gistfile1.rb

require 'tempfile'

class Diff
  KDIFF3 = '/usr/bin/kdiff3 --display NONE' unless defined?(KDIFF3)
  DIFF3 = '/usr/bin/diff3' unless defined?(DIFF3)

  class << self
    def diff(content1, content2, content3)
      file1_path = make_tmpfile('file1', content1)
      file2_path = make_tmpfile('file2', content2)
      file3_path = make_tmpfile('file3', content3)

      output = kdiff3(file1_path, file2_path, file3_path)
      output ||= diff3(file1_path, file2_path, file3_path)

      File.delete(file1_path, file2_path, file3_path)

      output
    end

    def kdiff3(file1_path, file2_path, file3_path)
      output = make_tmpfile('output')
      `#{KDIFF3} #{file1_path} #{file2_path} #{file3_path} --auto -o #{output} 2>&1`
      content = File.size?(output) ? File.read(output) : nil
      File.delete(output)
      content
    end

    def diff3(file1_path, file2_path, file3_path)
      `#{DIFF3} #{file2_path} #{file1_path} #{file3_path} -m`
    end

    # Returns path to the file
    def make_tmpfile(name, content = nil)
      tempfile = Tempfile.new(name)

      begin
        tempfile << content if content
      ensure
        tempfile.close
      end
      tempfile.path
    end
  end
end