diff.rb

def diff(a, b)
  a = a.clone # we will mutate a, so clone it
  operations = []

  # find and remove stuff which is in a, but not in b
  a.reverse.each do |item|
    unless b.include?(item)
      index = a.index(item)
      a.delete(item)
      operations.push(["delete", index])
    end
  end

  # find and insert stuff which is in b, but not in a
  b.each do |item|
    unless a.include?(item)
      index = b.index(item)
      a.insert(index, item)
      operations.push(["insert", index, item])
    end
  end

  # sort it
  a.each_with_index do |item, aindex|
    unless item == b[aindex]
      bindex = b.index(item)
      a[aindex], a[bindex] = a[bindex], a[aindex]
      operations.push(["swap", aindex, bindex])
    end
  end

  operations
end