From 01dba7f8ce5a4e5ab1a1c3b45a4ada4c7dbbf425 Mon Sep 17 00:00:00 2001
From: Dave Halter <davidhalter88@gmail.com>
Date: Sun, 13 Jan 2019 22:38:53 +0100
Subject: [PATCH] Fix diff parser: Need to calculate the prefix for the diff
 tokenizer better

---
 parso/python/diff.py     |  4 ++++
 test/test_diff_parser.py | 22 ++++++++++++++++++++++
 2 files changed, 26 insertions(+)

diff --git a/parso/python/diff.py b/parso/python/diff.py
index d9650b7..fc499bc 100644
--- a/parso/python/diff.py
+++ b/parso/python/diff.py
@@ -388,6 +388,10 @@ class DiffParser(object):
                     typ, string, start_pos, prefix = next(tokens)
                     if '\n' in prefix or '\r' in prefix:
                         prefix = re.sub(r'[^\n\r]+\Z', '', prefix)
+                    else:
+                        assert start_pos[1] >= len(prefix), repr(prefix)
+                        if start_pos[1] - len(prefix) == 0:
+                            prefix = ''
                     yield PythonToken(
                         PythonTokenTypes.ENDMARKER, '',
                         (start_pos[0] + line_offset, 0),
diff --git a/test/test_diff_parser.py b/test/test_diff_parser.py
index 924af01..a5eb4b9 100644
--- a/test/test_diff_parser.py
+++ b/test/test_diff_parser.py
@@ -1012,3 +1012,25 @@ def test_special_no_newline_ending(differ):
     differ.initialize(code1)
     differ.parse(code2, copies=1, parsers=1, expect_error_leaves=True)
     differ.parse(code1, copies=1, parsers=0)
+
+
+def test_random_character_insertion(differ):
+    code1 = dedent('''\
+            def create(self):
+                1
+                if self.path is not None:
+                    return
+                # 3
+                # 4
+        ''')
+    code2 = dedent('''\
+            def create(self):
+                1
+                if 2:
+             x       return
+                # 3
+                # 4
+        ''')
+    differ.initialize(code1)
+    differ.parse(code2, copies=1, parsers=3, expect_error_leaves=True)
+    differ.parse(code1, copies=1, parsers=1)