ryanmce created this revision.
Herald added a subscriber: mercurial-devel.
Herald added a reviewer: hg-reviewers.

REVISION SUMMARY
  Let's switch to xdiff for its better diff quality and faster performance!
  
  bdiff is still used as a fallback for cases xdiff isn't built, or the pure
  Python version.

TEST PLAN
  Added the "patience" test case mentioned in previous commit. It fails
  with a huge diff output before this change.
  
  I expected some annotate/diff output changes due to diff blocks being
  shifted around. Let's see what sandcastle says. I'll make adjustments
  accordingly.

REPOSITORY
  rHG Mercurial

REVISION DETAIL
  https://phab.mercurial-scm.org/D2577

AFFECTED FILES
  mercurial/mdiff.py
  tests/hghave.py
  tests/test-diff-antipatience.t

CHANGE DETAILS

diff --git a/tests/test-diff-antipatience.t b/tests/test-diff-antipatience.t
new file mode 100644
--- /dev/null
+++ b/tests/test-diff-antipatience.t
@@ -0,0 +1,41 @@
+#require xdiff
+
+Test case that makes use of the weakness of patience diff algorithm
+
+  $ hg init
+  >>> open('a', 'w').write('\n'.join(list('a' + 'x' * 300 + 'u' + 'x' * 700 + 
'a\n')))
+  $ hg commit -m 1 -A a
+  >>> open('a', 'w').write('\n'.join(list('b' + 'x' * 700 + 'u' + 'x' * 300 + 
'b\n')))
+  $ hg diff
+  diff -r 6c45a8fe8cb6 a
+  --- a/a      Thu Jan 01 00:00:00 1970 +0000
+  +++ b/a      Thu Jan 01 00:00:00 1970 +0000
+  @@ -1,4 +1,4 @@
+  -a
+  +b
+   x
+   x
+   x
+  @@ -299,7 +299,6 @@
+   x
+   x
+   x
+  -u
+   x
+   x
+   x
+  @@ -700,6 +699,7 @@
+   x
+   x
+   x
+  +u
+   x
+   x
+   x
+  @@ -1000,5 +1000,5 @@
+   x
+   x
+   x
+  -a
+  +b
+   
diff --git a/tests/hghave.py b/tests/hghave.py
--- a/tests/hghave.py
+++ b/tests/hghave.py
@@ -708,3 +708,11 @@
         # libfuzzer is new in clang 6
         return int(mat.group(1)) > 5
     return False
+
+@check("xdiff", "xdiff algorithm")
+def has_xdiff():
+    try:
+        from mercurial.cext import xdiff
+        return xdiff.blocks('', '') == [(0, 0, 0, 0)]
+    except ImportError:
+        return False
diff --git a/mercurial/mdiff.py b/mercurial/mdiff.py
--- a/mercurial/mdiff.py
+++ b/mercurial/mdiff.py
@@ -25,13 +25,18 @@
 bdiff = policy.importmod(r'bdiff')
 mpatch = policy.importmod(r'mpatch')
 
-blocks = bdiff.blocks
 fixws = bdiff.fixws
 patches = mpatch.patches
 patchedsize = mpatch.patchedsize
 textdiff = bdiff.bdiff
 splitnewlines = bdiff.splitnewlines
 
+try:
+    from .cext import xdiff
+    blocks = xdiff.blocks
+except ImportError:
+    blocks = bdiff.blocks
+
 class diffopts(object):
     '''context is the number of context lines
     text treats all files as text
@@ -200,7 +205,7 @@
     if opts.ignorews or opts.ignorewsamount or opts.ignorewseol:
         text1 = wsclean(opts, text1, False)
         text2 = wsclean(opts, text2, False)
-    diff = bdiff.blocks(text1, text2)
+    diff = blocks(text1, text2)
     for i, s1 in enumerate(diff):
         # The first match is special.
         # we've either found a match starting at line 0 or a match later
@@ -508,7 +513,7 @@
 
 # similar to difflib.SequenceMatcher.get_matching_blocks
 def get_matching_blocks(a, b):
-    return [(d[0], d[2], d[1] - d[0]) for d in bdiff.blocks(a, b)]
+    return [(d[0], d[2], d[1] - d[0]) for d in blocks(a, b)]
 
 def trivialdiffheader(length):
     return struct.pack(">lll", 0, 0, length) if length else ''



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