module DateAndTime::Calculations

def all_month
  beginning_of_month..end_of_month
end

def all_quarter
  beginning_of_quarter..end_of_quarter
end

def all_week(start_day = Date.beginning_of_week)
  beginning_of_week(start_day)..end_of_week(start_day)
end

def all_year
  beginning_of_year..end_of_year
end

def beginning_of_month
  first_hour(change(:day => 1))
end

def beginning_of_quarter
  first_quarter_month = [10, 7, 4, 1].detect { |m| m <= month }
  beginning_of_month.change(:month => first_quarter_month)
end

def beginning_of_week(start_day = Date.beginning_of_week)
  result = days_ago(days_to_week_start(start_day))
  acts_like?(:time) ? result.midnight : result
end

def beginning_of_year
  change(:month => 1).beginning_of_month
end

def copy_time_to(other)
  other.change(hour: hour, min: min, sec: sec, nsec: try(:nsec))
end

def days_ago(days)
  advance(:days => -days)
end

def days_since(days)
  advance(:days => days)
end

def days_span(day)
  (DAYS_INTO_WEEK[day] - DAYS_INTO_WEEK[Date.beginning_of_week]) % 7
end

def days_to_week_start(start_day = Date.beginning_of_week)
  start_day_number = DAYS_INTO_WEEK[start_day]
  current_day_number = wday != 0 ? wday - 1 : 6
  (current_day_number - start_day_number) % 7
end

def end_of_month
  last_day = ::Time.days_in_month(month, year)
  last_hour(days_since(last_day - day))
end

def end_of_quarter
  last_quarter_month = [3, 6, 9, 12].detect { |m| m >= month }
  beginning_of_month.change(:month => last_quarter_month).end_of_month
end

def end_of_week(start_day = Date.beginning_of_week)
  last_hour(days_since(6 - days_to_week_start(start_day)))
end

def end_of_year
  change(:month => 12).end_of_month
end

def first_hour(date_or_time)
  date_or_time.acts_like?(:time) ? date_or_time.beginning_of_day : date_or_time
end

def future?
  self > self.class.current
end

def last_hour(date_or_time)
  date_or_time.acts_like?(:time) ? date_or_time.end_of_day : date_or_time
end

def monday
  beginning_of_week(:monday)
end

def months_ago(months)
  advance(:months => -months)
end

def months_since(months)
  advance(:months => months)
end

def next_day
  advance(days: 1)
end

def next_month
  months_since(1)
end

def next_quarter
  months_since(3)
end

def next_week(given_day_in_next_week = Date.beginning_of_week, same_time: false)
  result = first_hour(weeks_since(1).beginning_of_week.days_since(days_span(given_day_in_next_week)))
  same_time ? copy_time_to(result) : result
end

def next_weekday
  if next_day.on_weekend?
    next_week(:monday, same_time: true)
  else
    next_day
  end
end

def next_year
  years_since(1)
end

def on_weekday?
  !WEEKEND_DAYS.include?(wday)
end

def on_weekend?
  WEEKEND_DAYS.include?(wday)
end

def past?
  self < self.class.current
end

def prev_day
  advance(days: -1)
end

def prev_month
  months_ago(1)
end

def prev_quarter
  months_ago(3)
end

def prev_week(start_day = Date.beginning_of_week, same_time: false)
  result = first_hour(weeks_ago(1).beginning_of_week.days_since(days_span(start_day)))
  same_time ? copy_time_to(result) : result
end

def prev_weekday
  if prev_day.on_weekend?
    copy_time_to(beginning_of_week(:friday))
  else
    prev_day
  end
end

def prev_year
  years_ago(1)
end

def sunday
  end_of_week(:monday)
end

def today?
  to_date == ::Date.current
end

def tomorrow
  advance(days: 1)
end

def weeks_ago(weeks)
  advance(:weeks => -weeks)
end

def weeks_since(weeks)
  advance(:weeks => weeks)
end

def years_ago(years)
  advance(:years => -years)
end

def years_since(years)
  advance(:years => years)
end

def yesterday
  advance(days: -1)
end