module Pagy::Calendar::MonthMixin

def add_months_to(time, months)
  months += months_in(time) - 1             # remove 1 month for 0-11 calculations
  new_time(months / 12, (months % 12) + 1)  # add 1 month back for 1-12 conversion
end

def months_in(time)
  (time.year * 12) + time.month
end

def setup_unit_vars
  super
  @months  = self.class::MONTHS  # number of months in the unit
  @initial = new_time(@starting.year, starting_month_including(@starting.month))
  @final   = add_months_to(new_time(@ending.year, starting_month_including(@ending.month)), @months)
  @pages   = @last = (months_in(@final) - months_in(@initial)) / @months
  @from    = starting_time_for(@page)
  @to      = add_months_to(@from, @months)
end

def starting_month_including(month)
  (@months * ((month - 1) / @months)) + 1  # remove 1 month for 0-11 calculations and add it back for 1-12 conversion
end

def starting_time_for(page)
  add_months_to(@initial, snap(page) * @months)
end