jinja2: add 'tn' filter, a numerical translation

The possibly plural version of `tr`, `tn` provides a way to translate
strings into singular or plural form based on a given integer
being 1 or not 1.

Example use:

```
{{ 1 | tn("%d package found.", "%d packages found.") | format(1) }}
```

Signed-off-by: Kevin Morris <kevr@0cost.org>

aurweb/l10n.py

@@ -93,3 +93,21 @@ def tr(context: typing.Any, value: str):
     """ A translation filter; example: {{ "Hello" | tr("de") }}. """
     _ = get_translator_for_request(context.get("request"))
     return _(value)
+
+
+@pass_context
+def tn(context: typing.Dict[str, typing.Any], count: int,
+       singular: str, plural: str) -> str:
+    """ A singular and plural translation filter.
+
+    Example:
+        {{ some_integer | tn("singular %d", "plural %d") }}
+
+    :param context: Response context
+    :param count: The number used to decide singular or plural state
+    :param singular: The singular translation
+    :param plural: The plural translation
+    :return: Translated string
+    """
+    gettext = get_raw_translator_for_request(context.get("request"))
+    return gettext.ngettext(singular, plural, count)

aurweb/templates.py

@@ -23,8 +23,9 @@ loader = jinja2.FileSystemLoader(os.path.join(
 env = jinja2.Environment(loader=loader, autoescape=True,
                          extensions=["jinja2.ext.i18n"])
 
-# Add tr translation filter.
+# Add t{r,n} translation filters.
 env.filters["tr"] = l10n.tr
+env.filters["tn"] = l10n.tn
 
 # Utility filters.
 env.filters["dt"] = util.timestamp_to_datetime

test/test_l10n.py

@@ -36,3 +36,15 @@ def test_get_translator_for_request():
 
     translate = l10n.get_translator_for_request(request)
     assert translate("Home") == "Startseite"
+
+
+def test_tn_filter():
+    request = Request()
+    request.cookies["AURLANG"] = "en"
+    context = {"language": "en", "request": request}
+
+    translated = l10n.tn(context, 1, "%d package found.", "%d packages found.")
+    assert translated == "%d package found."
+
+    translated = l10n.tn(context, 2, "%d package found.", "%d packages found.")
+    assert translated == "%d packages found."