Mercurial > hg
annotate mercurial/pvec.py @ 47072:4c041c71ec01
revlog: introduce an explicit tracking of what the revlog is about
Since the dawn of time, people have been forced to rely to lossy introspection
of the index filename to determine what the purpose and role of the revlog they
encounter is. This is hacky, error prone, inflexible, abstraction-leaky,
<insert-your-own-complaints-here>.
In f63299ee7e4d Raphaël introduced a new attribute to track this information:
`revlog_kind`. However it is initialized in an odd place and various instances
end up not having it set. In addition is only tracking some of the information
we end up having to introspect in various pieces of code.
So we add a new attribute that holds more data and is more strictly enforced.
This work is done in collaboration with Raphaël.
The `revlog_kind` one will be removed/adapted in the next changeset. We expect
to be able to clean up various existing piece of code and to simplify coming
work around the newer revlog format.
Differential Revision: https://phab.mercurial-scm.org/D10352
author | Pierre-Yves David <pierre-yves.david@octobus.net> |
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date | Tue, 06 Apr 2021 05:20:24 +0200 |
parents | d4ba4d51f85f |
children | 6000f5b25c9b |
rev | line source |
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16249 | 1 # pvec.py - probabilistic vector clocks for Mercurial |
2 # | |
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3 # Copyright 2012 Olivia Mackall <olivia@selenic.com> |
16249 | 4 # |
5 # This software may be used and distributed according to the terms of the | |
6 # GNU General Public License version 2 or any later version. | |
7 | |
8 ''' | |
9 A "pvec" is a changeset property based on the theory of vector clocks | |
10 that can be compared to discover relatedness without consulting a | |
11 graph. This can be useful for tasks like determining how a | |
12 disconnected patch relates to a repository. | |
13 | |
14 Currently a pvec consist of 448 bits, of which 24 are 'depth' and the | |
15 remainder are a bit vector. It is represented as a 70-character base85 | |
16 string. | |
17 | |
18 Construction: | |
19 | |
20 - a root changeset has a depth of 0 and a bit vector based on its hash | |
21 - a normal commit has a changeset where depth is increased by one and | |
22 one bit vector bit is flipped based on its hash | |
23 - a merge changeset pvec is constructed by copying changes from one pvec into | |
24 the other to balance its depth | |
25 | |
26 Properties: | |
27 | |
28 - for linear changes, difference in depth is always <= hamming distance | |
29 - otherwise, changes are probably divergent | |
30 - when hamming distance is < 200, we can reliably detect when pvecs are near | |
31 | |
32 Issues: | |
33 | |
34 - hamming distance ceases to work over distances of ~ 200 | |
35 - detecting divergence is less accurate when the common ancestor is very close | |
36 to either revision or total distance is high | |
37 - this could probably be improved by modeling the relation between | |
38 delta and hdist | |
39 | |
40 Uses: | |
41 | |
42 - a patch pvec can be used to locate the nearest available common ancestor for | |
43 resolving conflicts | |
44 - ordering of patches can be established without a DAG | |
45 - two head pvecs can be compared to determine whether push/pull/merge is needed | |
46 and approximately how many changesets are involved | |
47 - can be used to find a heuristic divergence measure between changesets on | |
48 different branches | |
49 ''' | |
50 | |
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51 from __future__ import absolute_import |
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52 |
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53 from .node import nullrev |
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54 from . import ( |
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55 pycompat, |
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56 util, |
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57 ) |
16249 | 58 |
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59 _size = 448 # 70 chars b85-encoded |
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60 _bytes = _size // 8 |
16249 | 61 _depthbits = 24 |
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62 _depthbytes = _depthbits // 8 |
16249 | 63 _vecbytes = _bytes - _depthbytes |
64 _vecbits = _vecbytes * 8 | |
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65 _radius = (_vecbits - 30) // 2 # high probability vectors are related |
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66 |
16249 | 67 |
68 def _bin(bs): | |
69 '''convert a bytestring to a long''' | |
70 v = 0 | |
71 for b in bs: | |
72 v = v * 256 + ord(b) | |
73 return v | |
74 | |
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75 |
16249 | 76 def _str(v, l): |
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77 # type: (int, int) -> bytes |
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78 bs = b"" |
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79 for p in pycompat.xrange(l): |
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80 bs = pycompat.bytechr(v & 255) + bs |
16249 | 81 v >>= 8 |
82 return bs | |
83 | |
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84 |
16249 | 85 def _split(b): |
86 '''depth and bitvec''' | |
87 return _bin(b[:_depthbytes]), _bin(b[_depthbytes:]) | |
88 | |
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89 |
16249 | 90 def _join(depth, bitvec): |
91 return _str(depth, _depthbytes) + _str(bitvec, _vecbytes) | |
92 | |
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93 |
16249 | 94 def _hweight(x): |
95 c = 0 | |
96 while x: | |
97 if x & 1: | |
98 c += 1 | |
99 x >>= 1 | |
100 return c | |
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101 |
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102 |
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103 _htab = [_hweight(x) for x in pycompat.xrange(256)] |
16249 | 104 |
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105 |
16249 | 106 def _hamming(a, b): |
107 '''find the hamming distance between two longs''' | |
108 d = a ^ b | |
109 c = 0 | |
110 while d: | |
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111 c += _htab[d & 0xFF] |
16249 | 112 d >>= 8 |
113 return c | |
114 | |
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115 |
16249 | 116 def _mergevec(x, y, c): |
117 # Ideally, this function would be x ^ y ^ ancestor, but finding | |
118 # ancestors is a nuisance. So instead we find the minimal number | |
119 # of changes to balance the depth and hamming distance | |
120 | |
121 d1, v1 = x | |
122 d2, v2 = y | |
123 if d1 < d2: | |
124 d1, d2, v1, v2 = d2, d1, v2, v1 | |
125 | |
126 hdist = _hamming(v1, v2) | |
127 ddist = d1 - d2 | |
128 v = v1 | |
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129 m = v1 ^ v2 # mask of different bits |
16249 | 130 i = 1 |
131 | |
132 if hdist > ddist: | |
133 # if delta = 10 and hdist = 100, then we need to go up 55 steps | |
134 # to the ancestor and down 45 | |
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135 changes = (hdist - ddist + 1) // 2 |
16249 | 136 else: |
137 # must make at least one change | |
138 changes = 1 | |
139 depth = d1 + changes | |
140 | |
141 # copy changes from v2 | |
142 if m: | |
143 while changes: | |
144 if m & i: | |
145 v ^= i | |
146 changes -= 1 | |
147 i <<= 1 | |
148 else: | |
149 v = _flipbit(v, c) | |
150 | |
151 return depth, v | |
152 | |
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153 |
16249 | 154 def _flipbit(v, node): |
155 # converting bit strings to longs is slow | |
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156 bit = (hash(node) & 0xFFFFFFFF) % _vecbits |
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157 return v ^ (1 << bit) |
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158 |
16249 | 159 |
160 def ctxpvec(ctx): | |
161 '''construct a pvec for ctx while filling in the cache''' | |
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162 r = ctx.repo() |
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163 if not util.safehasattr(r, "_pveccache"): |
16249 | 164 r._pveccache = {} |
165 pvc = r._pveccache | |
166 if ctx.rev() not in pvc: | |
167 cl = r.changelog | |
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168 for n in pycompat.xrange(ctx.rev() + 1): |
16249 | 169 if n not in pvc: |
170 node = cl.node(n) | |
171 p1, p2 = cl.parentrevs(n) | |
172 if p1 == nullrev: | |
173 # start with a 'random' vector at root | |
174 pvc[n] = (0, _bin((node * 3)[:_vecbytes])) | |
175 elif p2 == nullrev: | |
176 d, v = pvc[p1] | |
177 pvc[n] = (d + 1, _flipbit(v, node)) | |
178 else: | |
179 pvc[n] = _mergevec(pvc[p1], pvc[p2], node) | |
180 bs = _join(*pvc[ctx.rev()]) | |
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181 return pvec(util.b85encode(bs)) |
16249 | 182 |
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183 |
16249 | 184 class pvec(object): |
185 def __init__(self, hashorctx): | |
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186 if isinstance(hashorctx, bytes): |
16249 | 187 self._bs = hashorctx |
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188 self._depth, self._vec = _split(util.b85decode(hashorctx)) |
16249 | 189 else: |
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190 self._vec = ctxpvec(hashorctx) |
16249 | 191 |
192 def __str__(self): | |
193 return self._bs | |
194 | |
195 def __eq__(self, b): | |
196 return self._vec == b._vec and self._depth == b._depth | |
197 | |
198 def __lt__(self, b): | |
199 delta = b._depth - self._depth | |
200 if delta < 0: | |
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201 return False # always correct |
16249 | 202 if _hamming(self._vec, b._vec) > delta: |
203 return False | |
204 return True | |
205 | |
206 def __gt__(self, b): | |
207 return b < self | |
208 | |
209 def __or__(self, b): | |
210 delta = abs(b._depth - self._depth) | |
211 if _hamming(self._vec, b._vec) <= delta: | |
212 return False | |
213 return True | |
214 | |
215 def __sub__(self, b): | |
216 if self | b: | |
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217 raise ValueError(b"concurrent pvecs") |
16249 | 218 return self._depth - b._depth |
219 | |
220 def distance(self, b): | |
221 d = abs(b._depth - self._depth) | |
222 h = _hamming(self._vec, b._vec) | |
223 return max(d, h) | |
224 | |
225 def near(self, b): | |
226 dist = abs(b.depth - self._depth) | |
227 if dist > _radius or _hamming(self._vec, b._vec) > _radius: | |
228 return False |