sparql pagination

class CurrencyQuery
  def self.all(search_prefix = nil)
    %(
      PREFIX dc: <#{PrefixesService::PREFIXES[:dc]}>
      PREFIX skos: <#{PrefixesService::PREFIXES[:skos]}>
      PREFIX sali: <#{PrefixesService::PREFIXES[:sali]}>
      SELECT ?currency ?prefix ?label (REPLACE(?definition, "Locations used: ", "") AS ?cleanedDefinition) ?totalCount
      WHERE {
        {
          SELECT (COUNT(?currency) AS ?totalCount)
          WHERE {
            ?currency rdfs:subClassOf sali:R767niCLQVC5zIcO5WDQMSl .
            ?currency dc:identifier ?prefix .
            ?currency rdfs:label ?label .
            ?currency skos:definition ?definition .
          }
        }
        ?currency rdfs:subClassOf sali:R767niCLQVC5zIcO5WDQMSl .
        ?currency dc:identifier ?prefix .
        ?currency rdfs:label ?label .
        ?currency skos:definition ?definition .
        #{'FILTER(CONTAINS(?prefix, "' + search_prefix.upcase + '"))' if search_prefix.present?}
      }
      ORDER BY ?prefix
    )
  end
end

def self.all_currencies(search_prefix = nil, page:, per_page:)
    sparql_query = CurrencyQuery.all(search_prefix)
    sparql_query += "LIMIT #{per_page} OFFSET #{page}"
    result = GraphDBConnector.instance.query(sparql_query)
    return nil unless result[:success]
    return [] unless result[:data].any?

    [result[:data].first[:totalCount], result[:data]]
end

class PaginationService
  DEFAULT_PAGE = 1
  PER_PAGE = 20
  MAX_PER_PAGE = 100

  def initialize(params)
    @page = params[:page].to_i.abs.positive? ? params[:page].to_i.abs : DEFAULT_PAGE
    @per_page = validate_per_page(params[:per_page])
  end

  def call
    meta = {
      page: @page,
      per_page: @per_page,
      total_elements: nil
    }
    [@per_page, @page, meta]
  end

  private

  def validate_per_page(per_page)
    per_page = per_page.to_i.abs
    per_page = per_page.positive? ? per_page : PER_PAGE
    per_page = MAX_PER_PAGE if per_page > MAX_PER_PAGE
    per_page
  end
end

def index
 per_page, page, meta = PaginationService.new(page: index_params[:page], per_page: index_params[:per_page]).call
 total_elements, records = Amount.all_currencies(index_params[:prefix_search], page: page, per_page: per_page)
 meta[:total_elements] = total_elements.value.to_i
 render json: CurrencySerializer.new(records: records, root_key: :data, meta: meta).serialized_json, status: :ok
end