module Pagy::CalendarExtra::Backend

def pagy_calendar(collection, conf)
  unless conf.is_a?(Hash) && (conf.keys - CONF_KEYS).empty? && conf.all? { |k, v| v.is_a?(Hash) || k == :active }
    raise ArgumentError, "keys must be in #{CONF_KEYS.inspect} and object values must be Hashes; got #{conf.inspect}"
  end
  conf[:pagy] = {} unless conf[:pagy]  # use default Pagy object when omitted
  unless conf.key?(:active) && !conf[:active]
    calendar, from, to = Calendar::Helper.send(:init, conf, pagy_calendar_period(collection), params)
    collection         = pagy_calendar_filter(collection, from, to)
  end
  pagy, results = send(conf[:pagy][:backend] || :pagy, collection, conf[:pagy])  # use backend: :pagy when omitted
  [calendar, pagy, results]
end

def pagy_calendar_filter(*)
  raise NoMethodError, 'the pagy_calendar_filter method must be implemented by the application ' \
                       '(see https://ddnexus.github.io/pagy/docs/extras/calendar/#pagy-calendar-filter-collection-from-to)'
end

def pagy_calendar_period(*)
  raise NoMethodError, 'the pagy_calendar_period method must be implemented by the application ' \
                       '(see https://ddnexus.github.io/pagy/docs/extras/calendar/#pagy-calendar-period-collection)'
end

def pagy_calendar_url_at(calendar, time)
  pagy_url_for(calendar.send(:last_object_at, time), 1)
end