Mercurial > hg
view mercurial/pvec.py @ 49275:c6a3243567b6
chg: replace mercurial.util.recvfds() by simpler pure Python implementation
On Python 3, we have socket.socket.recvmsg(). This makes it possible to receive
FDs in pure Python code. The new code behaves like the previous
implementations, except that it’s more strict about the format of the ancillary
data. This works because we know in which format the FDs are passed.
Because the code is (and always has been) specific to chg (payload is 1 byte,
number of passed FDs is limited) and we now have only one implementation and
the code is very short, I decided to stop exposing a function in
mercurial.util.
Note on terminology: The SCM_RIGHTS mechanism is used to share open file
descriptions to another process over a socket. The sending side passes an array
of file descriptors and the receiving side receives an array of file
descriptors. The file descriptors are different in general on both sides but
refer to the same open file descriptions. The two terms are often conflated,
even in the official documentation. That’s why I used “FD” above, which could
mean both “file descriptor” and “file description”.
author | Manuel Jacob <me@manueljacob.de> |
---|---|
date | Thu, 02 Jun 2022 23:57:56 +0200 |
parents | 642e31cb55f0 |
children | d44e3c45f0e4 |
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# pvec.py - probabilistic vector clocks for Mercurial # # Copyright 2012 Olivia Mackall <olivia@selenic.com> # # This software may be used and distributed according to the terms of the # GNU General Public License version 2 or any later version. ''' A "pvec" is a changeset property based on the theory of vector clocks that can be compared to discover relatedness without consulting a graph. This can be useful for tasks like determining how a disconnected patch relates to a repository. Currently a pvec consist of 448 bits, of which 24 are 'depth' and the remainder are a bit vector. It is represented as a 70-character base85 string. Construction: - a root changeset has a depth of 0 and a bit vector based on its hash - a normal commit has a changeset where depth is increased by one and one bit vector bit is flipped based on its hash - a merge changeset pvec is constructed by copying changes from one pvec into the other to balance its depth Properties: - for linear changes, difference in depth is always <= hamming distance - otherwise, changes are probably divergent - when hamming distance is < 200, we can reliably detect when pvecs are near Issues: - hamming distance ceases to work over distances of ~ 200 - detecting divergence is less accurate when the common ancestor is very close to either revision or total distance is high - this could probably be improved by modeling the relation between delta and hdist Uses: - a patch pvec can be used to locate the nearest available common ancestor for resolving conflicts - ordering of patches can be established without a DAG - two head pvecs can be compared to determine whether push/pull/merge is needed and approximately how many changesets are involved - can be used to find a heuristic divergence measure between changesets on different branches ''' from .node import nullrev from . import ( pycompat, util, ) _size = 448 # 70 chars b85-encoded _bytes = _size // 8 _depthbits = 24 _depthbytes = _depthbits // 8 _vecbytes = _bytes - _depthbytes _vecbits = _vecbytes * 8 _radius = (_vecbits - 30) // 2 # high probability vectors are related def _bin(bs): '''convert a bytestring to a long''' v = 0 for b in bs: v = v * 256 + ord(b) return v def _str(v, l): # type: (int, int) -> bytes bs = b"" for p in pycompat.xrange(l): bs = pycompat.bytechr(v & 255) + bs v >>= 8 return bs def _split(b): '''depth and bitvec''' return _bin(b[:_depthbytes]), _bin(b[_depthbytes:]) def _join(depth, bitvec): return _str(depth, _depthbytes) + _str(bitvec, _vecbytes) def _hweight(x): c = 0 while x: if x & 1: c += 1 x >>= 1 return c _htab = [_hweight(x) for x in pycompat.xrange(256)] def _hamming(a, b): '''find the hamming distance between two longs''' d = a ^ b c = 0 while d: c += _htab[d & 0xFF] d >>= 8 return c def _mergevec(x, y, c): # Ideally, this function would be x ^ y ^ ancestor, but finding # ancestors is a nuisance. So instead we find the minimal number # of changes to balance the depth and hamming distance d1, v1 = x d2, v2 = y if d1 < d2: d1, d2, v1, v2 = d2, d1, v2, v1 hdist = _hamming(v1, v2) ddist = d1 - d2 v = v1 m = v1 ^ v2 # mask of different bits i = 1 if hdist > ddist: # if delta = 10 and hdist = 100, then we need to go up 55 steps # to the ancestor and down 45 changes = (hdist - ddist + 1) // 2 else: # must make at least one change changes = 1 depth = d1 + changes # copy changes from v2 if m: while changes: if m & i: v ^= i changes -= 1 i <<= 1 else: v = _flipbit(v, c) return depth, v def _flipbit(v, node): # converting bit strings to longs is slow bit = (hash(node) & 0xFFFFFFFF) % _vecbits return v ^ (1 << bit) def ctxpvec(ctx): '''construct a pvec for ctx while filling in the cache''' r = ctx.repo() if not util.safehasattr(r, "_pveccache"): r._pveccache = {} pvc = r._pveccache if ctx.rev() not in pvc: cl = r.changelog for n in pycompat.xrange(ctx.rev() + 1): if n not in pvc: node = cl.node(n) p1, p2 = cl.parentrevs(n) if p1 == nullrev: # start with a 'random' vector at root pvc[n] = (0, _bin((node * 3)[:_vecbytes])) elif p2 == nullrev: d, v = pvc[p1] pvc[n] = (d + 1, _flipbit(v, node)) else: pvc[n] = _mergevec(pvc[p1], pvc[p2], node) bs = _join(*pvc[ctx.rev()]) return pvec(util.b85encode(bs)) class pvec: def __init__(self, hashorctx): if isinstance(hashorctx, bytes): self._bs = hashorctx self._depth, self._vec = _split(util.b85decode(hashorctx)) else: self._vec = ctxpvec(hashorctx) def __str__(self): return self._bs def __eq__(self, b): return self._vec == b._vec and self._depth == b._depth def __lt__(self, b): delta = b._depth - self._depth if delta < 0: return False # always correct if _hamming(self._vec, b._vec) > delta: return False return True def __gt__(self, b): return b < self def __or__(self, b): delta = abs(b._depth - self._depth) if _hamming(self._vec, b._vec) <= delta: return False return True def __sub__(self, b): if self | b: raise ValueError(b"concurrent pvecs") return self._depth - b._depth def distance(self, b): d = abs(b._depth - self._depth) h = _hamming(self._vec, b._vec) return max(d, h) def near(self, b): dist = abs(b.depth - self._depth) if dist > _radius or _hamming(self._vec, b._vec) > _radius: return False