Implemented new HTML diffing class to produce an HTMLized diff of two HTML

strings. Not perfect yet. For instance, if string a contains "foo bar baz" and
string b contains "foo <i>bar</i> baz", the resulting output does not show
"bar" in strikeout and "<i>bar</i>" in green, as one might expect.

controller/Html_differ.py

@@ -0,0 +1,87 @@
+import re
+from difflib import SequenceMatcher
+from htmllib import HTMLParser
+from formatter import AbstractFormatter, NullWriter
+from xml.sax.saxutils import quoteattr
+
+
+class Html_differ( HTMLParser ):
+  """
+  Generates an HTML diff for two HTML strings. It assumed that the input HTML is already cleaned.
+  """
+  def __init__( self ):
+    HTMLParser.__init__( self, AbstractFormatter( NullWriter() ) )
+    self.result = []
+    self.requires_no_close = [ 'img', 'br' ]
+
+  WORD_AND_WHITESPACE_PATTERN = re.compile( "\S*\s*" )
+
+  def handle_data( self, data ):
+    self.result.extend( self.WORD_AND_WHITESPACE_PATTERN.findall( data ) )
+
+  def handle_charref( self, ref ):
+    self.result.append( '&#%s;' % ref )
+
+  def handle_entityref( self, ref ):
+    self.result.append( '&%s;' % ref )
+
+  def handle_comment( self, comment ):
+    pass # ignore comments
+
+  def handle_starttag( self, tag, method, attrs ):
+    self.result.append( self.get_starttag_text() )
+      
+  def handle_endtag( self, tag, attrs ):
+    if tag not in self.requires_no_close:
+      bracketed = "</%s>" % tag
+      self.result.append( bracketed )
+      
+  def unknown_starttag( self, tag, attr ):
+    self.handle_starttag( tag, None, attr )
+
+  def unknown_endtag( self, tag ):
+    self.handle_endtag( tag, None )
+
+  # used to replace, for instance, "<br/>" with "<br />"
+  INVALID_TAG_PATTERN = re.compile( "(\S)/>" )
+  INVALID_TAG_FIX = "\\1 />"
+
+  def diff( self, html_a, html_b ):
+    """
+    Return a composite HTML diff of the given HTML input strings.
+    """
+    # parse html_a into a list
+    self.reset()
+    self.result = []
+    html_a = self.INVALID_TAG_PATTERN.sub( self.INVALID_TAG_FIX, html_a )
+    self.feed( html_a )
+    a = [ x for x in self.result if x != "" ]
+
+    # parse html_b into a list
+    self.reset()
+    self.result = []
+    html_b = self.INVALID_TAG_PATTERN.sub( self.INVALID_TAG_FIX, html_b )
+    self.feed( html_b )
+    b = [ x for x in self.result if x != "" ]
+
+    return self.__diff_lists( a, b )
+
+  def __diff_lists( self, a, b ):
+    matcher = SequenceMatcher( None, a, b )
+    result = []
+
+    # inspired by http://www.aaronsw.com/2002/diff/
+    for ( tag, i1, i2, j1, j2 ) in matcher.get_opcodes():
+      if tag == "replace":
+        result.append(
+          '<del class="diff modified">' + ''.join( a[ i1:i2 ] ) + '</del>' + \
+          '<ins class="diff modified">' + ''.join( b[ j1:j2 ] ) + '</ins>'
+        )
+      elif tag == "delete":
+        result.append( '<del class="diff">' + ''.join( a[ i1:i2 ] ) + '</del>' )
+      elif tag == "insert":
+        result.append( '<ins class="diff">' + ''.join( b[ j1:j2 ] ) + '</ins>' )
+      elif tag == "equal":
+        result.append( ''.join( b[ j1:j2 ] ) )
+
+    return "".join( result )