module Jekyll
  module Generators
    class Pagination < Generator
      # This generator is safe from arbitrary code execution.
      safe true

      # Generate paginated pages if necessary.
      #
      # site - The Site.
      #
      # Returns nothing.
      def generate(site)
        site.pages.dup.each do |page|
          paginate(site, page) if Pager.pagination_enabled?(site.config, page.name)
        end
      end

      # Paginates the blog's posts. Renders the index.html file into paginated
      # directories, e.g.: page2/index.html, page3/index.html, etc and adds more
      # site-wide data.
      #
      # site - The Site.
      # page - The index.html Page that requires pagination.
      #
      # {"paginator" => { "page" => <Number>,
      #                   "per_page" => <Number>,
      #                   "posts" => [<Post>],
      #                   "total_posts" => <Number>,
      #                   "total_pages" => <Number>,
      #                   "previous_page" => <Number>,
      #                   "next_page" => <Number> }}
      def paginate(site, page)
        all_posts = site.site_payload['site']['posts']
        pages = Pager.calculate_pages(all_posts, site.config['paginate'].to_i)
        (1..pages).each do |num_page|
          pager = Pager.new(site.config, num_page, all_posts, pages)
          if num_page > 1
            newpage = Page.new(site, site.source, page.dir, page.name)
            newpage.pager = pager
            newpage.dir = File.join(page.dir, paginate_path(site, num_page))
            site.pages << newpage
          else
            page.pager = pager
          end
        end
      end

      private
        def paginate_path(site, num_page)
          format = site.config['paginate_path']
          format.sub(':num', num_page.to_s)
        end
    end
  end

  class Pager
    attr_reader :page, :per_page, :posts, :total_posts, :total_pages, :previous_page, :next_page

    # Calculate the number of pages.
    #
    # all_posts - The Array of all Posts.
    # per_page  - The Integer of entries per page.
    #
    # Returns the Integer number of pages.
    def self.calculate_pages(all_posts, per_page)
      (all_posts.size.to_f / per_page.to_i).ceil
    end

    # Determine if pagination is enabled for a given file.
    #
    # config - The configuration Hash.
    # file   - The String filename of the file.
    #
    # Returns true if pagination is enabled, false otherwise.
    def self.pagination_enabled?(config, file)
      file == 'index.html' && !config['paginate'].nil?
    end

    # Initialize a new Pager.
    #
    # config    - The Hash configuration of the site.
    # page      - The Integer page number.
    # all_posts - The Array of all the site's Posts.
    # num_pages - The Integer number of pages or nil if you'd like the number
    #             of pages calculated.
    def initialize(config, page, all_posts, num_pages = nil)
      @page = page
      @per_page = config['paginate'].to_i
      @total_pages = num_pages || Pager.calculate_pages(all_posts, @per_page)

      if @page > @total_pages
        raise RuntimeError, "page number can't be greater than total pages: #{@page} > #{@total_pages}"
      end

      init = (@page - 1) * @per_page
      offset = (init + @per_page - 1) >= all_posts.size ? all_posts.size : (init + @per_page - 1)

      @total_posts = all_posts.size
      @posts = all_posts[init..offset]
      @previous_page = @page != 1 ? @page - 1 : nil
      @next_page = @page != @total_pages ? @page + 1 : nil
    end

    # Convert this Pager's data to a Hash suitable for use by Liquid.
    #
    # Returns the Hash representation of this Pager.
    def to_liquid
      {
        'page' => page,
        'per_page' => per_page,
        'posts' => posts,
        'total_posts' => total_posts,
        'total_pages' => total_pages,
        'previous_page' => previous_page,
        'next_page' => next_page
      }
    end
  end
end