# HG changeset patch
# User Durham Goode <durham@fb.com>
# Date 1446778600 28800
# Node ID 1cbf144fd8a170b2a5e2a7f68357368c5793d7a4
# Parent  ab798d1a230f58d38b4d541527fda4e47aa8eae0
manifest: skip fastdelta if the change is large

In large repos, the existing manifest fastdelta computation (which performs a
bisect on the raw manifest for every file that is changing), is excessively
slow. This patch makes fastdelta fallback to the normal string delta algorithm
if the number of changes is large.

On a large repo with a commit of 8000 files, this reduces the commit time by 7
seconds (fastdelta goes from 8 seconds to 1).

I tested this change by modifying the function to compare the old and the new
values and running the test suite. The only difference is that the pure
text-diff algorithm sometimes produces smaller (but functionaly identical)
deltatexts than the bisect algorithm.

diff -r ab798d1a230f -r 1cbf144fd8a1 mercurial/manifest.py
--- a/mercurial/manifest.py	Wed Nov 04 23:44:51 2015 -0800
+++ b/mercurial/manifest.py	Thu Nov 05 18:56:40 2015 -0800
@@ -334,36 +334,44 @@
         # zero copy representation of base as a buffer
         addbuf = util.buffer(base)
 
-        # start with a readonly loop that finds the offset of
-        # each line and creates the deltas
-        for f, todelete in changes:
-            # bs will either be the index of the item or the insert point
-            start, end = _msearch(addbuf, f, start)
-            if not todelete:
-                h, fl = self._lm[f]
-                l = "%s\0%s%s\n" % (f, revlog.hex(h), fl)
-            else:
-                if start == end:
-                    # item we want to delete was not found, error out
-                    raise AssertionError(
-                            _("failed to remove %s from manifest") % f)
-                l = ""
-            if dstart is not None and dstart <= start and dend >= start:
-                if dend < end:
+        changes = list(changes)
+        if len(changes) < 1000:
+            # start with a readonly loop that finds the offset of
+            # each line and creates the deltas
+            for f, todelete in changes:
+                # bs will either be the index of the item or the insert point
+                start, end = _msearch(addbuf, f, start)
+                if not todelete:
+                    h, fl = self._lm[f]
+                    l = "%s\0%s%s\n" % (f, revlog.hex(h), fl)
+                else:
+                    if start == end:
+                        # item we want to delete was not found, error out
+                        raise AssertionError(
+                                _("failed to remove %s from manifest") % f)
+                    l = ""
+                if dstart is not None and dstart <= start and dend >= start:
+                    if dend < end:
+                        dend = end
+                    if l:
+                        dline.append(l)
+                else:
+                    if dstart is not None:
+                        delta.append([dstart, dend, "".join(dline)])
+                    dstart = start
                     dend = end
-                if l:
-                    dline.append(l)
-            else:
-                if dstart is not None:
-                    delta.append([dstart, dend, "".join(dline)])
-                dstart = start
-                dend = end
-                dline = [l]
+                    dline = [l]
 
-        if dstart is not None:
-            delta.append([dstart, dend, "".join(dline)])
-        # apply the delta to the base, and get a delta for addrevision
-        deltatext, arraytext = _addlistdelta(base, delta)
+            if dstart is not None:
+                delta.append([dstart, dend, "".join(dline)])
+            # apply the delta to the base, and get a delta for addrevision
+            deltatext, arraytext = _addlistdelta(base, delta)
+        else:
+            # For large changes, it's much cheaper to just build the text and
+            # diff it.
+            arraytext = array.array('c', self.text())
+            deltatext = mdiff.textdiff(base, arraytext)
+
         return arraytext, deltatext
 
 def _msearch(m, s, lo=0, hi=None):