How to add next and previous pagination without any gem in Rails

How to add next and previous pagination without any gem in Rails?

So! in the controller

class ProductsController < ApplicationController
   PRODUCTS_SIZE = 20

   def index
     @page = (params[:page] || 0).to_i
     @products = Product.offset(PRODUCTS_SIZE * @page).limit(PRODUCTS_SIZE)
   end
end

now in the view

<div>
   <ul class="pager">
      <li class="previous <%= @page == 0 ? 'disabled' : '' %>">
          <%= link_to_if @page > 0, "<< Previous", products_path(page: @page - 1) %>
      </li>

      <li class="next">
          <%= link_to "Next >>", products_path(page: @page + 1) %>
      </li>
   </ul>
</div>

Done!