# frozen_string_literal: true
module Blacklight
  # Pagination for facet values -- works by setting the limit to max
  # displayable. You have to ask Solr for limit+1, to get enough
  # results to see if 'more' are available'. That is, the all_facet_values
  # arg in constructor should be the result of asking solr for limit+1
  # values.
  # This is a workaround for the fact that Solr itself can't compute
  # the total values for a given facet field,
  # so we cannot know how many "pages" there are.
  #
  class FacetPaginator
    # What request keys will we use for the parameters need. Need to
    # make sure they do NOT conflict with catalog/index request params,
    # and need to make them accessible in a list so we can easily
    # strip em out before redirecting to catalog/index.
    mattr_accessor :request_keys do
      { sort: :'facet.sort', page: :'facet.page', prefix: :'facet.prefix' }
    end

    attr_reader :offset, :limit, :sort, :prefix

    # all_facet_values is a list of facet value objects returned by solr,
    # asking solr for n+1 facet values.
    # options:
    # :limit =>  number to display per page, or (default) ?. Nil means
    #            display all with no previous or next.
    # :offset => current item offset, default 0
    # :sort => 'count' or 'index', solr tokens for facet value sorting, default 'count'.
    def initialize(all_facet_values, arguments = {})
      # to_s.to_i will conveniently default to 0 if nil
      @offset = arguments[:offset].to_s.to_i
      @limit = arguments[:limit]
      @sort = arguments[:sort]
      @prefix = arguments[:prefix]

      @all = all_facet_values
    end

    # The number of records solr gave us when we asked for limit + 1 records at the current offset
    def total_count
      @all.size
    end

    def items
      items_for_limit(@all)
    end

    def prev_page
      current_page - 1 unless first_page?
    end

    def current_page
      # A nil limit is unlimited, thus only one page.
      if limit.nil? || limit.zero? #check for divide by zero
        1
      else
        @offset / limit + 1
      end
    end

    def next_page
      current_page + 1 unless last_page?
    end

    def last_page?
      limit.nil? || total_count <= limit
    end

    def first_page?
      current_page == 1
    end

    # We're implementing total_pages so that this matches the API from kaminari, even though we can't
    # know the total number of pages.
    # https://github.com/amatsuda/kaminari/blob/v0.16.1/lib/kaminari/models/page_scope_methods.rb#L21
    def total_pages
      -1
    end

    # Pass in a desired solr facet solr key ('count' or 'index', see
    # http://wiki.apache.org/solr/SimpleFacetParameters#facet.limit
    # under facet.sort ), and your current request params.
    # Get back params suitable to passing to an ActionHelper method for
    # creating a url, to resort by that method.
    # @param [String] sort_method
    # @param [HashWithIndifferentAccess] params
    def params_for_resort_url(sort_method, params = {})
      # When resorting, we've got to reset the offset to start at beginning,
      # no way to make it make sense otherwise.
      resort_params = params.merge(request_keys[:sort] => sort_method, request_keys[:page] => nil)

      if sort_method == 'count'
        resort_params.except!(request_keys[:prefix])
      end

      resort_params
    end

    def as_json(_ = nil)
      { 'items' => items.as_json, 'limit' => limit, 'offset' => offset, 'sort' => sort }
    end

    private

    # setting limit to nil implies no limit
    # @return an array of facets on the page
    def items_for_limit(values)
      limit.nil? ? values : values.take(limit)
    end
  end
end