module Kadmin
  # Simple Pager structure, used to paginate collections
  class Pager
    # @return [Integer] number of items per page
    attr_reader :size

    # @return [Integer] current index offset (determines current page)
    attr_reader :offset

    # @return [Integer] total number of pages
    attr_reader :pages

    # @return [Integer] the current page
    attr_reader :current_page

    # @return [Integer] total number of items in the collection
    attr_reader :total

    # @param [Integer] size size of a page
    # @param [Integer] offset offset/index for the current page
    def initialize(size:, offset:)
      @size = size
      @offset = offset

      raise(Kadmin::Error, 'Page size must be greater than 0!') unless @size.positive?
      raise(Kadmin::Error, 'Offset must be at least 0!') unless @offset >= 0

      @current_page = (@offset / @size.to_f).floor
      self.total = @size # assume page size is maximum initially
    end

    # @param [ActiveRecord::Relation] collection relation to paginate
    # @return [ActiveRecord::Relation] paginated collection
    def page(collection)
      self.total = collection.count

      collection = collection.offset(@offset)
      collection = collection.limit(@size)

      return collection
    end

    # @param [Integer] page the page to get the offset for; if not given, uses the current page
    # @return [Integer] start offset for the given page
    def offset_at(page = nil)
      page ||= @current_page
      return @size * page.to_i
    end

    # @param [Integer] page the page to check for
    # @return [Boolean] true if `page` is the current page
    def current_page?(page)
      return page == @current_page
    end

    # @param [Integer] page the page to check for
    # @return [Boolean] true if `page` exists (i.e. would have any data)
    def contains?(page)
      page.in?(0...@pages)
    end

    # @param [Integer] total sets the number of total collection items (not total loaded)
    def total=(total)
      @total = total
      @pages = (@total / @size.to_f).ceil
    end

    # @param [Integer] page optional; if not given, uses the current page
    # @return [Integer] the number of items that are on this page
    def page_size(page = nil)
      page ||= @current_page
      page_start = offset_at(page)
      page_end = [offset_at(page.to_i + 1), @total].min

      return page_end - page_start
    end

    # @param [Integer] page optional; if given, checks if the page after would have any data, otherwise checks based on the current page
    # @return [Boolean] true if there is a next page
    def next_page?(page = nil)
      page ||= @current_page
      return offset_at(page.to_i + 1) < @total
    end

    # @param [Integer] page optional; if given, checks if the page after would have any data, otherwise checks based on the current page
    # @return [Boolean] true if there is a previous page
    def previous_page?(page = nil)
      page ||= @current_page

      return page.to_i.positive?
    end
  end
end