diff a ruby string or array

For arrays, use the minus operator. For example:

>> foo = [1, 2, 3]
=> [1, 2, 3]
>> goo = [2, 3, 4]
=> [2, 3, 4]
>> foo - goo
=> [1]

Here the last line removes everything from foo that is also in goo, leaving just the element 1. I don't know how to do this for two strings, but until somebody who knows posts about it, you could just convert each string to an array, use the minus operator, and then convert the result back.

diff.rb is what you want, which is available at http://users.cybercity.dk/~dsl8950/ruby/diff.html via internet archive:

http://web.archive.org/web/20140421214841/http://users.cybercity.dk:80/~dsl8950/ruby/diff.html

For strings, I would first try out the Ruby Gem that @sam-saffron mentioned below. It's easier to install: http://github.com/pvande/differ/tree/master

gem install differ

irb
require 'differ'

one = "one two three"
two = "one two 3"

Differ.format = :color
puts Differ.diff_by_word(one, two).to_s

Differ.format = :html
puts Differ.diff_by_word(one, two).to_s

There is also diff-lcs which is available as a gem. It hasn't been updated since 2004 but we have been using it without any problem.

Edit: A new version was released in 2011. Looks like it's back in active development.

http://rubygems.org/gems/diff-lcs

The HTMLDiff that @da01 mentions above worked for me.

script/plugin install git://github.com/myobie/htmldiff.git

# bottom of environment.rb
require 'htmldiff'

# in model
class Page < ActiveRecord::Base
  extend HTMLDiff
end

# in view
<h1>Revisions for <%= @page.name %></h1>
<ul>
<% @page.revisions.each do |revision| %>
  <li>
    <b>Revised <%= distance_of_time_in_words_to_now revision.created_at %> ago</b><BR>
      <%= Page.diff(
        revision.changes['description'][0],
        revision.changes['description'][1]
      ) %>
      <BR><BR>
  </li>
<% end %>

# in style.css
ins.diffmod, ins.diffins { background: #d4fdd5; text-decoration: none; }
del.diffmod, del.diffdel { color: #ff9999; }

Looks pretty good. By the way I used this with the acts_as_audited plugin.

t=s2.chars; s1.chars.map{|c| c == t.shift ? c : '^'}.join

This simple line gives a ^ in the positions that don't match. That's often enough and it's copy/paste-able.

I just found a new project that seems pretty flexible:

http://github.com/pvande/differ/tree/master

Trying it out and will try to post some sort of report.