require 'longleaf/events/event_names'
require 'longleaf/errors'
require 'longleaf/logging'
require 'time'

module Longleaf
  # Iterator for getting file candidates which have services which need to be run.
  # Implementation uses an index of file metadata to determine if the file needs any
  # services run.
  class ServiceCandidateIndexIterator
    include Longleaf::Logging

    def initialize(file_selector, event, app_config, force = false)
      @file_selector = file_selector
      @event = event
      @app_config = app_config
      @force = force
      @index_manager = @app_config.index_manager
      @stale_datetime = Time.now.utc
      @result_set = nil
    end

    # Get the file record for the next candidate which needs services run which match the
    # provided file_selector
    # @return [FileRecord] file record of the next candidate with services needing to be run,
    #    or nil if there are no more candidates.
    def next_candidate
      file_rec = nil
      # loop until a candidate with metadata is retrieved
      loop do
        # Get the next page of results if the previous page has been processed
        fetch_next_page if @result_set.nil? || @result_set.empty?

        # Retrieve the next possible candidate path from the page
        next_path = @result_set.shift
        # given that the page was just retrieved, getting a nil path indicates that the retrieved page of
        # candidates is empty and there are no more candidates to iterate at this time.
        return nil if next_path.nil?

        logger.debug("Retrieved candidate #{next_path}")
        storage_loc = @app_config.location_manager.get_location_by_path(next_path)
        file_rec = FileRecord.new(next_path, storage_loc)

        # Keep seeking until a registered candidate is found, according to the file system.
        if file_rec.metadata_present?
          break
        else
          logger.warn("Encountered #{next_path} in index, but path is not registered. Clearing out of synch entry.")
          @index_manager.remove(file_rec)
        end
      end

      @app_config.md_manager.load(file_rec)

      file_rec
    end

    # Iterate through the candidates in this object and execute the provided block with each
    # candidate. A block is required.
    def each
      file_rec = next_candidate
      until file_rec.nil?
        yield file_rec

        file_rec = next_candidate
      end
    end

    private
    def fetch_next_page
      if @force
        @result_set = @index_manager.registered_paths(@file_selector)
      else
        case @event
        when EventNames::PRESERVE
          @result_set = @index_manager.paths_with_stale_services(@file_selector, @stale_datetime)
        when EventNames::CLEANUP
          # TODO
        end
      end
      logger.debug("Retrieve result set with #{@result_set&.length} entries")
    end
  end
end