defmodule GettextPseudolocalize.Plural do
  @moduledoc """
  A `Gettext.Plural` implementation that teaches Gettext about the `xx`
  pseudo-locale produced by `gettext_pseudolocalize`.

  `xx` is not a real CLDR locale, so the default `Gettext.Plural` raises
  `Gettext.Plural.UnknownLocaleError` for it. That breaks `mix gettext.merge`,
  `mix compile` (once a backend compiles against `priv/gettext/xx/`), and
  runtime plural lookups. This module resolves `xx` to two plural forms — the
  same shape as English (`nplurals=2; plural=(n != 1)`) — and delegates every
  other locale to `Gettext.Plural`, so it is safe to register as the global
  pluralizer:

      config :gettext, :plural_forms, GettextPseudolocalize.Plural

  If you already have a custom pluralizer, add the `"xx"` clauses to it instead
  and keep delegating the rest to `Gettext.Plural`.
  """
  @behaviour Gettext.Plural

  # `Gettext.Plural.init/1` normalises the pluralization context into either the
  # bare locale string (when the PO has no `Plural-Forms` header) or a
  # `{locale, plural_forms}` tuple (when it does). Both shapes flow into the
  # other callbacks, so every clause below accepts both for "xx".
  @impl true
  def init(context), do: Gettext.Plural.init(context)

  @impl true
  def nplurals("xx"), do: 2
  def nplurals({"xx", _plural_forms}), do: 2
  def nplurals(other), do: Gettext.Plural.nplurals(other)

  @impl true
  def plural("xx", n), do: english_plural(n)
  def plural({"xx", _plural_forms}, n), do: english_plural(n)
  def plural(other, n), do: Gettext.Plural.plural(other, n)

  @impl true
  def plural_forms_header("xx"), do: "nplurals=2; plural=(n != 1);"
  def plural_forms_header(other), do: Gettext.Plural.plural_forms_header(other)

  # English-style: singular for exactly one, plural otherwise.
  defp english_plural(1), do: 0
  defp english_plural(_), do: 1
end