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
      page += 1


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

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}"

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.