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"""

https://en.wikipedia.org/wiki/Computus#Gauss'_Easter_algorithm

"""

import math

from datetime import datetime, timedelta

def gauss_easter(year: int) -> datetime:

"""

Calculation Gregorian easter date for given year

>>> gauss_easter(2007)

datetime.datetime(2007, 4, 8, 0, 0)

>>> gauss_easter(2008)

datetime.datetime(2008, 3, 23, 0, 0)

>>> gauss_easter(2020)

datetime.datetime(2020, 4, 12, 0, 0)

>>> gauss_easter(2021)

datetime.datetime(2021, 4, 4, 0, 0)

"""

metonic_cycle = year % 19

julian_leap_year = year % 4

non_leap_year = year % 7

leap_day_inhibits = math.floor(year / 100)

lunar_orbit_correction = math.floor((13 + 8 * leap_day_inhibits) / 25)

leap_day_reinstall_number = leap_day_inhibits / 4

secular_moon_shift = (

15 - lunar_orbit_correction + leap_day_inhibits - leap_day_reinstall_number

) % 30

century_starting_point = (4 + leap_day_inhibits - leap_day_reinstall_number) % 7

# days to be added to March 21

days_to_add = (19 * metonic_cycle + secular_moon_shift) % 30

# PHM -> Paschal Full Moon

days_from_phm_to_sunday = (

2 * julian_leap_year

+ 4 * non_leap_year

+ 6 * days_to_add

+ century_starting_point

) % 7

if days_to_add == 29 and days_from_phm_to_sunday == 6:

return datetime(year, 4, 19)

elif days_to_add == 28 and days_from_phm_to_sunday == 6:

return datetime(year, 4, 18)

else:

return datetime(year, 3, 22) + timedelta(

days=int(days_to_add + days_from_phm_to_sunday)

)

if __name__ == "__main__":

for year in (1994, 2000, 2010, 2021, 2023):

tense = "will be" if year > datetime.now().year else "was"

print(f"Easter in {year} {tense} {gauss_easter(year)}")

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