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
  calendar, collection = pagy_setup_calendar(collection, conf) unless conf.key?(:active) && !conf[:active]
  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/extras/calendar#pagy_calendar_filtercollection-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/extras/calendar#pagy_calendar_periodcollection)'
end

def pagy_setup_calendar(collection, conf)
  units = Calendar::UNITS & conf.keys # get the units in time length desc order
  raise ArgumentError, 'no calendar unit found in pagy_calendar configuration' if units.empty?
  page_param = conf[:pagy][:page_param] || DEFAULT[:page_param]
  units.each do |unit|  # set all the :page_param vars for later deletion
    unit_page_param         = :"#{unit}_#{page_param}"
    conf[unit][:page_param] = unit_page_param
    conf[unit][:page]       = params[unit_page_param]
  end
  calendar = {}
  last_obj = nil
  units.each_with_index do |unit, index|
    params_to_delete    = units[(index + 1), units.size].map { |sub| conf[sub][:page_param] } + [page_param]
    conf[unit][:params] = lambda do |params|  # delete page_param from the sub-units
                            params_to_delete.each { |p| params.delete(p.to_s) } # Hash#except missing from ruby 2.5 baseline
                            params
                          end
    conf[unit][:period] = last_obj&.send(:active_period) || pagy_calendar_period(collection)
    calendar[unit]      = last_obj = Calendar.send(:create, unit, conf[unit])
  end
  [calendar, pagy_calendar_filter(collection, last_obj.from, last_obj.to)]
end