Supply plurals forms header to Gettext.Plural callbacks

Closes #318

This makes it possible to react precisely to the header instead of
using predefined formats.

lib/gettext/compiler.ex

@@ -6,6 +6,7 @@ defmodule Gettext.Compiler do
   alias Expo.Message
   alias Expo.Messages
   alias Expo.PO
+  alias Gettext.Plural
 
   @default_priv "priv/gettext"
   @default_domain "default"
@@ -501,7 +502,12 @@ defmodule Gettext.Compiler do
   # lngettext/7 (for plural messages) clauses.
   defp compile_po_file(kind, po_file, env, plural_mod, interpolation_module) do
     %{locale: locale, domain: domain, path: path} = po_file
-    %Messages{messages: messages, file: file} = PO.parse_file!(path)
+    %Messages{messages: messages, file: file} = messages_struct = PO.parse_file!(path)
+
+    plural_forms_fun = :"{locale}_#{domain}_plural"
+
+    plural_forms = compile_plural_forms(locale, messages_struct, plural_mod, plural_forms_fun)
+    nplurals = nplurals(locale, messages_struct, plural_mod)
 
     singular_fun = :"#{locale}_#{domain}_lgettext"
     plural_fun = :"#{locale}_#{domain}_lngettext"
@@ -516,14 +522,16 @@ defmodule Gettext.Compiler do
         singular_fun,
         plural_fun,
         file,
-        plural_mod,
+        {plural_forms_fun, nplurals},
         interpolation_module
       )
 
     messages = block(Enum.map(messages, mapper))
 
     quoted =
       quote do
+        unquote(plural_forms)
+
         unquote(messages)
 
         Kernel.unquote(kind)(unquote(singular_fun)(msgctxt, msgid, bindings)) do
@@ -552,6 +560,21 @@ defmodule Gettext.Compiler do
     {locale, domain, singular_fun, plural_fun, quoted}
   end
 
+  defp nplurals(locale, messages_struct, plural_mod) do
+    plural_mod.nplurals(Plural.plural_info(locale, messages_struct, plural_mod))
+  end
+
+  defp compile_plural_forms(locale, messages_struct, plural_mod, plural_fun) do
+    quote do
+      defp unquote(plural_fun)(n) do
+        unquote(plural_mod).plural(
+          unquote(Macro.escape(Plural.plural_info(locale, messages_struct, plural_mod))),
+          n
+        )
+      end
+    end
+  end
+
   defp locale_and_domain_from_path(path) do
     [file, "LC_MESSAGES", locale | _rest] = path |> Path.split() |> Enum.reverse()
     domain = Path.rootname(file, ".po")
@@ -565,7 +588,7 @@ defmodule Gettext.Compiler do
          singular_fun,
          _plural_fun,
          _file,
-         _plural_mod,
+         _pluralization,
          interpolation_module
        ) do
     msgid = IO.iodata_to_binary(message.msgid)
@@ -602,10 +625,10 @@ defmodule Gettext.Compiler do
          _singular_fun,
          plural_fun,
          file,
-         plural_mod,
+         {plural_forms_fun, nplurals},
          interpolation_module
        ) do
-    warn_if_missing_plural_forms(locale, plural_mod, message, file)
+    warn_if_missing_plural_forms(locale, nplurals, message, file)
 
     msgid = IO.iodata_to_binary(message.msgid)
     msgid_plural = IO.iodata_to_binary(message.msgid_plural)
@@ -652,7 +675,8 @@ defmodule Gettext.Compiler do
             bindings
           )
         ) do
-          plural_form = unquote(plural_mod).plural(unquote(locale), n)
+          plural_form = unquote(plural_forms_fun)(n)
+
           var!(bindings) = Map.put(bindings, :count, n)
 
           case plural_form, do: unquote(clauses ++ error_clause)
@@ -661,8 +685,8 @@ defmodule Gettext.Compiler do
     end
   end
 
-  defp warn_if_missing_plural_forms(locale, plural_mod, message, file) do
-    Enum.each(0..(plural_mod.nplurals(locale) - 1), fn form ->
+  defp warn_if_missing_plural_forms(locale, nplurals, message, file) do
+    Enum.each(0..(nplurals - 1), fn form ->
       unless Map.has_key?(message.msgstr, form) do
         _ =
           Logger.error([

lib/gettext/merger.ex

@@ -5,6 +5,7 @@ defmodule Gettext.Merger do
   alias Expo.Message
   alias Expo.Messages
   alias Gettext.Fuzzy
+  alias Gettext.Plural
 
   @new_po_informative_comment """
   # "msgid"s in this file come from POT (.pot) files.
@@ -53,7 +54,7 @@ defmodule Gettext.Merger do
           {Messages.t(), map()}
   def merge(%Messages{} = old, %Messages{} = new, locale, opts)
       when is_binary(locale) and is_list(opts) do
-    opts = put_plural_forms_opt(opts, old.headers, locale)
+    opts = put_plural_forms_opt(opts, old, locale)
     stats = %{new: 0, exact_matches: 0, fuzzy_matches: 0, removed: 0, marked_as_obsolete: 0}
 
     {messages, stats} = merge_messages(old.messages, new.messages, opts, stats)
@@ -220,12 +221,13 @@ defmodule Gettext.Merger do
   """
   def new_po_file(po_file, pot_file, locale, opts) when is_binary(locale) and is_list(opts) do
     pot = PO.parse_file!(pot_file)
-    opts = put_plural_forms_opt(opts, pot.headers, locale)
+    opts = put_plural_forms_opt(opts, pot, locale)
     plural_forms = Keyword.fetch!(opts, :plural_forms)
+    plural_forms_header = Keyword.fetch!(opts, :plural_forms_header)
 
     po = %Messages{
       top_comments: String.split(@new_po_informative_comment, "\n", trim: true),
-      headers: headers_for_new_po_file(locale, plural_forms),
+      headers: headers_for_new_po_file(locale, plural_forms_header),
       file: po_file,
       messages: Enum.map(pot.messages, &prepare_new_message(&1, plural_forms))
     }
@@ -277,11 +279,11 @@ defmodule Gettext.Merger do
     Enum.reject(unique_refs, &match?([], &1))
   end
 
-  defp headers_for_new_po_file(locale, plural_forms) do
+  defp headers_for_new_po_file(locale, plural_forms_header) do
     [
       "",
       ~s(Language: #{locale}\n),
-      ~s(Plural-Forms: nplurals=#{plural_forms}\n)
+      ~s(Plural-Forms: #{plural_forms_header}\n)
     ]
   end
 
@@ -295,21 +297,27 @@ defmodule Gettext.Merger do
     %{message | comments: Enum.reject(comments, &match?("#" <> _, &1))}
   end
 
-  defp put_plural_forms_opt(opts, headers, locale) do
-    Keyword.put_new_lazy(opts, :plural_forms, fn ->
-      read_plural_forms_from_headers(headers) ||
-        Application.get_env(:gettext, :plural_forms, Gettext.Plural).nplurals(locale)
-    end)
-  end
+  defp put_plural_forms_opt(opts, messages, locale) do
+    plural_mod = Application.get_env(:gettext, :plural_forms, Gettext.Plural)
+
+    opts =
+      Keyword.put_new_lazy(opts, :plural_forms, fn ->
+        plural_mod.nplurals(Plural.plural_info(locale, messages, plural_mod))
+      end)
+
+    Keyword.put_new_lazy(opts, :plural_forms_header, fn ->
+      requested_nplurals = Keyword.fetch!(opts, :plural_forms)
+
+      default_nplurals = plural_mod.nplurals(Plural.plural_info(locale, messages, plural_mod))
 
-  defp read_plural_forms_from_headers(headers) do
-    Enum.find_value(headers, fn header ->
-      with "Plural-Forms:" <> rest <- header,
-           "nplurals=" <> rest <- String.trim(rest),
-           {plural_forms, _rest} <- Integer.parse(rest) do
-        plural_forms
+      # If nplurals is overridden to a non-default value by the user the
+      # implementation will not be able to provide a correct header therefore
+      # the header is just set to `nplurals=#{n}` and it is up to the user to
+      # put a complete plural forms header himself.
+      if requested_nplurals == default_nplurals do
+        Plural.plural_forms_header_impl(locale, messages, plural_mod)
       else
-        _other -> nil
+        "nplurals=#{requested_nplurals}"
       end
     end)
   end

lib/gettext/plural.ex

@@ -122,18 +122,52 @@ defmodule Gettext.Plural do
 
   """
 
+  alias Expo.Messages
+
+  # Types
+
+  @type locale :: String.t()
+
+  @type pluralization_context :: %{
+          optional(:plural_forms_header) => String.t(),
+          required(:locale) => locale()
+        }
+
+  @type plural_info :: term()
+
   # Behaviour definition.
 
+  @doc """
+  Initialize context for `nplurals/1` / `plurals/2`.
+
+  Perform all preparations (for example parsing the plural forms header) per
+  for the provided locale to offload into compile time.
+
+  If the `init/1` callback is not defined, the `plural_info` will be set to the
+  `locale`.
+  """
+  @callback init(pluralization_context()) :: plural_info()
+
   @doc """
   Returns the number of possible plural forms in the given `locale`.
   """
-  @callback nplurals(locale :: String.t()) :: pos_integer
+  @callback nplurals(plural_info()) :: pos_integer()
 
   @doc """
   Returns the plural form in the given `locale` for the given `count` of
   elements.
   """
-  @callback plural(locale :: String.t(), count :: integer) :: plural_form :: non_neg_integer
+  @callback plural(plural_info(), count :: integer()) :: plural_form :: non_neg_integer()
+
+  @doc """
+  Returns the plural forms header value for the given `locale`.
+
+
+  Fallback if not implemented: `"nplurals={nplurals};"`.
+  """
+  @callback plural_forms_header(locale()) :: String.t() | nil
+
+  @optional_callbacks init: 1, plural_forms_header: 1
 
   defmodule UnknownLocaleError do
     @moduledoc """
@@ -434,10 +468,22 @@ defmodule Gettext.Plural do
     "sk"
   ]
 
+  @doc false
+  def init(%{locale: locale, plural_forms_header: plural_forms_header}) do
+    case read_plural_forms_from_headers(plural_forms_header) do
+      nil -> locale
+      nplurals -> {locale, nplurals}
+    end
+  end
+
+  def init(%{locale: locale}), do: locale
+
   # Number of plural forms.
 
   def nplurals(locale)
 
+  def nplurals({_locale, nplurals}), do: nplurals
+
   # All the groupable forms.
 
   for l <- @one_form do
@@ -511,6 +557,8 @@ defmodule Gettext.Plural do
 
   def plural(locale, count)
 
+  def plural({locale, _nplurals}, count), do: plural(locale, count)
+
   # All the `x_Y` languages that have different pluralization rules than `x`.
 
   def plural("pt_BR", n) when n in [0, 1], do: 0
@@ -681,4 +729,69 @@ defmodule Gettext.Plural do
       _other -> raise UnknownLocaleError, locale
     end
   end
+
+  @doc false
+  def plural_info(locale, messages_struct, plural_mod) do
+    ensure_loaded!(plural_mod)
+
+    if function_exported?(plural_mod, :init, 1) do
+      pluralization_context = %{locale: locale}
+
+      pluralization_context =
+        case Messages.get_header(messages_struct, "Plural-Forms") do
+          [] ->
+            pluralization_context
+
+          plural_forms ->
+            Map.put(
+              pluralization_context,
+              :plural_forms_header,
+              IO.iodata_to_binary(plural_forms)
+            )
+        end
+
+      plural_mod.init(pluralization_context)
+    else
+      locale
+    end
+  end
+
+  @doc false
+  def plural_forms_header_impl(locale, messages_struct, plural_mod) do
+    ensure_loaded!(plural_mod)
+
+    plural_forms_header =
+      if function_exported?(plural_mod, :plural_forms_header, 1) do
+        plural_mod.plural_forms_header(locale)
+      end
+
+    if plural_forms_header do
+      plural_forms_header
+    else
+      nplurals = plural_mod.nplurals(plural_info(locale, messages_struct, plural_mod))
+
+      "nplurals=#{nplurals}"
+    end
+  end
+
+  defp read_plural_forms_from_headers(header) do
+    with "nplurals=" <> rest <- String.trim(header),
+         {plural_forms, _rest} <- Integer.parse(rest) do
+      plural_forms
+    else
+      _other -> nil
+    end
+  end
+
+  # TODO: remove when we depend on Elixir 1.12+
+  if function_exported?(Code, :ensure_loaded!, 1) do
+    defp ensure_loaded!(mod), do: Code.ensure_loaded!(mod)
+  else
+    defp ensure_loaded!(mod) do
+      case Code.ensure_loaded(mod) do
+        {:module, ^mod} -> :ok
+        _other -> raise "<good error message>"
+      end
+    end
+  end
 end

lib/mix/tasks/gettext.merge.ex

@@ -76,8 +76,8 @@ defmodule Mix.Tasks.Gettext.Merge do
   by checking the value of `nplurals` in the `Plural-Forms` header in the existing `.po` file. If
   a `.po` file doesn't already exist and Gettext is creating a new one or if the `Plural-Forms`
   header is not in the `.po` file, Gettext will use the number of plural forms that
-  `Gettext.Plural` returns for the locale of the file being created. The number of plural forms
-  can be forced through the `--plural-forms` option (see below).
+  `Gettext.Plural` returns for the locale of the file being created. The content of the plural forms
+  header can be forced through the `--plural-forms-header` option (see below).
 
   ## Options
 
@@ -98,6 +98,10 @@ defmodule Mix.Tasks.Gettext.Merge do
       new messages in the target PO files will have this number of empty
       plural forms. See the "Plural forms" section above.
 
+    * `--plural-forms-header` - `Plural-Forms` header content as string.
+      If this is passed, new messages in the target PO files will have this
+      number of empty plural forms. See the "Plural forms" section above.
+
     * `--on-obsolete` - controls what happens when obsolete messages are found.
       If `mark_as_obsolete`, messages are kept and marked as obsolete.
       If `delete`, obsolete messages are deleted. Defaults to `delete`.
@@ -117,6 +121,7 @@ defmodule Mix.Tasks.Gettext.Merge do
     fuzzy: :boolean,
     fuzzy_threshold: :float,
     plural_forms: :integer,
+    plural_forms_header: :string,
     on_obsolete: :string,
     store_previous_message_on_fuzzy_match: :boolean
   ]
@@ -273,6 +278,7 @@ defmodule Mix.Tasks.Gettext.Merge do
         :fuzzy,
         :fuzzy_threshold,
         :plural_forms,
+        :plural_forms_header,
         :on_obsolete,
         :store_previous_message_on_fuzzy_match
       ])

test/fixtures/single_messages/it/LC_MESSAGES/default.po

@@ -1,3 +1,8 @@
+msgid ""
+msgstr ""
+"Language: it\n"
+"Plural-Forms: nplurals=2; plural=(n != 1);\n"
+
 msgid "Hello world"
 msgstr "Ciao mondo"