related_posts.rb

require 'jekyll/post'

module Jekyll
  module RelatedPostsByTags
    # Used to remove #related_posts so that it can be overridden
    def self.included(klass)
      klass.class_eval do
        remove_method :related_posts
      end
    end

    # Calculate related posts.
    # Returns [<Post>]
    def related_posts(posts)
      return [] unless posts.size > 1
      highest_freq = tag_freq(posts).values.max
      related_scores = Hash.new(0)

      posts.each do |post|
        post.tags.each do |tag|
          if self.tags.include?(tag) && post != self
            cat_freq = tag_freq(posts)[tag]
            related_scores[post] += (1+highest_freq-cat_freq)
          end
        end
      end

      sort_related_posts(related_scores)
    end

    # Calculate the frequency of each tag.
    # Returns {tag => freq, tag => freq, ...}
    def tag_freq(posts)
      return @tag_freq if @tag_freq
      @tag_freq = Hash.new(0)
      posts.each do |post|
        post.tags.each {|tag| @tag_freq[tag] += 1}
      end
      @tag_freq
    end

    # Sort the related posts in order of their score and date
    # and return just the posts
    def sort_related_posts(related_scores)
      related_scores.sort do |a,b|
        if a[1] < b[1]
          1
        elsif a[1] > b[1]
          -1
        else
          b[0].date <=> a[0].date
        end
      end.collect {|post,freq| post}
    end
  end

  class Post
    include RelatedPostsByTags
  end
end