html_diff: Show context lines at the beginning/end of a file correctly.

third_party/WebKit/Tools/Scripts/webkitpy/common/html_diff.py

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Utility for outputting a HTML diff of two multi-line strings.
 
 The main purpose of this utility is to show the difference between
 text baselines (-expected.txt files) and actual text results.
 
 Note, in the standard library module difflib, there is also a HtmlDiff class,
@@ skipping 32 matching lines @@
 
     def __init__(self):
         self.a_line_no = None
         self.b_line_no = None
         self.a_lines_len = None
 
     def generate_tbody(self, a_lines, b_lines):
         self.a_line_no = 0
         self.b_line_no = 0
         self.a_lines_len = len(a_lines)
+        self.b_lines_len = len(b_lines)
         matcher = difflib.SequenceMatcher(None, a_lines, b_lines)
         output = []
         for tag, a_start, a_end, b_start, b_end in matcher.get_opcodes():
             output.append(self._format_chunk(tag, a_lines[a_start:a_end], b_lines[b_start:b_end]))
         return ''.join(output)
 
     def _format_chunk(self, tag, a_chunk, b_chunk):
         if tag == 'delete':
             return self._format_delete(a_chunk)
         if tag == 'insert':
             return self._format_insert(b_chunk)
         if tag == 'replace':
             return self._format_delete(a_chunk) + self._format_insert(b_chunk)
         assert tag == 'equal'
         return self._format_equal(a_chunk)
 
     def _format_equal(self, common_chunk):
         output = ''
         if len(common_chunk) <= 7:
             for line in common_chunk:
                 output += self._format_equal_line(line)
         else:
             # Do not show context lines at the beginning of the file.
-            if self.a_line_no == 0:
+            if self.a_line_no == 0 and self.b_line_no == 0:
                 self.a_line_no += 3
                 self.b_line_no += 3
             else:
                 for line in common_chunk[0:3]:
                     output += self._format_equal_line(line)
             self.a_line_no += len(common_chunk) - 6
             self.b_line_no += len(common_chunk) - 6
             output += '<tr><td colspan=3>\n\n</tr>'
             # Do not show context lines at the end of the file.
-            if self.a_line_no + 3 != self.a_lines_len:
+            if self.a_line_no + 3 != self.a_lines_len or self.b_line_no + 3 != self.b_lines_len:
                 for line in common_chunk[len(common_chunk) - 3:len(common_chunk)]:
                     output += self._format_equal_line(line)
         return output
 
     def _format_equal_line(self, line):
         self.a_line_no += 1
         self.b_line_no += 1
         return '<tr><th>%d<th>%d<td>%s</tr>' % (self.a_line_no, self.b_line_no, cgi.escape(line))
 
     def _format_insert(self, chunk):
         output = ''
         for line in chunk:
             self.b_line_no += 1
             output += '<tr><th><th>%d<td class="add">%s</tr>' % (self.b_line_no, cgi.escape(line))
         return output
 
     def _format_delete(self, chunk):
         output = ''
         for line in chunk:
             self.a_line_no += 1
             output += '<tr><th>%d<th><td class="del">%s</tr>' % (self.a_line_no, cgi.escape(line))
         return output