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Easter date gauss algorithm #2010

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59 changes: 59 additions & 0 deletions other/gauss_easter.py
Original file line number Diff line number Diff line change
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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)}")