Add doomsday algorithm

other/doomsday.py

@@ -0,0 +1,51 @@
+#!/bin/python3
+# Doomsday algorithm info: https://en.wikipedia.org/wiki/Doomsday_rule
+
+_doomsday_leap = [4, 1, 7, 4, 2, 6, 4, 1, 5, 3, 7, 5]
+_doomsday_not_leap = [3, 7, 7, 4, 2, 6, 4, 1, 5, 3, 7, 5]
+_week_day_names = {
+    0: "Sunday",
+    1: "Monday",
+    2: "Tuesday",
+    3: "Wednesday",
+    4: "Thursday",
+    5: "Friday",
+    6: "Saturday",
+}
+
+
+def get_week_day(year: int, month: int, day: int) -> str:
+    """Returns the week-day name out of a given date.
+
+    >>> get_week_day(2020, 10, 24)
+    'Saturday'
+    >>> get_week_day(2017, 10, 24)
+    'Tuesday'
+
+    """
+    # minimal input check:
+    assert len(str(year)) > 2, "year should be in YYYY format"
+    assert 1 <= month <= 12, "month should be between 1 to 12"
+    assert 1 <= day <= 31, "day should be between 1 to 31"
+
+    # Doomsday algorithm:
+    century = year // 100
+    century_anchor = (5 * (century % 4) + 2) % 7
+    centurian = year % 100
+    centurian_m = centurian % 12
+    dooms_day = (
+        (centurian // 12) + centurian_m + (centurian_m // 4) + century_anchor
+    ) % 7
+    day_anchor = (
+        _doomsday_not_leap[month - 1]
+        if (year % 4 != 0) or (centurian == 0 and (year % 400) == 0)
+        else _doomsday_leap[month - 1]
+    )
+    week_day = (dooms_day + day - day_anchor) % 7
+    return _week_day_names[week_day]
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()