SequenceMatcher class in ruby. credit to testunit folks.

diff.rb

# port of Python's difflib.
#
# Copyright (c) 2001-2008 Python Software Foundation; All Rights Reserved
# Copyright (c) 2008-2011 Kouhei Sutou; All Rights Reserved
#
# It is free software, and is distributed under the Ruby
# license and/or the PSF license. See the COPYING file and
# PSFL file.

# Carry this code from testunit/testunit project. 
# https://github.com/test-unit/test-unit/blob/master/lib/test/unit/diff.rb

module Diff
  class SequenceMatcher
    def initialize(from, to, &junk_predicate)
      @from = from
      @to = to
      @junk_predicate = junk_predicate
      update_to_indexes
    end

    def longest_match(from_start, from_end, to_start, to_end)
      best_info = find_best_match_position(from_start, from_end,
                                           to_start, to_end)
      unless @junks.empty?
        args = [from_start, from_end, to_start, to_end]
            best_info = adjust_best_info_with_junk_predicate(false, best_info,
                                                             *args)
            best_info = adjust_best_info_with_junk_predicate(true, best_info,
                                                             *args)
    end

      best_info
    end

    def blocks
      @blocks ||= compute_blocks
    end

    def operations
      @operations ||= compute_operations
    end

    def grouped_operations(context_size=nil)
      context_size ||= 3
      _operations = operations.dup
      _operations = [[:equal, 0, 0, 0, 0]] if _operations.empty?
      expand_edge_equal_operations!(_operations, context_size)

      group_window = context_size * 2
      groups = []
      group = []
      _operations.each do |tag, from_start, from_end, to_start, to_end|
        if tag == :equal and from_end - from_start > group_window
          group << [tag,
            from_start,
            [from_end, from_start + context_size].min,
            to_start,
            [to_end, to_start + context_size].min]
          groups << group
          group = []
          from_start = [from_start, from_end - context_size].max
          to_start = [to_start, to_end - context_size].max
        end
        group << [tag, from_start, from_end, to_start, to_end]
      end
      groups << group unless group.empty?
      groups
    end

    def ratio
      @ratio ||= compute_ratio
    end

    private
    def update_to_indexes
      @to_indexes = {}
      @junks = {}
      if @to.is_a?(String)
        each = " "[0].is_a?(Integer) ? :each_byte : :each_char
      else
        each = :each
      end
      i = 0
      @to.send(each) do |item|
        @to_indexes[item] ||= []
        @to_indexes[item] << i
        i += 1
      end

      return if @junk_predicate.nil?
      @to_indexes = @to_indexes.reject do |key, value|
        junk = @junk_predicate.call(key)
        @junks[key] = true if junk
        junk
      end
    end

    def find_best_match_position(from_start, from_end, to_start, to_end)
      best_from, best_to, best_size = from_start, to_start, 0
      sizes = {}
      from_start.upto(from_end) do |from_index|
        _sizes = {}
        (@to_indexes[@from[from_index]] || []).each do |to_index|
          next if to_index < to_start
          break if to_index > to_end
          size = _sizes[to_index] = (sizes[to_index - 1] || 0) + 1
          if size > best_size
            best_from = from_index - size + 1
            best_to = to_index - size + 1
            best_size = size
          end
        end
        sizes = _sizes
      end
      [best_from, best_to, best_size]
    end

    def adjust_best_info_with_junk_predicate(should_junk, best_info,
                                             from_start, from_end,
                                             to_start, to_end)
      best_from, best_to, best_size = best_info
      while best_from > from_start and best_to > to_start and
        (should_junk ?
         @junks.has_key?(@to[best_to - 1]) :
         !@junks.has_key?(@to[best_to - 1])) and
        @from[best_from - 1] == @to[best_to - 1]
        best_from -= 1
        best_to -= 1
        best_size += 1
      end

      while best_from + best_size < from_end and
        best_to + best_size < to_end and
        (should_junk ?
         @junks.has_key?(@to[best_to + best_size]) :
         !@junks.has_key?(@to[best_to + best_size])) and
        @from[best_from + best_size] == @to[best_to + best_size]
        best_size += 1
      end

      [best_from, best_to, best_size]
    end

    def matches
      @matches ||= compute_matches
    end

    def compute_matches
      matches = []
      queue = [[0, @from.size, 0, @to.size]]
      until queue.empty?
        from_start, from_end, to_start, to_end = queue.pop
        match = longest_match(from_start, from_end - 1, to_start, to_end - 1)
        match_from_index, match_to_index, size = match
        unless size.zero?
          if from_start < match_from_index and
            to_start < match_to_index
            queue.push([from_start, match_from_index,
                       to_start, match_to_index])
          end
          matches << match
          if match_from_index + size < from_end and
            match_to_index + size < to_end
            queue.push([match_from_index + size, from_end,
                       match_to_index + size, to_end])
          end
        end
      end
      matches.sort_by do |(from_index, _, _)|
        from_index
      end
    end

    def compute_blocks
      blocks = []
      current_from_index = current_to_index = current_size = 0
      matches.each do |from_index, to_index, size|
        if current_from_index + current_size == from_index and
          current_to_index + current_size == to_index
          current_size += size
        else
          unless current_size.zero?
            blocks << [current_from_index, current_to_index, current_size]
          end
          current_from_index = from_index
          current_to_index = to_index
          current_size = size
        end
      end
      unless current_size.zero?
        blocks << [current_from_index, current_to_index, current_size]
      end

      blocks << [@from.size, @to.size, 0]
      blocks
    end

    def compute_operations
      from_index = to_index = 0
      operations = []
      blocks.each do |match_from_index, match_to_index, size|
        tag = determine_tag(from_index, to_index,
                            match_from_index, match_to_index)
        if tag != :equal
          operations << [tag,
            from_index, match_from_index,
            to_index, match_to_index]
        end

        from_index, to_index = match_from_index + size, match_to_index + size
        if size > 0
          operations << [:equal,
            match_from_index, from_index,
            match_to_index, to_index]
        end
      end
      operations
    end

    def compute_ratio
      matches = blocks.inject(0) {|result, block| result + block[-1]}
      length = @from.length + @to.length
      if length.zero?
        1.0
      else
        2.0 * matches / length
      end
    end

    def determine_tag(from_index, to_index,
                      match_from_index, match_to_index)
      if from_index < match_from_index and to_index < match_to_index
        :replace
      elsif from_index < match_from_index
        :delete
      elsif to_index < match_to_index
        :insert
      else
        :equal
      end
    end

    def expand_edge_equal_operations!(_operations, context_size)
      tag, from_start, from_end, to_start, to_end = _operations[0]
      if tag == :equal
        _operations[0] = [tag,
          [from_start, from_end - context_size].max,
          from_end,
          [to_start, to_end - context_size].max,
          to_end]
      end

      tag, from_start, from_end, to_start, to_end = _operations[-1]
      if tag == :equal
        _operations[-1] = [tag,
          from_start,
          [from_end, from_start + context_size].min,
          to_start,
          [to_end, to_start + context_size].min]
      end
    end
  end

  class Differ
    def initialize(from, to)
      @from = from
      @to = to
    end

    private
    def tag(mark, contents)
      contents.collect {|content| "#{mark}#{content}"}
    end
  end

  class UTF8Line
    class << self
      # from http://unicode.org/reports/tr11/
      WIDE_CHARACTERS =
        [0x1100..0x1159, 0x115F..0x115F, 0x2329..0x232A,
          0x2E80..0x2E99, 0x2E9B..0x2EF3, 0x2F00..0x2FD5,
          0x2FF0..0x2FFB, 0x3000..0x303E, 0x3041..0x3096,
          0x3099..0x30FF, 0x3105..0x312D, 0x3131..0x318E,
          0x3190..0x31B7, 0x31C0..0x31E3, 0x31F0..0x321E,
          0x3220..0x3243, 0x3250..0x32FE, 0x3300..0x4DB5,
          0x4E00..0x9FC3, 0xA000..0xA48C, 0xA490..0xA4C6,
          0xAC00..0xD7A3, 0xF900..0xFA2D, 0xFA30..0xFA6A,
          0xFA70..0xFAD9, 0xFE10..0xFE19, 0xFE30..0xFE52,
          0xFE54..0xFE66, 0xFE68..0xFE6B, 0xFF01..0xFF60,
          0xFFE0..0xFFE6, 0x20000..0x2FFFD, 0x30000..0x3FFFD,
      ]

      AMBIGUOUS =
        [0x00A1..0x00A1, 0x00A4..0x00A4, 0x00A7..0x00A8,
          0x00AA..0x00AA, 0x00AD..0x00AE, 0x00B0..0x00B4,
          0x00B6..0x00BA, 0x00BC..0x00BF, 0x00C6..0x00C6,
          0x00D0..0x00D0, 0x00D7..0x00D8, 0x00DE..0x00E1,
          0x00E6..0x00E6, 0x00E8..0x00EA, 0x00EC..0x00ED,
          0x00F0..0x00F0, 0x00F2..0x00F3, 0x00F7..0x00FA,
          0x00FC..0x00FC, 0x00FE..0x00FE, 0x0101..0x0101,
          0x0111..0x0111, 0x0113..0x0113, 0x011B..0x011B,
          0x0126..0x0127, 0x012B..0x012B, 0x0131..0x0133,
          0x0138..0x0138, 0x013F..0x0142, 0x0144..0x0144,
          0x0148..0x014B, 0x014D..0x014D, 0x0152..0x0153,
          0x0166..0x0167, 0x016B..0x016B, 0x01CE..0x01CE,
          0x01D0..0x01D0, 0x01D2..0x01D2, 0x01D4..0x01D4,
          0x01D6..0x01D6, 0x01D8..0x01D8, 0x01DA..0x01DA,
          0x01DC..0x01DC, 0x0251..0x0251, 0x0261..0x0261,
          0x02C4..0x02C4, 0x02C7..0x02C7, 0x02C9..0x02CB,
          0x02CD..0x02CD, 0x02D0..0x02D0, 0x02D8..0x02DB,
          0x02DD..0x02DD, 0x02DF..0x02DF, 0x0300..0x036F,
          0x0391..0x03A1, 0x03A3..0x03A9, 0x03B1..0x03C1,
          0x03C3..0x03C9, 0x0401..0x0401, 0x0410..0x044F,
          0x0451..0x0451, 0x2010..0x2010, 0x2013..0x2016,
          0x2018..0x2019, 0x201C..0x201D, 0x2020..0x2022,
          0x2024..0x2027, 0x2030..0x2030, 0x2032..0x2033,
          0x2035..0x2035, 0x203B..0x203B, 0x203E..0x203E,
          0x2074..0x2074, 0x207F..0x207F, 0x2081..0x2084,
          0x20AC..0x20AC, 0x2103..0x2103, 0x2105..0x2105,
          0x2109..0x2109, 0x2113..0x2113, 0x2116..0x2116,
          0x2121..0x2122, 0x2126..0x2126, 0x212B..0x212B,
          0x2153..0x2154, 0x215B..0x215E, 0x2160..0x216B,
          0x2170..0x2179, 0x2190..0x2199, 0x21B8..0x21B9,
          0x21D2..0x21D2, 0x21D4..0x21D4, 0x21E7..0x21E7,
          0x2200..0x2200, 0x2202..0x2203, 0x2207..0x2208,
          0x220B..0x220B, 0x220F..0x220F, 0x2211..0x2211,
          0x2215..0x2215, 0x221A..0x221A, 0x221D..0x2220,
          0x2223..0x2223, 0x2225..0x2225, 0x2227..0x222C,
          0x222E..0x222E, 0x2234..0x2237, 0x223C..0x223D,
          0x2248..0x2248, 0x224C..0x224C, 0x2252..0x2252,
          0x2260..0x2261, 0x2264..0x2267, 0x226A..0x226B,
          0x226E..0x226F, 0x2282..0x2283, 0x2286..0x2287,
          0x2295..0x2295, 0x2299..0x2299, 0x22A5..0x22A5,
          0x22BF..0x22BF, 0x2312..0x2312, 0x2460..0x24E9,
          0x24EB..0x254B, 0x2550..0x2573, 0x2580..0x258F,
          0x2592..0x2595, 0x25A0..0x25A1, 0x25A3..0x25A9,
          0x25B2..0x25B3, 0x25B6..0x25B7, 0x25BC..0x25BD,
          0x25C0..0x25C1, 0x25C6..0x25C8, 0x25CB..0x25CB,
          0x25CE..0x25D1, 0x25E2..0x25E5, 0x25EF..0x25EF,
          0x2605..0x2606, 0x2609..0x2609, 0x260E..0x260F,
          0x2614..0x2615, 0x261C..0x261C, 0x261E..0x261E,
          0x2640..0x2640, 0x2642..0x2642, 0x2660..0x2661,
          0x2663..0x2665, 0x2667..0x266A, 0x266C..0x266D,
          0x266F..0x266F, 0x273D..0x273D, 0x2776..0x277F,
          0xE000..0xF8FF, 0xFE00..0xFE0F, 0xFFFD..0xFFFD,
          0xE0100..0xE01EF, 0xF0000..0xFFFFD, 0x100000..0x10FFFD,
      ]

      def wide_character?(character)
        binary_search_ranges(character, WIDE_CHARACTERS) or
        binary_search_ranges(character, AMBIGUOUS)
      end

      private
      def binary_search_ranges(character, ranges)
        if ranges.size.zero?
          false
        elsif ranges.size == 1
          ranges[0].include?(character)
        else
          half = ranges.size / 2
          range = ranges[half]
          if range.include?(character)
            true
          elsif character < range.begin
            binary_search_ranges(character, ranges[0...half])
          else
            binary_search_ranges(character, ranges[(half + 1)..-1])
          end
        end
      end
    end

    def initialize(line)
      @line = line
      @characters = @line.unpack("U*")
    end

    def [](*args)
      result = @characters[*args]
      if result.respond_to?(:pack)
        result.pack("U*")
      else
        result
      end
    end

    def each(&block)
      @characters.each(&block)
    end

    def size
      @characters.size
    end

    def to_s
      @line
    end

    def compute_width(start, _end)
      width = 0
      start.upto(_end - 1) do |i|
        if self.class.wide_character?(@characters[i])
          width += 2
        else
          width += 1
        end
      end
      width
    end
  end

  class ReadableDiffer < Differ
    def diff(options={})
      @result = []
      operations.each do |tag, from_start, from_end, to_start, to_end|
        case tag
        when :replace
          diff_lines(from_start, from_end, to_start, to_end)
        when :delete
          tag_deleted(@from[from_start...from_end])
        when :insert
          tag_inserted(@to[to_start...to_end])
        when :equal
          tag_equal(@from[from_start...from_end])
        else
          raise "unknown tag: #{tag}"
        end
      end
      @result
    end

    private
    def operations
      @operations ||= nil
      if @operations.nil?
        matcher = SequenceMatcher.new(@from, @to)
        @operations = matcher.operations
      end
      @operations
    end

    def default_ratio
      0.74
    end

    def cut_off_ratio
      0.75
    end

    def tag(mark, contents)
      contents.each do |content|
        @result << "#{mark}#{content}"
      end
    end

    def tag_deleted(contents)
      tag("- ", contents)
    end

    def tag_inserted(contents)
      tag("+ ", contents)
    end

    def tag_equal(contents)
      tag("  ", contents)
    end

    def tag_difference(contents)
      tag("? ", contents)
    end

    def find_diff_line_info(from_start, from_end, to_start, to_end)
      best_ratio = default_ratio
      from_equal_index = to_equal_index = nil
      from_best_index = to_best_index = nil

      to_start.upto(to_end - 1) do |to_index|
        from_start.upto(from_end - 1) do |from_index|
          if @from[from_index] == @to[to_index]
            from_equal_index ||= from_index
            to_equal_index ||= to_index
            next
          end

          matcher = SequenceMatcher.new(@from[from_index], @to[to_index],
                                        &method(:space_character?))
          if matcher.ratio > best_ratio
            best_ratio = matcher.ratio
            from_best_index = from_index
            to_best_index = to_index
          end
        end
      end

      [best_ratio,
        from_equal_index, to_equal_index,
        from_best_index,  to_best_index]
    end

    def diff_lines(from_start, from_end, to_start, to_end)
      info = find_diff_line_info(from_start, from_end, to_start, to_end)
      best_ratio, from_equal_index, to_equal_index, *info = info
      from_best_index, to_best_index = info
      from_best_index ||= from_start
      to_best_index ||= to_start

      if best_ratio < cut_off_ratio
        if from_equal_index.nil?
          if to_end - to_start < from_end - from_start
            tag_inserted(@to[to_start...to_end])
            tag_deleted(@from[from_start...from_end])
          else
            tag_deleted(@from[from_start...from_end])
            tag_inserted(@to[to_start...to_end])
          end
          return
        end
        from_best_index = from_equal_index
        to_best_index = to_equal_index
        best_ratio = 1.0
      end

      _diff_lines(from_start, from_best_index, to_start, to_best_index)
      diff_line(@from[from_best_index], @to[to_best_index])
      _diff_lines(from_best_index + 1, from_end, to_best_index + 1, to_end)
    end

    def _diff_lines(from_start, from_end, to_start, to_end)
      if from_start < from_end
        if to_start < to_end
          diff_lines(from_start, from_end, to_start, to_end)
        else
          tag_deleted(@from[from_start...from_end])
        end
      else
        tag_inserted(@to[to_start...to_end])
      end
    end

    def line_operations(from_line, to_line)
      if !from_line.respond_to?(:force_encoding) and $KCODE == "UTF8"
        from_line = UTF8Line.new(from_line)
        to_line = UTF8Line.new(to_line)
      end
      matcher = SequenceMatcher.new(from_line, to_line,
                                    &method(:space_character?))
      [from_line, to_line, matcher.operations]
    end

    def compute_width(line, start, _end)
      if line.respond_to?(:encoding) and
        Encoding.compatible?(Encoding::UTF_8, line.encoding)
        utf8_line = line[start..._end].encode(Encoding::UTF_8)
        width = 0
        utf8_line.each_codepoint do |unicode_codepoint|
          if UTF8Line.wide_character?(unicode_codepoint)
            width += 2
          else
            width += 1
          end
        end
        width
      elsif line.is_a?(UTF8Line)
        line.compute_width(start, _end)
      else
        _end - start
      end
    end

    def diff_line(from_line, to_line)
      from_tags = ""
      to_tags = ""
      from_line, to_line, _operations = line_operations(from_line, to_line)
      _operations.each do |tag, from_start, from_end, to_start, to_end|
        from_width = compute_width(from_line, from_start, from_end)
        to_width = compute_width(to_line, to_start, to_end)
        case tag
        when :replace
          from_tags << "^" * from_width
          to_tags << "^" * to_width
        when :delete
          from_tags << "-" * from_width
        when :insert
          to_tags << "+" * to_width
        when :equal
          from_tags << " " * from_width
          to_tags << " " * to_width
        else
          raise "unknown tag: #{tag}"
        end
      end
      format_diff_point(from_line, to_line, from_tags, to_tags)
    end

    def format_diff_point(from_line, to_line, from_tags, to_tags)
      common = [n_leading_characters(from_line, ?\t),
        n_leading_characters(to_line, ?\t)].min
      common = [common,
        n_leading_characters(from_tags[0, common], " "[0])].min
      from_tags = from_tags[common..-1].rstrip
      to_tags = to_tags[common..-1].rstrip

      tag_deleted([from_line])
      unless from_tags.empty?
        tag_difference(["#{"\t" * common}#{from_tags}"])
      end
      tag_inserted([to_line])
      unless to_tags.empty?
        tag_difference(["#{"\t" * common}#{to_tags}"])
      end
    end

    def n_leading_characters(string, character)
      n = 0
      while string[n] == character
        n += 1
      end
      n
    end

    def space_character?(character)
      [" "[0], "\t"[0]].include?(character)
    end
  end

  class UnifiedDiffer < Differ
    def diff(options={})
      groups = SequenceMatcher.new(@from, @to).grouped_operations
      return [] if groups.empty?
      return [] if same_content?(groups)

      show_context = options[:show_context]
      show_context = true if show_context.nil?
      result = ["--- #{options[:from_label]}".rstrip,
        "+++ #{options[:to_label]}".rstrip]
        groups.each do |operations|
          result << format_summary(operations, show_context)
          operations.each do |args|
            operation_tag, from_start, from_end, to_start, to_end = args
            case operation_tag
            when :replace
              result.concat(tag("-", @from[from_start...from_end]))
              result.concat(tag("+", @to[to_start...to_end]))
            when :delete
              result.concat(tag("-", @from[from_start...from_end]))
            when :insert
              result.concat(tag("+", @to[to_start...to_end]))
            when :equal
              result.concat(tag(" ", @from[from_start...from_end]))
            end
          end
        end
        result
    end

    private
    def same_content?(groups)
      return false if groups.size != 1
      group = groups[0]
      return false if group.size != 1
      tag, from_start, from_end, to_start, to_end = group[0]

      tag == :equal and [from_start, from_end] == [to_start, to_end]
    end

    def format_summary(operations, show_context)
      _, first_from_start, _, first_to_start, _ = operations[0]
      _, _, last_from_end, _, last_to_end = operations[-1]
      summary = "@@ -%d,%d +%d,%d @@" % [first_from_start + 1,
        last_from_end - first_from_start,
        first_to_start + 1,
        last_to_end - first_to_start,]
      if show_context
        interesting_line = find_interesting_line(first_from_start,
                                                 first_to_start,
                                                 :define_line?)
        summary << " #{interesting_line}" if interesting_line
      end
      summary
    end

    def find_interesting_line(from_start, to_start, predicate)
      from_index = from_start
      to_index = to_start
      while from_index >= 0 or to_index >= 0
        [@from[from_index], @to[to_index]].each do |line|
          return line if line and send(predicate, line)
        end

        from_index -= 1
        to_index -= 1
      end
      nil
    end

    def define_line?(line)
      /\A(?:[_a-zA-Z$]|\s*(?:class|module|def)\b)/ =~ line
    end
  end

  module_function
  def need_fold?(diff)
    /^[-+].{79}/ =~ diff
  end

  def fold(string)
    string.split(/\r?\n/).collect do |line|
      line.gsub(/(.{78})/, "\\1\n")
    end.join("\n")
  end

  def folded_readable(from, to, options={})
    readable(fold(from), fold(to), options)
  end

  def readable(from, to, options={})
    diff(ReadableDiffer, from, to, options)
  end

  def unified(from, to, options={})
    diff(UnifiedDiffer, from, to, options)
  end

  def diff(differ_class, from, to, options={})
    if from.respond_to?(:valid_encoding?) and not from.valid_encoding?
      from = from.dup.force_encoding("ASCII-8BIT")
    end
    if to.respond_to?(:valid_encoding?) and not to.valid_encoding?
      to = to.dup.force_encoding("ASCII-8BIT")
    end
    differ = differ_class.new(from.split(/\r?\n/), to.split(/\r?\n/))
    lines = differ.diff(options)
    if Object.const_defined?(:EncodingError)
      begin
        lines.join("\n")
      rescue EncodingError
        lines.collect {|line| line.force_encoding("ASCII-8BIT")}.join("\n")
      end
    else
      lines.join("\n")
    end
  end
end