book_search_paginator.rb

class BookSearchPaginator
  DEFAULT_CURSOR = {
    current_query_index: 0,
    current_page_number: 1,
    current_item: 0
  }.freeze

  def initialize(ordered_queries, &auth_block)
    @ordered_queries = ordered_queries

    @auth_block = auth_block
  end

  def next(page_size: 12, cursor: DEFAULT_CURSOR)
    results = []
    new_cursor = cursor

    auth_block.call do
      (page_results, new_cursor) = fetch_results_from_cursor(number_needed: page_size, cursor:)
      results += page_results

      while results.size < page_size
        break if ordered_queries[new_cursor.fetch(:current_query_index)].nil?

        (page_results, new_cursor) = fetch_results_from_cursor(number_needed: page_size - results.size, cursor: new_cursor)
        break if results.empty?

        results += page_results
      end
    end

    [results, new_cursor]
  end

  private

  attr_reader :ordered_queries, :auth_block

  def fetch_results_from_cursor(number_needed:, cursor:)
    current_query_index = cursor.fetch(:current_query_index)
    current_item = cursor.fetch(:current_item)

    current_page = ordered_queries[current_query_index]
      .page(cursor.fetch(:current_page_number))
      .all

    results = current_page.slice(cursor.fetch(:current_item), number_needed) || []

    new_cursor = if results.size < number_needed
                   if current_page.pages.next_page
                     {
                       current_query_index:,
                       current_page_number: current_page.pages.next_page,
                       current_item: 0
                     }
                   else
                     {
                       current_query_index: current_query_index + 1,
                       current_page_number: 1,
                       current_item: 0
                     }
                   end
                 else
                   {
                     current_query_index:,
                     current_page_number: current_page.pages.current_page,
                     current_item: current_item + results.size
                   }
                 end

    [results, new_cursor]
  end
end