utils: add function to get weekday index from date (#45159)

chrono/utils/date.py

@@ -0,0 +1,2 @@
+def get_weekday_index(datetime):
+    return (datetime.day - 1) // 7 + 1

tests/test_utils.py

@@ -0,0 +1,23 @@
+import datetime
+
+from chrono.utils.date import get_weekday_index
+
+
+def test_get_weekday_index():
+    for date in (
+        datetime.date(2021, 11, 1),  # month starting a Monday
+        datetime.date(2021, 12, 1),  # month starting a Wednesday
+        datetime.date(2021, 5, 1),  # month starting a Sunday
+    ):
+        assert get_weekday_index(date) == 1
+        assert get_weekday_index(date.replace(day=3)) == 1
+        assert get_weekday_index(date.replace(day=7)) == 1
+        assert get_weekday_index(date.replace(day=8)) == 2
+        assert get_weekday_index(date.replace(day=10)) == 2
+        assert get_weekday_index(date.replace(day=14)) == 2
+        assert get_weekday_index(date.replace(day=15)) == 3
+        assert get_weekday_index(date.replace(day=21)) == 3
+        assert get_weekday_index(date.replace(day=22)) == 4
+        assert get_weekday_index(date.replace(day=28)) == 4
+        assert get_weekday_index(date.replace(day=29)) == 5
+        assert get_weekday_index(date.replace(day=30)) == 5