symbol, to_string(code)]
          |> Kernel.++(if currency.count, do: Map.values(currency.count), else: [])
          |> Enum.reject(&is_nil/1)
          |> Enum.map(&String.downcase/1)
          |> Enum.map(&String.trim_trailing(&1, @rtl_mark))
          |> Enum.map(&String.trim_trailing(&1, "."))
          |> Enum.uniq()

        Enum.map(strings, fn string -> {string, code} end)
      end)

    string_map =
      currency_string_pairs
      |> resolve_duplicate_strings(currencies)
      |> Map.new()

    add_unique_narrow_symbols(string_map, currencies)
  end

  defp resolve_duplicate_strings(pairs, currencies) do
    pairs
    |> Enum.sort_by(fn {string, _code} -> string end)
    |> do_resolve_duplicates(currencies)
  end

  defp do_resolve_duplicates([], _currencies), do: []
  defp do_resolve_duplicates([pair], _currencies), do: [pair]

  defp do_resolve_duplicates(
         [{string, code1}, {string, code2} | rest],
         currencies
       ) do
    currency1 = Map.get(currencies, code1)
    currency2 = Map.get(currencies, code2)

    cond do
      currency1 != nil and currency2 != nil and historic?(currency1) and current?(currency2) ->
        do_resolve_duplicates([{string, code2} | rest], currencies)

      currency1 != nil and currency2 != nil and current?(currency1) and historic?(currency2) ->
        do_resolve_duplicates([{string, code1} | rest], currencies)

      true ->
        do_resolve_duplicates(rest, currencies)
    end
  end

  defp do_resolve_duplicates([pair | rest], currencies) do
    [pair | do_resolve_duplicates(rest, currencies)]
  end

  defp add_unique_narrow_symbols(string_map, currencies) do
    Enum.reduce(currencies, string_map, fn {code, currency}, acc ->
      cond do
        is_nil(currency.narrow_symbol) ->
          acc

        Map.has_key?(acc, String.downcase(currency.narrow_symbol)) ->
          acc

        true ->
          Map.put(acc, String.downcase(currency.narrow_symbol), code)
      end
    end)
  end
end