commit: increase perf by building a new addlist instead of editing the old one

When commiting to a repo with lots of files (>170000),
manifest.py:addlistdelta takes some time because it's editing a large
array many times. Changing it to build a new array instead of editing
the old one saves around 0.04 seconds on a 1.64 second commit. A 2.5%
gain.

The gain here is pretty minor, but it was blatantly at the top of the
profiler report and the fix is straight forward.

I tested it by comparing the arrays produced by the new and old logic
while running all of the tests.

mercurial/manifest.py

@@ -1,204 +1,213 b''
 # manifest.py - manifest revision class for mercurial
 #
 # Copyright 2005-2007 Matt Mackall <[e-mail]>
 #
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
 from i18n import _
 import mdiff, parsers, error, revlog, util
 import array, struct
 
 class manifestdict(dict):
     def __init__(self, mapping=None, flags=None):
         if mapping is None:
             mapping = {}
         if flags is None:
             flags = {}
         dict.__init__(self, mapping)
         self._flags = flags
     def flags(self, f):
         return self._flags.get(f, "")
     def withflags(self):
         return set(self._flags.keys())
     def set(self, f, flags):
         self._flags[f] = flags
     def copy(self):
         return manifestdict(self, dict.copy(self._flags))
 
 class manifest(revlog.revlog):
     def __init__(self, opener):
         self._mancache = None
         revlog.revlog.__init__(self, opener, "00manifest.i")
 
     def parse(self, lines):
         mfdict = manifestdict()
         parsers.parse_manifest(mfdict, mfdict._flags, lines)
         return mfdict
 
     def readdelta(self, node):
         r = self.rev(node)
         return self.parse(mdiff.patchtext(self.revdiff(self.deltaparent(r), r)))
 
     def readfast(self, node):
         '''use the faster of readdelta or read'''
         r = self.rev(node)
         deltaparent = self.deltaparent(r)
         if deltaparent != revlog.nullrev and deltaparent in self.parentrevs(r):
             return self.readdelta(node)
         return self.read(node)
 
     def read(self, node):
         if node == revlog.nullid:
             return manifestdict() # don't upset local cache
         if self._mancache and self._mancache[0] == node:
             return self._mancache[1]
         text = self.revision(node)
         arraytext = array.array('c', text)
         mapping = self.parse(text)
         self._mancache = (node, mapping, arraytext)
         return mapping
 
     def _search(self, m, s, lo=0, hi=None):
         '''return a tuple (start, end) that says where to find s within m.
 
         If the string is found m[start:end] are the line containing
         that string.  If start == end the string was not found and
         they indicate the proper sorted insertion point.
 
         m should be a buffer or a string
         s is a string'''
         def advance(i, c):
             while i < lenm and m[i] != c:
                 i += 1
             return i
         if not s:
             return (lo, lo)
         lenm = len(m)
         if not hi:
             hi = lenm
         while lo < hi:
             mid = (lo + hi) // 2
             start = mid
             while start > 0 and m[start - 1] != '\n':
                 start -= 1
             end = advance(start, '\0')
             if m[start:end] < s:
                 # we know that after the null there are 40 bytes of sha1
                 # this translates to the bisect lo = mid + 1
                 lo = advance(end + 40, '\n') + 1
             else:
                 # this translates to the bisect hi = mid
                 hi = start
         end = advance(lo, '\0')
         found = m[lo:end]
         if s == found:
             # we know that after the null there are 40 bytes of sha1
             end = advance(end + 40, '\n')
             return (lo, end + 1)
         else:
             return (lo, lo)
 
     def find(self, node, f):
         '''look up entry for a single file efficiently.
         return (node, flags) pair if found, (None, None) if not.'''
         if self._mancache and self._mancache[0] == node:
             return self._mancache[1].get(f), self._mancache[1].flags(f)
         text = self.revision(node)
         start, end = self._search(text, f)
         if start == end:
             return None, None
         l = text[start:end]
         f, n = l.split('\0')
         return revlog.bin(n[:40]), n[40:-1]
 
     def add(self, map, transaction, link, p1=None, p2=None,
             changed=None):
         # apply the changes collected during the bisect loop to our addlist
         # return a delta suitable for addrevision
         def addlistdelta(addlist, x):
-            # start from the bottom up
-            # so changes to the offsets don't mess things up.
-            for start, end, content in reversed(x):
+            # for large addlist arrays, building a new array is cheaper
+            # than repeatedly modifying the existing one
+            currentposition = 0
+            newaddlist = array.array('c')
+
+            for start, end, content in x:
+                newaddlist += addlist[currentposition:start]
                 if content:
-                    addlist[start:end] = array.array('c', content)
-                else:
-                    del addlist[start:end]
-            return "".join(struct.pack(">lll", start, end, len(content))
+                    newaddlist += array.array('c', content)
+
+                currentposition = end
+
+            newaddlist += addlist[currentposition:]
+
+            deltatext = "".join(struct.pack(">lll", start, end, len(content))
                            + content for start, end, content in x)
+            return deltatext, newaddlist
 
         def checkforbidden(l):
             for f in l:
                 if '\n' in f or '\r' in f:
                     raise error.RevlogError(
                         _("'\\n' and '\\r' disallowed in filenames: %r") % f)
 
         # if we're using the cache, make sure it is valid and
         # parented by the same node we're diffing against
         if not (changed and self._mancache and p1 and self._mancache[0] == p1):
             files = sorted(map)
             checkforbidden(files)
 
             # if this is changed to support newlines in filenames,
             # be sure to check the templates/ dir again (especially *-raw.tmpl)
             hex, flags = revlog.hex, map.flags
             text = ''.join("%s\0%s%s\n" % (f, hex(map[f]), flags(f))
                            for f in files)
             arraytext = array.array('c', text)
             cachedelta = None
         else:
             added, removed = changed
             addlist = self._mancache[2]
 
             checkforbidden(added)
             # combine the changed lists into one list for sorting
             work = [(x, False) for x in added]
             work.extend((x, True) for x in removed)
             # this could use heapq.merge() (from Python 2.6+) or equivalent
             # since the lists are already sorted
             work.sort()
 
             delta = []
             dstart = None
             dend = None
             dline = [""]
             start = 0
             # zero copy representation of addlist as a buffer
             addbuf = util.buffer(addlist)
 
             # start with a readonly loop that finds the offset of
             # each line and creates the deltas
             for f, todelete in work:
                 # bs will either be the index of the item or the insert point
                 start, end = self._search(addbuf, f, start)
                 if not todelete:
                     l = "%s\0%s%s\n" % (f, revlog.hex(map[f]), map.flags(f))
                 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
                     dline = [l]
 
             if dstart is not None:
                 delta.append([dstart, dend, "".join(dline)])
             # apply the delta to the addlist, and get a delta for addrevision
-            cachedelta = (self.rev(p1), addlistdelta(addlist, delta))
+            deltatext, addlist = addlistdelta(addlist, delta)
+            cachedelta = (self.rev(p1), deltatext)
             arraytext = addlist
             text = util.buffer(arraytext)
 
         n = self.addrevision(text, transaction, link, p1, p2, cachedelta)
         self._mancache = (n, map, arraytext)
 
         return n