--- a/Allura/allura/tests/unit/test_utils.py
+++ b/Allura/allura/tests/unit/test_utils.py
@@ -3,6 +3,8 @@
 
 from alluratest.controller import setup_unit_test
 from allura.lib.utils import generate_code_stats, chunked_list
+from allura.lib.diff import HtmlSideBySideDiff
+
 
 class TestCodeStats(unittest.TestCase):
 
@@ -35,3 +37,120 @@
         self.assertEqual(len(chunks), 4)
         self.assertEqual(len(chunks[0]), 3)
         self.assertEqual([el for sublist in chunks for el in sublist], l)
+
+
+class TestHtmlSideBySideDiff(unittest.TestCase):
+
+    def setUp(self):
+        self.diff = HtmlSideBySideDiff()
+
+    def test_render_change(self):
+        html = self.diff._render_change(
+                'aline', 'aline <span class="diff-add">bline</span>',
+                1, 2,'aclass', 'bclass')
+        expected = '''
+<tr>
+  <td class="lineno">1</td>
+  <td class="aclass"><pre>aline</pre></td>
+  <td class="lineno">2</td>
+  <td class="bclass"><pre>aline <span class="diff-add">bline</span></pre></td>
+</tr>'''.strip()
+        self.assertEqual(html, expected)
+
+    def test_render_change_default_args(self):
+        html = self.diff._render_change('aline', 'bline')
+        expected = '''
+<tr>
+  <td class="lineno"></td>
+  <td><pre>aline</pre></td>
+  <td class="lineno"></td>
+  <td><pre>bline</pre></td>
+</tr>'''.strip()
+        self.assertEqual(html, expected)
+
+    def test_preprocess(self):
+        d = self.diff
+        self.assertEquals(d._preprocess(None), None)
+        self.assertEquals(d._preprocess('<br>&nbsp;'), '&lt;br&gt;&amp;nbsp;')
+        self.assertEquals(d._preprocess('\ttabbed'), '    tabbed')
+        # test non default tab size
+        d = HtmlSideBySideDiff(2)
+        self.assertEquals(d._preprocess('\ttabbed'), '  tabbed')
+
+    def test_replace_marks(self):
+        line, flag = self.diff._replace_marks('\0+line added\1')
+        self.assertEquals(line, 'line added')
+        self.assertEquals(flag, 'diff-add')
+        line, flag = self.diff._replace_marks('\0-line removed\1')
+        self.assertEquals(line, 'line removed')
+        self.assertEquals(flag, 'diff-rem')
+        line, flag = self.diff._replace_marks('\0^line changed\1')
+        self.assertEquals(line, '<span class="diff-chg">line changed</span>')
+        self.assertEquals(flag, 'diff-chg')
+        line, flag = self.diff._replace_marks('chunk \0+add\1ed')
+        self.assertEquals(line, 'chunk <span class="diff-add">add</span>ed')
+        self.assertEquals(flag, 'diff-chg')
+        line, flag = self.diff._replace_marks('chunk \0-remov\1ed')
+        self.assertEquals(line, 'chunk <span class="diff-rem">remov</span>ed')
+        self.assertEquals(flag, 'diff-chg')
+        line, flag = self.diff._replace_marks('chunk \0^chang\1ed')
+        self.assertEquals(line, 'chunk <span class="diff-chg">chang</span>ed')
+        self.assertEquals(flag, 'diff-chg')
+
+    def test_make_line(self):
+        # context separation
+        d = (None, None, None)
+        expected = '''
+<tr>
+  <td class="lineno"></td>
+  <td class="diff-gap"><pre>...</pre></td>
+  <td class="lineno"></td>
+  <td class="diff-gap"><pre>...</pre></td>
+</tr>'''.strip()
+        self.assertEquals(self.diff._make_line(d), expected)
+        # no change
+        d = ((1, 'aline'), (1, 'aline'), False)
+        expected = '''
+<tr>
+  <td class="lineno">1</td>
+  <td><pre>aline</pre></td>
+  <td class="lineno">1</td>
+  <td><pre>aline</pre></td>
+</tr>'''.strip()
+        self.assertEquals(self.diff._make_line(d), expected)
+        # has change
+        d = ((1, '\0^a\1line'), (1, '\0^b\1line'), True)
+        expected = '''
+<tr>
+  <td class="lineno">1</td>
+  <td class="diff-chg"><pre><span class="diff-chg">a</span>line</pre></td>
+  <td class="lineno">1</td>
+  <td class="diff-chg"><pre><span class="diff-chg">b</span>line</pre></td>
+</tr>'''.strip()
+        self.assertEquals(self.diff._make_line(d), expected)
+
+    def test_make_table(self):
+        a = 'line 1\nline 2'.split('\n')
+        b = 'changed line 1\nchanged line 2'.split('\n')
+        expected = '''
+<table class="side-by-side-diff">
+  <thead>
+    <th colspan="2">file a</th>
+    <th colspan="2">file b</th>
+  </thead>
+<tr>
+  <td class="lineno">1</td>
+  <td class="diff-rem"><pre>line 1</pre></td>
+  <td class="lineno">1</td>
+  <td class="diff-add"><pre>changed line 1</pre></td>
+</tr>
+<tr>
+  <td class="lineno">2</td>
+  <td class="diff-rem"><pre>line 2</pre></td>
+  <td class="lineno">2</td>
+  <td class="diff-add"><pre>changed line 2</pre></td>
+</tr>
+</table>
+'''.strip()
+        html = self.diff.make_table(a, b, 'file a', 'file b')
+        self.assertEquals(html, expected)