Jekyll Plugin Generator - Next and previous pagination by category

category_prev_next_generator.rb

# A plugin to obtain category pagination with Jekyll
#
# Usage:
# <ul class="pagination">
# {% if page.blog_previous %}
#   <li class="prev"><a href="{{ page.blog_previous.url | prepend: site.baseurl }}" title="{{ page.blog_previous.title }}">&laquo; Previous</a></li>
# {% else %}
#   <li class="prev disabled"><a>&larr; Previous</a></li>
# {% endif %}
#   <li><a href="{{ "/blog/" | prepend: site.baseurl }}">Blog</a></li>
# {% if page.blog_next %}
#   <li class="next"><a href="{{ page.blog_next.url | prepend: site.baseurl }}" title="{{ page.blog_next.title }}">Next &raquo;</a></li>
# {% else %}
#   <li class="next disabled"><a>Next &rarr;</a>
# {% endif %}
# </ul>
module Jekyll
  class CategoryPrevNextGenerator < Generator

    safe true
    priority :high

    def generate(site)
      site.categories.each_pair do |category_name, posts|

        # print "> " + category_name + ":\n"

        posts.sort! { |a, b| b <=> a }

        posts.each_with_index do |post, index|
          # print "-> " + post.data['date'].to_s + ": " + post.data['title'] + "\n"
          # print "--: " + index.to_s + "\n"

          # Previous
          if index < posts.length - 1
            category_prev = posts[index + 1]
          else
            category_prev = nil
          end

          # Next
          if index > 0 && posts.length > 1
            category_next = posts[index - 1]
          else
            category_next = nil
          end

          post.data["#{category_name}_previous"] = category_prev unless category_prev.nil?
          post.data["#{category_name}_next"] = category_next unless category_next.nil?

        end # / posts each

      end # / categories each
    end # / method generate
  end # / class
end # / module