truncating and adding ellipsis to string in ruby

gistfile1.rb

# this is probably a fairly common use case
# but there must be a more elegant solution?
# shortened_url returns a bit.ly link or nil in development

class Post
  def tweet_text
    link_length = shortened_url ? shortened_url.length : ""
    separator = "-"
    available_chars = link_length - separator.length

    str = if available_chars < title.length
      title.first(available_chars)
    else
      title.first(available_chars - 3) + "..."
    end

    return "#{str} #{separator} #{shortened_url}"
  end
end