class Pagy::Calendar

def active_period
  [[@starting, @from].max, [@to - 1, @ending].min] # -1 sec: include only last unit day
end

def create(unit, vars)
  raise InternalError, "unit must be in #{UNITS.inspect}; got #{unit}" unless UNITS.include?(unit)
  name    = unit.to_s
  name[0] = name[0].capitalize
  Object.const_get("Pagy::Calendar::#{name}").new(vars)
end

def initialize(vars) # rubocop:disable Lint/MissingSuper
  raise InternalError, 'Pagy::Calendar is a base class; use one of its subclasses' if instance_of?(Pagy::Calendar)
  vars = self.class::DEFAULT.merge(vars)  # subclass specific default
  normalize_vars(vars)                    # general default
  setup_vars(page: 1)
  setup_unit_vars
  setup_params_var
  raise OverflowError.new(self, :page, "in 1..#{@last}", @page) if @page > @last
  @prev = (@page - 1 unless @page == 1)
  @next = @page == @last ? (1 if @vars[:cycle]) : @page + 1
end

def label(opts = {})
  label_for(@page, opts)
end

def label_for(page, opts = {})
  opts[:format] ||= @vars[:format]
  localize(starting_time_for(page.to_i), opts)  # page could be a string
end

def localize(time, opts)
  time.strftime(opts[:format])
end

def page_at(time, **opts)
  fit_time  = time
  fit_final = @final - 1
  unless time.between?(@initial, fit_final)
    raise OutOfRangeError.new(self, :time, "between #{@initial} and #{fit_final}", time) unless opts[:fit_time]
    if time < @final
      fit_time = @initial
      ordinal  = 'first'
    else
      fit_time = fit_final
      ordinal  = 'last'
    end
    Warning.warn "Pagy::Calendar#page_at: Rescued #{time} out of range by returning the #{ordinal} page."
  end
  offset = page_offset_at(fit_time)   # offset starts from 0
  @order == :asc ? offset + 1 : @pages - offset
end

def page_offset_at(*)
  raise NoMethodError, 'the page_offset_at method must be implemented by the unit subclass'
end

def setup_unit_vars
  raise VariableError.new(self, :format, 'to be a strftime format', @vars[:format]) unless @vars[:format].is_a?(String)
  raise VariableError.new(self, :order, 'to be in [:asc, :desc]', @order) \
        unless %i[asc desc].include?(@order = @vars[:order])
  @starting, @ending = @vars[:period]
  raise VariableError.new(self, :period, 'to be a an Array of min and max TimeWithZone instances', @vars[:period]) \
        unless @starting.is_a?(ActiveSupport::TimeWithZone) \
            && @ending.is_a?(ActiveSupport::TimeWithZone) && @starting <= @ending
end

def starting_time_for(*)
  raise NoMethodError, 'the starting_time_for method must be implemented by the unit subclass'
end

def time_offset_for(page)
  @order == :asc ? page - 1 : @pages - page
end