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When you’re a Twitter'er you will often be in a situation where someone follows you, and you’re wondering, ‘Who is this person? Do I know them?’ Well, I can’t answer that question for you. But, I have found that one thing that tells you about your new follower is who they follow that you also follow. Follow?

I want to be able to ask the question ‘Who do we know in common?’, in short. A useful question, but one that can take quite a while to answer using the web site. I asked the lazy twitterverse if there was already an app for this, but my twitterverse is too small to get an answer. So, I wrote my own script. I don’t have any handy web space to run this from, so you’ll have to grab it and run it yourself. You will need to install the twitter4r gem first:

sudo gem install twitter4r

Then paste the following code into a Ruby file, and run. It takes two parameters, the names of the two users for who you want to find common ground.

require 'rubygems'
require 'open-uri'
require 'rexml/document'
require 'twitter'

class Twitter::User

  def all_friends
    users = friends.map { |f| f.screen_name }
    # If there's more than one page of users, we've already got the
    # first one
    page = 2
    found_users = friends.length
    while found_users >= 100
      found_users = 0
      open("http://twitter.com/statuses/friends/#{screen_name}.xml?page=#{page}") do |f|
        users_doc = REXML::Document.new(f.readlines.join(''))
        users_doc.elements.each('/users/user/screen_name') do |friend_name|
          users << friend_name.text
          found_users += 1
        end
      end
      page += 1
    end
    users
  end

end

def in_common(my_friends, other_friends)
  my_friends.select { |m_n| m_n if other_friends.member? m_n }
end

def main(me, other)
  c = Twitter::Client.new
  me_friends = c.user(me).all_friends
  other_friends = c.user(other).all_friends
  in_common(me_friends, other_friends).each do |f|
    puts "  #{f}"
  end
end

main(ARGV.shift, ARGV.shift)

Enjoy, and please let me know how it works out for you, or if you make any changes. And by the way, this is why RESTful APIs rock.