sure/app/controllers/concerns/safe_pagination.rb

# frozen_string_literal: true

module SafePagination
  extend ActiveSupport::Concern

  private
    def safe_per_page(default = 10)
      allowed_values = [ 10, 20, 30, 50, 100 ]
      per_page = params[:per_page].to_i

      return default if per_page <= 0

      allowed_values.include?(per_page) ? per_page : allowed_values.min_by { |v| (v - per_page).abs }
    end
end