Module: UkWorkingDays::Easter

Included in:
Date
Defined in:
lib/uk_working_days/easter.rb

Instance Method Summary collapse

Instance Method Details

#easter(year) ⇒ Object

Calculate easter sunday for the given year



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# File 'lib/uk_working_days/easter.rb', line 15

def easter(year)
  golden_number = (year % 19) + 1
  
  if year <= 1752 then
    # Julian calendar
    dominical_number = (year + (year / 4) + 5) % 7
    paschal_full_moon = (3 - (11 * golden_number) - 7) % 30
  else
    # Gregorian calendar
    dominical_number = (year + (year / 4) - (year / 100) + (year / 400)) % 7
    solar_correction = (year - 1600) / 100 - (year - 1600) / 400
    lunar_correction = (((year - 1400) / 100) * 8) / 25
    paschal_full_moon = (3 - 11 * golden_number + solar_correction - lunar_correction) % 30
  end

  dominical_number += 7 until dominical_number > 0 
  
  paschal_full_moon += 30 until paschal_full_moon > 0 
  paschal_full_moon -= 1 if paschal_full_moon == 29 or (paschal_full_moon == 28 and golden_number > 11)
  
  difference = (4 - paschal_full_moon - dominical_number) % 7
  difference += 7 if difference < 0 
  
  day_easter = paschal_full_moon + difference + 1
  
  day_easter < 11 ? new(year, 3, day_easter + 21) : new(year, 4, day_easter - 10)
end