Zeller's Congruence Algorithm

maths/zellers_congruence.py

@@ -0,0 +1,157 @@
+from __future__ import annotations
+import datetime
+import argparse
+
+
+def zeller(date_input: str) -> str:
+
+    """
+    Zellers Congruence Algorithm
+    Find the day of the week for nearly any Gregorian or Julian calendar date 
+
+    >>> zeller('01-31-2010')
+    'Your date 01-31-2010, is a Sunday!'
+
+    Validate out of range month
+    >>> zeller('13-31-2010')
+    Traceback (most recent call last):
+    ...
+    ValueError: Month must be between 1 - 12
+    >>> zeller('.2-31-2010')
+    Traceback (most recent call last):
+    ...
+    ValueError: invalid literal for int() with base 10: '.2'
+
+    Validate out of range date:
+    >>> zeller('01-33-2010')
+    Traceback (most recent call last):
+        ...
+    ValueError: Date must be between 1 - 31
+    >>> zeller('01-.4-2010')
+    Traceback (most recent call last):
+        ...
+    ValueError: invalid literal for int() with base 10: '.4'
+
+    Validate second seperator:
+    >>> zeller('01-31*2010')
+    Traceback (most recent call last):
+        ...
+    ValueError: Date seperator must be '-' or '/'
+
+    Validate first seperator:
+    >>> zeller('01^31-2010')
+    Traceback (most recent call last):
+        ...
+    ValueError: Date seperator must be '-' or '/'
+
+    Validate out of range year:
+    >>> zeller('01-31-8999')
+    Traceback (most recent call last):
+        ...
+    ValueError: Year out of range. There has to be some sort of limit...right?
+
+    Test null input:
+    >>> zeller()
+    Traceback (most recent call last):
+        ...
+    TypeError: zeller() missing 1 required positional argument: 'date_input'
+
+    Test length fo date_input:
+    >>> zeller('')
+    Traceback (most recent call last):
+        ...
+    ValueError: Must be 10 characters long
+    >>> zeller('01-31-19082939')
+    Traceback (most recent call last):
+        ...
+    ValueError: Must be 10 characters long
+"""
+
+    # Days of the week for response
+    days = {
+        '0': 'Sunday',
+        '1': 'Monday',
+        '2': 'Tuesday',
+        '3': 'Wednesday',
+        '4': 'Thursday',
+        '5': 'Friday',
+        '6': 'Saturday'
+    }
+
+    convert_datetime_days = {
+        0:1,
+        1:2,
+        2:3,
+        3:4,
+        4:5,
+        5:6,
+        6:0
+    }
+
+    # Validate
+    if not 0 < len(date_input) < 11:
+        raise ValueError("Must be 10 characters long")
+
+    # Get month
+    m: int = int(date_input[0] + date_input[1])
+    # Validate
+    if not 0 < m < 13:
+        raise ValueError("Month must be between 1 - 12")
+
+    sep_1:str = date_input[2]
+    # Validate
+    if sep_1 not in ["-","/"]:
+        raise ValueError("Date seperator must be '-' or '/'")
+
+    # Get day
+    d: int = int(date_input[3] + date_input[4])
+    # Validate
+    if not 0 < d < 32:
+        raise ValueError("Date must be between 1 - 31")
+
+    # Get second seperator
+    sep_2: str = date_input[5]
+    # Validate
+    if sep_2 not in ["-","/"]:
+        raise ValueError("Date seperator must be '-' or '/'")
+
+    # Get year
+    y: int = int(date_input[6] + date_input[7] + date_input[8] + date_input[9])
+    # Arbitrary year range
+    if not 45 < y < 8500:
+        raise ValueError("Year out of range. There has to be some sort of limit...right?")
+
+    # Get datetime obj for validation
+    dt_ck = datetime.date(int(y), int(m), int(d))
+
+    # Start math
+    if m <= 2:
+        y = y - 1
+        m = m + 12
+    # maths var
+    c: int = int(str(y)[:2])
+    k: int = int(str(y)[2:])
+    t: int = int(2.6*m - 5.39)
+    u: int = int(c / 4)
+    v: int = int(k / 4)
+    x: int = int(d + k)
+    z: int = int(t + u + v + x)
+    w: int = int(z - (2 * c))
+    f: int = round(w%7)
+    # End math
+
+    # Validate math
+    if f != convert_datetime_days[dt_ck.weekday()]:
+        raise AssertionError("The date was evaluated incorrectly. Contact developer.")
+
+    # Response
+    response: str = f"Your date {date_input}, is a {days[str(f)]}!"
+    return response
+
+if __name__ == '__main__':
+    import doctest
+    doctest.testmod()
+    parser = argparse.ArgumentParser(description='Find out what day of the week nearly any date is or was. Enter date as a string in the mm-dd-yyyy or mm/dd/yyyy format')
+    parser.add_argument('date_input', type=str, help='Date as a string (mm-dd-yyyy or mm/dd/yyyy)')
+    args = parser.parse_args()
+    zeller(args.date_input)