author | Gregory Szorc <gregory.szorc@gmail.com> |
Wed, 07 Feb 2018 20:24:22 -0800 | |
changeset 36069 | 957e773614d0 |
parent 32201 | 4462a981e8df |
child 38783 | e7aa113b14f7 |
permissions | -rw-r--r-- |
16249 | 1 |
# pvec.py - probabilistic vector clocks for Mercurial |
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# |
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# Copyright 2012 Matt Mackall <mpm@selenic.com> |
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# |
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# This software may be used and distributed according to the terms of the |
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# GNU General Public License version 2 or any later version. |
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''' |
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A "pvec" is a changeset property based on the theory of vector clocks |
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that can be compared to discover relatedness without consulting a |
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graph. This can be useful for tasks like determining how a |
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disconnected patch relates to a repository. |
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Currently a pvec consist of 448 bits, of which 24 are 'depth' and the |
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remainder are a bit vector. It is represented as a 70-character base85 |
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string. |
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Construction: |
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- a root changeset has a depth of 0 and a bit vector based on its hash |
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- a normal commit has a changeset where depth is increased by one and |
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one bit vector bit is flipped based on its hash |
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- a merge changeset pvec is constructed by copying changes from one pvec into |
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the other to balance its depth |
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Properties: |
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- for linear changes, difference in depth is always <= hamming distance |
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- otherwise, changes are probably divergent |
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- when hamming distance is < 200, we can reliably detect when pvecs are near |
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Issues: |
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- hamming distance ceases to work over distances of ~ 200 |
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- detecting divergence is less accurate when the common ancestor is very close |
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to either revision or total distance is high |
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- this could probably be improved by modeling the relation between |
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delta and hdist |
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Uses: |
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- a patch pvec can be used to locate the nearest available common ancestor for |
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resolving conflicts |
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- ordering of patches can be established without a DAG |
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- two head pvecs can be compared to determine whether push/pull/merge is needed |
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and approximately how many changesets are involved |
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- can be used to find a heuristic divergence measure between changesets on |
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different branches |
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''' |
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27501
983e93d88193
pvec: use absolute_import
Gregory Szorc <gregory.szorc@gmail.com>
parents:
24339
diff
changeset
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from __future__ import absolute_import |
983e93d88193
pvec: use absolute_import
Gregory Szorc <gregory.szorc@gmail.com>
parents:
24339
diff
changeset
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983e93d88193
pvec: use absolute_import
Gregory Szorc <gregory.szorc@gmail.com>
parents:
24339
diff
changeset
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from .node import nullrev |
983e93d88193
pvec: use absolute_import
Gregory Szorc <gregory.szorc@gmail.com>
parents:
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diff
changeset
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from . import ( |
983e93d88193
pvec: use absolute_import
Gregory Szorc <gregory.szorc@gmail.com>
parents:
24339
diff
changeset
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util, |
983e93d88193
pvec: use absolute_import
Gregory Szorc <gregory.szorc@gmail.com>
parents:
24339
diff
changeset
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) |
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_size = 448 # 70 chars b85-encoded |
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_bytes = _size / 8 |
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_depthbits = 24 |
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_depthbytes = _depthbits / 8 |
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_vecbytes = _bytes - _depthbytes |
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_vecbits = _vecbytes * 8 |
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17424
e7cfe3587ea4
fix trivial spelling errors
Mads Kiilerich <mads@kiilerich.com>
parents:
16249
diff
changeset
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_radius = (_vecbits - 30) / 2 # high probability vectors are related |
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def _bin(bs): |
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'''convert a bytestring to a long''' |
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v = 0 |
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for b in bs: |
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v = v * 256 + ord(b) |
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return v |
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def _str(v, l): |
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bs = "" |
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for p in xrange(l): |
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bs = chr(v & 255) + bs |
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v >>= 8 |
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return bs |
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def _split(b): |
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'''depth and bitvec''' |
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return _bin(b[:_depthbytes]), _bin(b[_depthbytes:]) |
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def _join(depth, bitvec): |
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return _str(depth, _depthbytes) + _str(bitvec, _vecbytes) |
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def _hweight(x): |
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c = 0 |
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while x: |
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if x & 1: |
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c += 1 |
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x >>= 1 |
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return c |
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_htab = [_hweight(x) for x in xrange(256)] |
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def _hamming(a, b): |
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'''find the hamming distance between two longs''' |
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d = a ^ b |
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c = 0 |
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while d: |
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c += _htab[d & 0xff] |
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d >>= 8 |
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return c |
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def _mergevec(x, y, c): |
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# Ideally, this function would be x ^ y ^ ancestor, but finding |
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# ancestors is a nuisance. So instead we find the minimal number |
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# of changes to balance the depth and hamming distance |
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d1, v1 = x |
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d2, v2 = y |
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if d1 < d2: |
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d1, d2, v1, v2 = d2, d1, v2, v1 |
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hdist = _hamming(v1, v2) |
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ddist = d1 - d2 |
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v = v1 |
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m = v1 ^ v2 # mask of different bits |
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i = 1 |
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if hdist > ddist: |
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# if delta = 10 and hdist = 100, then we need to go up 55 steps |
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# to the ancestor and down 45 |
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changes = (hdist - ddist + 1) / 2 |
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else: |
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# must make at least one change |
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changes = 1 |
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depth = d1 + changes |
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# copy changes from v2 |
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if m: |
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while changes: |
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if m & i: |
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v ^= i |
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changes -= 1 |
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i <<= 1 |
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else: |
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v = _flipbit(v, c) |
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return depth, v |
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def _flipbit(v, node): |
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# converting bit strings to longs is slow |
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bit = (hash(node) & 0xffffffff) % _vecbits |
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return v ^ (1<<bit) |
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def ctxpvec(ctx): |
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'''construct a pvec for ctx while filling in the cache''' |
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24339
bcc319d936a3
pvec: replace 'ctx._repo' with 'ctx.repo()'
Matt Harbison <matt_harbison@yahoo.com>
parents:
18918
diff
changeset
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r = ctx.repo() |
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if not util.safehasattr(r, "_pveccache"): |
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r._pveccache = {} |
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pvc = r._pveccache |
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if ctx.rev() not in pvc: |
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cl = r.changelog |
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for n in xrange(ctx.rev() + 1): |
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if n not in pvc: |
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node = cl.node(n) |
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p1, p2 = cl.parentrevs(n) |
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if p1 == nullrev: |
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# start with a 'random' vector at root |
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pvc[n] = (0, _bin((node * 3)[:_vecbytes])) |
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elif p2 == nullrev: |
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d, v = pvc[p1] |
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pvc[n] = (d + 1, _flipbit(v, node)) |
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else: |
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pvc[n] = _mergevec(pvc[p1], pvc[p2], node) |
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bs = _join(*pvc[ctx.rev()]) |
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4462a981e8df
base85: proxy through util module
Yuya Nishihara <yuya@tcha.org>
parents:
27501
diff
changeset
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return pvec(util.b85encode(bs)) |
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class pvec(object): |
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def __init__(self, hashorctx): |
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if isinstance(hashorctx, str): |
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self._bs = hashorctx |
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4462a981e8df
base85: proxy through util module
Yuya Nishihara <yuya@tcha.org>
parents:
27501
diff
changeset
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self._depth, self._vec = _split(util.b85decode(hashorctx)) |
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else: |
18918
5093d2a87ff6
pvec: use the correct name for an identifier
Bryan O'Sullivan <bryano@fb.com>
parents:
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self._vec = ctxpvec(hashorctx) |
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def __str__(self): |
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return self._bs |
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def __eq__(self, b): |
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return self._vec == b._vec and self._depth == b._depth |
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def __lt__(self, b): |
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delta = b._depth - self._depth |
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if delta < 0: |
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return False # always correct |
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if _hamming(self._vec, b._vec) > delta: |
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return False |
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return True |
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def __gt__(self, b): |
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return b < self |
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def __or__(self, b): |
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delta = abs(b._depth - self._depth) |
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if _hamming(self._vec, b._vec) <= delta: |
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return False |
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return True |
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def __sub__(self, b): |
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if self | b: |
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raise ValueError("concurrent pvecs") |
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return self._depth - b._depth |
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def distance(self, b): |
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d = abs(b._depth - self._depth) |
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h = _hamming(self._vec, b._vec) |
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return max(d, h) |
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def near(self, b): |
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dist = abs(b.depth - self._depth) |
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if dist > _radius or _hamming(self._vec, b._vec) > _radius: |
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return False |