require 'diff-0.4/lib/algorithm/diff'

a = 'testing
sled
123
testing'
ab = a.dup
b = 'testing
sledding across the white snow
123error
testing'
diff = Diff.diff(a, b)
offset = 0
b_insert = '<b>'
a_insert = '</b>'
diff.each do |d|
  a.insert(d[1] + offset, b_insert + d[2])
  offset += b_insert.length
  a.insert(d[1] + d[2].length + offset, a_insert)
  offset += a_insert.length
end
puts "\nOriginal:"
puts ab
puts "\nChanged:"
puts a, "\n"