discourse/lib/onpdiff.rb

# Use "An O(NP) Sequence Comparison Algorithm" as described by Sun Wu, Udi Manber and Gene Myers
# in http://www.itu.dk/stud/speciale/bepjea/xwebtex/litt/an-onp-sequence-comparison-algorithm.pdf
class ONPDiff

  def initialize(a, b)
    @a, @b = a, b
    @m, @n = a.length, b.length
    @backtrack = []
    if @reverse = @m > @n
      @a, @b = @b, @a
      @m, @n = @n, @m
    end
    @offset = @m + 1
    @delta = @n - @m
  end

  def diff
    @diff ||= build_edit_script(compose)
  end

  def short_diff
    @short_diff ||= build_short_edit_script(compose)
  end

  private

  def compose
    return @shortest_path if @shortest_path

    size = @m + @n + 3
    fp = Array.new(size) { |i| -1 }
    @path = Array.new(size) { |i| -1 }
    p = -1

    begin
      p += 1

      k = -p
      while k <= @delta - 1
        fp[k + @offset] = snake(k, fp[k - 1 + @offset] + 1, fp[k + 1 + @offset])
        k += 1
      end

      k = @delta + p
      while k >= @delta + 1
        fp[k + @offset] = snake(k, fp[k - 1 + @offset] + 1, fp[k + 1 + @offset])
        k -= 1
      end

      fp[@delta + @offset] = snake(@delta, fp[@delta - 1 + @offset] + 1, fp[@delta + 1 + @offset])

    end until fp[@delta + @offset] == @n

    r = @path[@delta + @offset]

    @shortest_path = []
    while r != -1
      @shortest_path << [@backtrack[r][0], @backtrack[r][1]]
      r = @backtrack[r][2]
    end

    @shortest_path
  end

  def snake(k, p, pp)
    r = p > pp ? @path[k - 1 + @offset] : @path[k + 1 + @offset]
    y = [p, pp].max
    x = y - k

    while x < @m && y < @n && @a[x] == @b[y]
      x += 1
      y += 1
    end

    @path[k + @offset] = @backtrack.length
    @backtrack << [x, y, r]

    y
  end

  def build_edit_script(shortest_path)
    ses = []
    x, y = 1, 1
    px, py = 0, 0
    i = shortest_path.length - 1
    while i >= 0
      while px < shortest_path[i][0] || py < shortest_path[i][1]
        if shortest_path[i][1] - shortest_path[i][0] > py - px
          t = @reverse ? :delete : :add
          ses << [@b[py], t]
          y += 1
          py += 1
        elsif shortest_path[i][1] - shortest_path[i][0] < py - px
          t = @reverse ? :add : :delete
          ses << [@a[px], t]
          x += 1
          px += 1
        else
          ses << [@a[px], :common]
          x += 1
          y += 1
          px += 1
          py += 1
        end
      end
      i -= 1
    end
    ses
  end

  def build_short_edit_script(shortest_path)
    ses = []
    x, y = 1, 1
    px, py = 0, 0
    i = shortest_path.length - 1
    while i >= 0
      while px < shortest_path[i][0] || py < shortest_path[i][1]
        if shortest_path[i][1] - shortest_path[i][0] > py - px
          t = @reverse ? :delete : :add
          if ses.length > 0 && ses[-1][1] == t
            ses[-1][0] << @b[py]
          else
            ses << [@b[py], t]
          end
          y += 1
          py += 1
        elsif shortest_path[i][1] - shortest_path[i][0] < py - px
          t = @reverse ? :add : :delete
          if ses.length > 0 && ses[-1][1] == t
            ses[-1][0] << @a[px]
          else
            ses << [@a[px], t]
          end
          x += 1
          px += 1
        else
          if ses.length > 0 && ses[-1][1] == :common
            ses[-1][0] << @a[px]
          else
            ses << [@a[px], :common]
          end
          x += 1
          y += 1
          px += 1
          py += 1
        end
      end
      i -= 1
    end
    ses
  end

end