Mercurial > hg
annotate mercurial/pvec.py @ 29587:536eec443b4a
commandserver: rename _serveworker() to _runworker()
"run" sounds more natural as the function does never listen for new connection.
author | Yuya Nishihara <yuya@tcha.org> |
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date | Sat, 16 Jul 2016 14:46:31 +0900 |
parents | 983e93d88193 |
children | 4462a981e8df |
rev | line source |
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16249 | 1 # pvec.py - probabilistic vector clocks for Mercurial |
2 # | |
3 # Copyright 2012 Matt Mackall <mpm@selenic.com> | |
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 base85, |
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56 util, |
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57 ) |
16249 | 58 |
59 _size = 448 # 70 chars b85-encoded | |
60 _bytes = _size / 8 | |
61 _depthbits = 24 | |
62 _depthbytes = _depthbits / 8 | |
63 _vecbytes = _bytes - _depthbytes | |
64 _vecbits = _vecbytes * 8 | |
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65 _radius = (_vecbits - 30) / 2 # high probability vectors are related |
16249 | 66 |
67 def _bin(bs): | |
68 '''convert a bytestring to a long''' | |
69 v = 0 | |
70 for b in bs: | |
71 v = v * 256 + ord(b) | |
72 return v | |
73 | |
74 def _str(v, l): | |
75 bs = "" | |
76 for p in xrange(l): | |
77 bs = chr(v & 255) + bs | |
78 v >>= 8 | |
79 return bs | |
80 | |
81 def _split(b): | |
82 '''depth and bitvec''' | |
83 return _bin(b[:_depthbytes]), _bin(b[_depthbytes:]) | |
84 | |
85 def _join(depth, bitvec): | |
86 return _str(depth, _depthbytes) + _str(bitvec, _vecbytes) | |
87 | |
88 def _hweight(x): | |
89 c = 0 | |
90 while x: | |
91 if x & 1: | |
92 c += 1 | |
93 x >>= 1 | |
94 return c | |
95 _htab = [_hweight(x) for x in xrange(256)] | |
96 | |
97 def _hamming(a, b): | |
98 '''find the hamming distance between two longs''' | |
99 d = a ^ b | |
100 c = 0 | |
101 while d: | |
102 c += _htab[d & 0xff] | |
103 d >>= 8 | |
104 return c | |
105 | |
106 def _mergevec(x, y, c): | |
107 # Ideally, this function would be x ^ y ^ ancestor, but finding | |
108 # ancestors is a nuisance. So instead we find the minimal number | |
109 # of changes to balance the depth and hamming distance | |
110 | |
111 d1, v1 = x | |
112 d2, v2 = y | |
113 if d1 < d2: | |
114 d1, d2, v1, v2 = d2, d1, v2, v1 | |
115 | |
116 hdist = _hamming(v1, v2) | |
117 ddist = d1 - d2 | |
118 v = v1 | |
119 m = v1 ^ v2 # mask of different bits | |
120 i = 1 | |
121 | |
122 if hdist > ddist: | |
123 # if delta = 10 and hdist = 100, then we need to go up 55 steps | |
124 # to the ancestor and down 45 | |
125 changes = (hdist - ddist + 1) / 2 | |
126 else: | |
127 # must make at least one change | |
128 changes = 1 | |
129 depth = d1 + changes | |
130 | |
131 # copy changes from v2 | |
132 if m: | |
133 while changes: | |
134 if m & i: | |
135 v ^= i | |
136 changes -= 1 | |
137 i <<= 1 | |
138 else: | |
139 v = _flipbit(v, c) | |
140 | |
141 return depth, v | |
142 | |
143 def _flipbit(v, node): | |
144 # converting bit strings to longs is slow | |
145 bit = (hash(node) & 0xffffffff) % _vecbits | |
146 return v ^ (1<<bit) | |
147 | |
148 def ctxpvec(ctx): | |
149 '''construct a pvec for ctx while filling in the cache''' | |
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150 r = ctx.repo() |
16249 | 151 if not util.safehasattr(r, "_pveccache"): |
152 r._pveccache = {} | |
153 pvc = r._pveccache | |
154 if ctx.rev() not in pvc: | |
155 cl = r.changelog | |
156 for n in xrange(ctx.rev() + 1): | |
157 if n not in pvc: | |
158 node = cl.node(n) | |
159 p1, p2 = cl.parentrevs(n) | |
160 if p1 == nullrev: | |
161 # start with a 'random' vector at root | |
162 pvc[n] = (0, _bin((node * 3)[:_vecbytes])) | |
163 elif p2 == nullrev: | |
164 d, v = pvc[p1] | |
165 pvc[n] = (d + 1, _flipbit(v, node)) | |
166 else: | |
167 pvc[n] = _mergevec(pvc[p1], pvc[p2], node) | |
168 bs = _join(*pvc[ctx.rev()]) | |
169 return pvec(base85.b85encode(bs)) | |
170 | |
171 class pvec(object): | |
172 def __init__(self, hashorctx): | |
173 if isinstance(hashorctx, str): | |
174 self._bs = hashorctx | |
175 self._depth, self._vec = _split(base85.b85decode(hashorctx)) | |
176 else: | |
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177 self._vec = ctxpvec(hashorctx) |
16249 | 178 |
179 def __str__(self): | |
180 return self._bs | |
181 | |
182 def __eq__(self, b): | |
183 return self._vec == b._vec and self._depth == b._depth | |
184 | |
185 def __lt__(self, b): | |
186 delta = b._depth - self._depth | |
187 if delta < 0: | |
188 return False # always correct | |
189 if _hamming(self._vec, b._vec) > delta: | |
190 return False | |
191 return True | |
192 | |
193 def __gt__(self, b): | |
194 return b < self | |
195 | |
196 def __or__(self, b): | |
197 delta = abs(b._depth - self._depth) | |
198 if _hamming(self._vec, b._vec) <= delta: | |
199 return False | |
200 return True | |
201 | |
202 def __sub__(self, b): | |
203 if self | b: | |
204 raise ValueError("concurrent pvecs") | |
205 return self._depth - b._depth | |
206 | |
207 def distance(self, b): | |
208 d = abs(b._depth - self._depth) | |
209 h = _hamming(self._vec, b._vec) | |
210 return max(d, h) | |
211 | |
212 def near(self, b): | |
213 dist = abs(b.depth - self._depth) | |
214 if dist > _radius or _hamming(self._vec, b._vec) > _radius: | |
215 return False |