Change the size of the short hash representation

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

Change the size of the short hash representation

First note that this number doesn't really matter, as we always check
for ambiguous short hash ids.

Here's the math on collision probability:

>>> import math
>>> def p(f, n): return 1 - (1 / math.exp(n**2/(2*f)))
...
>>> p(2**32, 30000.0)
0.09947179164613551   # with 30000 changesets (BKCVS), we have a 9% chance
>>> p(2**32, 65000.0)
0.38850881217977273   # and with a full import from BK, we'd have a 39% chance
>>> p(2**40, 1e6)
0.36539171908447321   # we'd like to be "safe" for 1M csets, so 40 isn't enough
>>> p(2**48, 1e6)
0.001774780051374103  # But 48 looks good
>>> p(2**48, 1e7)
0.16275260939624481
>>> p(2**48, 5e6)
0.043437281083569146
>>> p(2**48, 2e6)
0.0070802434913129764
>>> p(2**48, 3e6)
0.01586009440574343

manifest hash: 24d9f928a463f46708b0e11fb781d5a241851424
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.0 (GNU/Linux)

iD8DBQFCsQoMywK+sNU5EO8RAoBBAJwII9GV6dT9QUOYAk3gZGw9z0JvjACfSI4q
IFnTu1F7P5OuLelO1GsM8Bs=
=CNWk
-----END PGP SIGNATURE-----

mercurial/revlog.py

@@ -1,535 +1,535 b''
 # revlog.py - storage back-end for mercurial
 #
 # This provides efficient delta storage with O(1) retrieve and append
 # and O(changes) merge between branches
 #
 # Copyright 2005 Matt Mackall <[e-mail]>
 #
 # This software may be used and distributed according to the terms
 # of the GNU General Public License, incorporated herein by reference.
 
 import zlib, struct, sha, binascii, heapq
 from mercurial import mdiff
 
 def hex(node): return binascii.hexlify(node)
 def bin(node): return binascii.unhexlify(node)
-def short(node): return hex(node[:4])
+def short(node): return hex(node[:6])
 
 def compress(text):
     if not text: return text
     if len(text) < 44:
         if text[0] == '\0': return text
         return 'u' + text
     bin = zlib.compress(text)
     if len(bin) > len(text):
         if text[0] == '\0': return text
         return 'u' + text
     return bin
 
 def decompress(bin):
     if not bin: return bin
     t = bin[0]
     if t == '\0': return bin
     if t == 'x': return zlib.decompress(bin)
     if t == 'u': return bin[1:]
     raise "unknown compression type %s" % t
 
 def hash(text, p1, p2):
     l = [p1, p2]
     l.sort()
     return sha.sha(l[0] + l[1] + text).digest()
 
 nullid = "\0" * 20
 indexformat = ">4l20s20s20s"
 
 class lazyparser:
     def __init__(self, data, revlog):
         self.data = data
         self.s = struct.calcsize(indexformat)
         self.l = len(data)/self.s
         self.index = [None] * self.l
         self.map = {nullid: -1}
         self.all = 0
         self.revlog = revlog
 
     def load(self, pos=None):
         if self.all: return
         if pos is not None:
             block = pos / 1000
             i = block * 1000
             end = min(self.l, i + 1000)
         else:
             self.all = 1
             i = 0
             end = self.l
             self.revlog.index = self.index
             self.revlog.nodemap = self.map
             
         while i < end:
             d = self.data[i * self.s: (i + 1) * self.s]
             e = struct.unpack(indexformat, d)
             self.index[i] = e
             self.map[e[6]] = i
             i += 1
         
 class lazyindex:
     def __init__(self, parser):
         self.p = parser
     def __len__(self):
         return len(self.p.index)
     def load(self, pos):
         self.p.load(pos)
         return self.p.index[pos]
     def __getitem__(self, pos):
         return self.p.index[pos] or self.load(pos)
     def append(self, e):
         self.p.index.append(e)
         
 class lazymap:
     def __init__(self, parser):
         self.p = parser
     def load(self, key):
         if self.p.all: return
         n = self.p.data.find(key)
         if n < 0: raise KeyError("node " + hex(key))
         pos = n / self.p.s
         self.p.load(pos)
     def __contains__(self, key):
         self.p.load()
         return key in self.p.map
     def __iter__(self):
         for i in xrange(self.p.l):
             try:
                 yield self.p.index[i][6]
             except:
                 self.p.load(i)
                 yield self.p.index[i][6]
     def __getitem__(self, key):
         try:
             return self.p.map[key]
         except KeyError:
             try:
                 self.load(key)
                 return self.p.map[key]
             except KeyError:
                 raise KeyError("node " + hex(key))
     def __setitem__(self, key, val):
         self.p.map[key] = val
 
 class revlog:
     def __init__(self, opener, indexfile, datafile):
         self.indexfile = indexfile
         self.datafile = datafile
         self.opener = opener
         self.cache = None
 
         try:
             i = self.opener(self.indexfile).read()
         except IOError:
             i = ""
 
         if len(i) > 10000:
             # big index, let's parse it on demand
             parser = lazyparser(i, self)
             self.index = lazyindex(parser)
             self.nodemap = lazymap(parser)
         else:
             s = struct.calcsize(indexformat)
             l = len(i) / s
             self.index = [None] * l
             m = [None] * l
 
             n = 0
             for f in xrange(0, len(i), s):
                 # offset, size, base, linkrev, p1, p2, nodeid
                 e = struct.unpack(indexformat, i[f:f + s])
                 m[n] = (e[6], n)
                 self.index[n] = e
                 n += 1
 
             self.nodemap = dict(m)
             self.nodemap[nullid] = -1
             
 
     def tip(self): return self.node(len(self.index) - 1)
     def count(self): return len(self.index)
     def node(self, rev): return (rev < 0) and nullid or self.index[rev][6]
     def rev(self, node): return self.nodemap[node]
     def linkrev(self, node): return self.index[self.nodemap[node]][3]
     def parents(self, node):
         if node == nullid: return (nullid, nullid)
         return self.index[self.nodemap[node]][4:6]
 
     def start(self, rev): return self.index[rev][0]
     def length(self, rev): return self.index[rev][1]
     def end(self, rev): return self.start(rev) + self.length(rev)
     def base(self, rev): return self.index[rev][2]
 
     def heads(self):
         p = {}
         h = []
         for r in range(self.count() - 1, -1, -1):
             n = self.node(r)
             if n not in p:
                 h.append(n)
             for pn in self.parents(n):
                 p[pn] = 1
         return h
 
     def children(self, node):
         c = []
         p = self.rev(node)
         for r in range(p + 1, self.count()):
             n = self.node(r)
             for pn in self.parents(n):
                 if pn == p:
                     c.append(p)
                     continue
                 elif pn == nullid:
                     continue
         return c
     
     def lookup(self, id):
         try:
             rev = int(id)
             return self.node(rev)
         except ValueError:
             c = []
             for n in self.nodemap:
                 if id in hex(n):
                     c.append(n)
             if len(c) > 1: raise KeyError("Ambiguous identifier")
             if len(c) < 1: raise KeyError("No match found")
             return c[0]
                 
         return None
 
     def diff(self, a, b):
         return mdiff.textdiff(a, b)
 
     def patches(self, t, pl):
         return mdiff.patches(t, pl)
 
     def delta(self, node):
         r = self.rev(node)
         b = self.base(r)
         if r == b:
             return self.diff(self.revision(self.node(r - 1)),
                              self.revision(node))
         else:
             f = self.opener(self.datafile)
             f.seek(self.start(r))
             data = f.read(self.length(r))
         return decompress(data)
 
     def revision(self, node):
         if node == nullid: return ""
         if self.cache and self.cache[0] == node: return self.cache[2]
 
         text = None
         rev = self.rev(node)
         start, length, base, link, p1, p2, node = self.index[rev]
         end = start + length
         if base != rev: start = self.start(base)
 
         if self.cache and self.cache[1] >= base and self.cache[1] < rev:
             base = self.cache[1]
             start = self.start(base + 1)
             text = self.cache[2]
             last = 0
 
         f = self.opener(self.datafile)
         f.seek(start)
         data = f.read(end - start)
 
         if not text:
             last = self.length(base)
             text = decompress(data[:last])
 
         bins = []
         for r in xrange(base + 1, rev + 1):
             s = self.length(r)
             bins.append(decompress(data[last:last + s]))
             last = last + s
 
         text = mdiff.patches(text, bins)
 
         if node != hash(text, p1, p2):
             raise IOError("integrity check failed on %s:%d"
                           % (self.datafile, rev))
 
         self.cache = (node, rev, text)
         return text  
 
     def addrevision(self, text, transaction, link, p1=None, p2=None):
         if text is None: text = ""
         if p1 is None: p1 = self.tip()
         if p2 is None: p2 = nullid
 
         node = hash(text, p1, p2)
 
         if node in self.nodemap:
             return node
 
         n = self.count()
         t = n - 1
 
         if n:
             base = self.base(t)
             start = self.start(base)
             end = self.end(t)
             prev = self.revision(self.tip())
             d = self.diff(prev, text)
             data = compress(d)
             dist = end - start + len(data)
 
         # full versions are inserted when the needed deltas
         # become comparable to the uncompressed text
         if not n or dist > len(text) * 2:
             data = compress(text)
             base = n
         else:
             base = self.base(t)
 
         offset = 0
         if t >= 0:
             offset = self.end(t)
 
         e = (offset, len(data), base, link, p1, p2, node)
         
         self.index.append(e)
         self.nodemap[node] = n
         entry = struct.pack(indexformat, *e)
 
         transaction.add(self.datafile, e[0])
         self.opener(self.datafile, "a").write(data)
         transaction.add(self.indexfile, n * len(entry))
         self.opener(self.indexfile, "a").write(entry)
 
         self.cache = (node, n, text)
         return node
 
     def ancestor(self, a, b):
         # calculate the distance of every node from root
         dist = {nullid: 0}
         for i in xrange(self.count()):
             n = self.node(i)
             p1, p2 = self.parents(n)
             dist[n] = max(dist[p1], dist[p2]) + 1
         
         # traverse ancestors in order of decreasing distance from root
         def ancestors(node):
             # we store negative distances because heap returns smallest member
             h = [(-dist[node], node)]
             seen = {}
             earliest = self.count()
             while h:
                 d, n = heapq.heappop(h)
                 r = self.rev(n)
                 if n not in seen:
                     seen[n] = 1
                     yield (-d, n)
                     for p in self.parents(n):
                         heapq.heappush(h, (-dist[p], p))
 
         x = ancestors(a)
         y = ancestors(b)
         lx = x.next()
         ly = y.next()
 
         # increment each ancestor list until it is closer to root than
         # the other, or they match
         while 1:
             if lx == ly:
                 return lx[1]
             elif lx < ly:
                 ly = y.next()
             elif lx > ly:
                 lx = x.next()
 
     def group(self, linkmap):
         # given a list of changeset revs, return a set of deltas and
         # metadata corresponding to nodes. the first delta is
         # parent(nodes[0]) -> nodes[0] the receiver is guaranteed to
         # have this parent as it has all history before these
         # changesets. parent is parent[0]
 
         revs = []
         needed = {}
 
         # find file nodes/revs that match changeset revs
         for i in xrange(0, self.count()):
             if self.index[i][3] in linkmap:
                 revs.append(i)
                 needed[i] = 1
 
         # if we don't have any revisions touched by these changesets, bail
         if not revs:
             yield struct.pack(">l", 0)
             return
 
         # add the parent of the first rev
         p = self.parents(self.node(revs[0]))[0]
         revs.insert(0, self.rev(p))
 
         # for each delta that isn't contiguous in the log, we need to
         # reconstruct the base, reconstruct the result, and then
         # calculate the delta. We also need to do this where we've
         # stored a full version and not a delta
         for i in xrange(0, len(revs) - 1):
             a, b = revs[i], revs[i + 1]
             if a + 1 != b or self.base(b) == b:
                 for j in xrange(self.base(a), a + 1):
                     needed[j] = 1
                 for j in xrange(self.base(b), b + 1):
                     needed[j] = 1
 
         # calculate spans to retrieve from datafile
         needed = needed.keys()
         needed.sort()
         spans = []
         oo = -1
         ol = 0
         for n in needed:
             if n < 0: continue
             o = self.start(n)
             l = self.length(n)
             if oo + ol == o: # can we merge with the previous?
                 nl = spans[-1][2]
                 nl.append((n, l))
                 ol += l
                 spans[-1] = (oo, ol, nl)
             else:
                 oo = o
                 ol = l
                 spans.append((oo, ol, [(n, l)]))
 
         # read spans in, divide up chunks
         chunks = {}
         for span in spans:
             # we reopen the file for each span to make http happy for now
             f = self.opener(self.datafile)
             f.seek(span[0])
             data = f.read(span[1])
 
             # divide up the span
             pos = 0
             for r, l in span[2]:
                 chunks[r] = decompress(data[pos: pos + l])
                 pos += l
 
         # helper to reconstruct intermediate versions
         def construct(text, base, rev):
             bins = [chunks[r] for r in xrange(base + 1, rev + 1)]
             return mdiff.patches(text, bins)
 
         # build deltas
         deltas = []
         for d in xrange(0, len(revs) - 1):
             a, b = revs[d], revs[d + 1]
             n = self.node(b)
 
             # do we need to construct a new delta?
             if a + 1 != b or self.base(b) == b:
                 if a >= 0:
                     base = self.base(a)
                     ta = chunks[self.base(a)]
                     ta = construct(ta, base, a)
                 else:
                     ta = ""
                     
                 base = self.base(b)
                 if a > base:
                     base = a
                     tb = ta
                 else:
                     tb = chunks[self.base(b)]
                 tb = construct(tb, base, b)
                 d = self.diff(ta, tb)
             else:
                 d = chunks[b]
 
             p = self.parents(n)
             meta = n + p[0] + p[1] + linkmap[self.linkrev(n)]
             l = struct.pack(">l", len(meta) + len(d) + 4)
             yield l
             yield meta
             yield d
 
         yield struct.pack(">l", 0)
 
     def addgroup(self, revs, linkmapper, transaction, unique = 0):
         # given a set of deltas, add them to the revision log. the
         # first delta is against its parent, which should be in our
         # log, the rest are against the previous delta.
 
         # track the base of the current delta log
         r = self.count()
         t = r - 1
         node = nullid
         
         base = prev = -1
         start = end = 0
         if r:
             start = self.start(self.base(t))
             end = self.end(t)
             measure = self.length(self.base(t))
             base = self.base(t)
             prev = self.tip()
 
         transaction.add(self.datafile, end)
         transaction.add(self.indexfile, r * struct.calcsize(indexformat))
         dfh = self.opener(self.datafile, "a")
         ifh = self.opener(self.indexfile, "a")
 
         # loop through our set of deltas
         chain = None
         for chunk in revs:
             node, p1, p2, cs = struct.unpack("20s20s20s20s", chunk[:80])
             link = linkmapper(cs)
             if node in self.nodemap:
                 # this can happen if two branches make the same change
                 if unique:
                     raise "already have %s" % hex(node[:4])
                 continue
             delta = chunk[80:]
 
             if not chain:
                 # retrieve the parent revision of the delta chain
                 chain = p1
                 if not chain in self.nodemap:
                     raise "unknown base %s" % short(chain[:4])
 
             # full versions are inserted when the needed deltas become
             # comparable to the uncompressed text or when the previous
             # version is not the one we have a delta against. We use
             # the size of the previous full rev as a proxy for the
             # current size.
 
             if chain == prev:
                 cdelta = compress(delta)
 
             if chain != prev or (end - start + len(cdelta)) > measure * 2:
                 # flush our writes here so we can read it in revision
                 dfh.flush()
                 ifh.flush()
                 text = self.revision(chain)
                 text = self.patches(text, [delta])
                 chk = self.addrevision(text, transaction, link, p1, p2)
                 if chk != node:
                     raise "consistency error adding group"
                 measure = len(text)
             else:
                 e = (end, len(cdelta), self.base(t), link, p1, p2, node)
                 self.index.append(e)
                 self.nodemap[node] = r
                 dfh.write(cdelta)
                 ifh.write(struct.pack(indexformat, *e))
 
             t, r, chain, prev = r, r + 1, node, node
             start = self.start(self.base(t))
             end = self.end(t)
 
         dfh.close()
         ifh.close()
         return node