Optimize rendering of very large calendars (#2406)

src/pretix/helpers/daterange.py

@@ -32,10 +32,11 @@
 # distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 # License for the specific language governing permissions and limitations under the License.
 
-from django.template.defaultfilters import date as _date
 from django.utils.html import format_html
 from django.utils.translation import get_language, gettext_lazy as _
 
+from pretix.helpers.templatetags.date_fast import date_fast as _date
+
 
 def daterange(df, dt, as_html=False):
     lng = get_language()

src/pretix/helpers/templatetags/date_fast.py

@@ -0,0 +1,61 @@
+#
+# This file is part of pretix (Community Edition).
+#
+# Copyright (C) 2014-2020 Raphael Michel and contributors
+# Copyright (C) 2020-2021 rami.io GmbH and contributors
+#
+# This program is free software: you can redistribute it and/or modify it under the terms of the GNU Affero General
+# Public License as published by the Free Software Foundation in version 3 of the License.
+#
+# ADDITIONAL TERMS APPLY: Pursuant to Section 7 of the GNU Affero General Public License, additional terms are
+# applicable granting you additional permissions and placing additional restrictions on your usage of this software.
+# Please refer to the pretix LICENSE file to obtain the full terms applicable to this work. If you did not receive
+# this file, see <https://pretix.eu/about/en/license>.
+#
+# This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied
+# warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Affero General Public License for more
+# details.
+#
+# You should have received a copy of the GNU Affero General Public License along with this program.  If not, see
+# <https://www.gnu.org/licenses/>.
+#
+import functools
+
+from django import template
+from django.utils import dateformat
+from django.utils.formats import get_format
+from django.utils.translation import get_language
+
+register = template.Library()
+
+
+@functools.lru_cache(maxsize=32)
+def _get_format(format_type, lang):
+    return get_format(format_type, lang)
+
+
+@register.filter(expects_localtime=True, is_safe=False)
+def date_fast(value, arg=None):
+    """
+    Slightly quicker version of |date if the filter is called a lot. The speedup is achieved through
+    LRU caching for formats.
+
+    Django's built-in |date filter has a caching mechanism if you call it with a named format,
+    i.e. ``|date_fast:"SHORT_DATE_FORMAT"`` will only be ~6% faster than ``|date:"SHORT_DATE_FORMAT"``.
+
+    However, Django's built-in caching has a flaw with unnamed formats, therefore ``|date_fast:"Y-m-d"``
+    will be ~12% faster than ``|date:"Y-m-d"``.
+    """
+    if value in (None, ''):
+        return ''
+
+    lang = get_language()
+    format = _get_format(arg, lang)
+
+    try:
+        return dateformat.format(value, format)
+    except AttributeError:
+        try:
+            return format(value, arg)
+        except AttributeError:
+            return ''