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 = unit_starting_time_for(@starting)
  @final   = add_months_to(unit_starting_time_for(@ending), @months)
  @pages   = @last = (months_in(@final) - months_in(@initial)) / @months
  @from    = starting_time_for(@page)
  @to      = add_months_to(@from, @months)
end

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

def unit_starting_time_for(time)
  # remove 1 month for 0-11 calculations and add it back for 1-12 conversion
  starting_month = (@months * ((time.month - 1) / @months)) + 1
  new_time(time.year, starting_month)
end