The Doomsday Algorithm

doomsday.rb

require 'date'

ANCHOR_DAYS = {
  1800 => 5,
  1900 => 3,
  2000 => 2,
  2100 => 0,
}

# supports only 1800 ~ 2199
def calc(year, month, day)
  base_wday = get_weekday_of(year)
  doomsday = get_doomsday_of_the_month(year, month)
  month_shift = day - doomsday

  (base_wday + month_shift) % 7
end

def get_weekday_of(year)
  last_two = year % 100

  nums = [
    ANCHOR_DAYS[year - last_two],
    last_two / 12,
    remainder = last_two % 12,
    remainder / 4,
  ]
  nums.reduce(&:+) % 7
end

def get_doomsday_of_the_month(year, month)
  # we have to consider leap year
  return janfeb_doomsday(year, month) if month < 3
  # pi day
  return 14 if month == 3
  # even numbers rule
  return month if month % 2 == 0

  # work from 9 to 5 at seven eleven rule
  {
    5 => 9,
    7 => 11,
    9 => 5,
    11 => 7,
  }[month]
end

def janfeb_doomsday(year, month)
  is_leap = check_leap(year)
  case month
  when 1 then
    is_leap ? 4 : 3
  when 2 then
    is_leap ? 29 : 28
  end
end

def check_leap(year)
  return true if year % 400 == 0
  return false if year % 100 == 0
  return true if year % 4 == 0
  false
end

puts calc(1976, 6, 18)