mercurial/revlog.py

Change the size of the short hash representation

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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
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# 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[: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