better revision history

lib/onpdiff.rb

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+# 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_diff_script(compose)
+  end
+
+  def short_diff
+    @short_diff ||= build_short_diff_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_diff_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_diff_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