Mercurial > hg
annotate mercurial/pvec.py @ 49576:d12446766a35 stable
upgrade: byteify requirement changes written to output
ui.write() expects bytes, and internally uses `b''` as the default when getting
the `label` keyword from `*args`. So either we're missing test coverage, or
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Also, slip in a type hint to flag this in the future.
author | Matt Harbison <matt_harbison@yahoo.com> |
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date | Sun, 06 Nov 2022 12:15:35 -0500 |
parents | d44e3c45f0e4 |
children | d718eddf01d9 |
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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 |
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52 from .node import nullrev |
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53 from . import ( |
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54 pycompat, |
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55 util, |
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56 ) |
16249 | 57 |
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58 _size = 448 # 70 chars b85-encoded |
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59 _bytes = _size // 8 |
16249 | 60 _depthbits = 24 |
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61 _depthbytes = _depthbits // 8 |
16249 | 62 _vecbytes = _bytes - _depthbytes |
63 _vecbits = _vecbytes * 8 | |
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64 _radius = (_vecbits - 30) // 2 # high probability vectors are related |
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65 |
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 | |
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74 |
16249 | 75 def _str(v, l): |
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76 # type: (int, int) -> bytes |
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77 bs = b"" |
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78 for p in range(l): |
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79 bs = pycompat.bytechr(v & 255) + bs |
16249 | 80 v >>= 8 |
81 return bs | |
82 | |
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83 |
16249 | 84 def _split(b): |
85 '''depth and bitvec''' | |
86 return _bin(b[:_depthbytes]), _bin(b[_depthbytes:]) | |
87 | |
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88 |
16249 | 89 def _join(depth, bitvec): |
90 return _str(depth, _depthbytes) + _str(bitvec, _vecbytes) | |
91 | |
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92 |
16249 | 93 def _hweight(x): |
94 c = 0 | |
95 while x: | |
96 if x & 1: | |
97 c += 1 | |
98 x >>= 1 | |
99 return c | |
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100 |
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101 |
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102 _htab = [_hweight(x) for x in range(256)] |
16249 | 103 |
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104 |
16249 | 105 def _hamming(a, b): |
106 '''find the hamming distance between two longs''' | |
107 d = a ^ b | |
108 c = 0 | |
109 while d: | |
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110 c += _htab[d & 0xFF] |
16249 | 111 d >>= 8 |
112 return c | |
113 | |
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114 |
16249 | 115 def _mergevec(x, y, c): |
116 # Ideally, this function would be x ^ y ^ ancestor, but finding | |
117 # ancestors is a nuisance. So instead we find the minimal number | |
118 # of changes to balance the depth and hamming distance | |
119 | |
120 d1, v1 = x | |
121 d2, v2 = y | |
122 if d1 < d2: | |
123 d1, d2, v1, v2 = d2, d1, v2, v1 | |
124 | |
125 hdist = _hamming(v1, v2) | |
126 ddist = d1 - d2 | |
127 v = v1 | |
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128 m = v1 ^ v2 # mask of different bits |
16249 | 129 i = 1 |
130 | |
131 if hdist > ddist: | |
132 # if delta = 10 and hdist = 100, then we need to go up 55 steps | |
133 # to the ancestor and down 45 | |
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134 changes = (hdist - ddist + 1) // 2 |
16249 | 135 else: |
136 # must make at least one change | |
137 changes = 1 | |
138 depth = d1 + changes | |
139 | |
140 # copy changes from v2 | |
141 if m: | |
142 while changes: | |
143 if m & i: | |
144 v ^= i | |
145 changes -= 1 | |
146 i <<= 1 | |
147 else: | |
148 v = _flipbit(v, c) | |
149 | |
150 return depth, v | |
151 | |
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152 |
16249 | 153 def _flipbit(v, node): |
154 # converting bit strings to longs is slow | |
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155 bit = (hash(node) & 0xFFFFFFFF) % _vecbits |
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156 return v ^ (1 << bit) |
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157 |
16249 | 158 |
159 def ctxpvec(ctx): | |
160 '''construct a pvec for ctx while filling in the cache''' | |
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161 r = ctx.repo() |
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162 if not util.safehasattr(r, "_pveccache"): |
16249 | 163 r._pveccache = {} |
164 pvc = r._pveccache | |
165 if ctx.rev() not in pvc: | |
166 cl = r.changelog | |
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167 for n in range(ctx.rev() + 1): |
16249 | 168 if n not in pvc: |
169 node = cl.node(n) | |
170 p1, p2 = cl.parentrevs(n) | |
171 if p1 == nullrev: | |
172 # start with a 'random' vector at root | |
173 pvc[n] = (0, _bin((node * 3)[:_vecbytes])) | |
174 elif p2 == nullrev: | |
175 d, v = pvc[p1] | |
176 pvc[n] = (d + 1, _flipbit(v, node)) | |
177 else: | |
178 pvc[n] = _mergevec(pvc[p1], pvc[p2], node) | |
179 bs = _join(*pvc[ctx.rev()]) | |
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180 return pvec(util.b85encode(bs)) |
16249 | 181 |
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182 |
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183 class pvec: |
16249 | 184 def __init__(self, hashorctx): |
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185 if isinstance(hashorctx, bytes): |
16249 | 186 self._bs = hashorctx |
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187 self._depth, self._vec = _split(util.b85decode(hashorctx)) |
16249 | 188 else: |
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189 self._vec = ctxpvec(hashorctx) |
16249 | 190 |
191 def __str__(self): | |
192 return self._bs | |
193 | |
194 def __eq__(self, b): | |
195 return self._vec == b._vec and self._depth == b._depth | |
196 | |
197 def __lt__(self, b): | |
198 delta = b._depth - self._depth | |
199 if delta < 0: | |
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200 return False # always correct |
16249 | 201 if _hamming(self._vec, b._vec) > delta: |
202 return False | |
203 return True | |
204 | |
205 def __gt__(self, b): | |
206 return b < self | |
207 | |
208 def __or__(self, b): | |
209 delta = abs(b._depth - self._depth) | |
210 if _hamming(self._vec, b._vec) <= delta: | |
211 return False | |
212 return True | |
213 | |
214 def __sub__(self, b): | |
215 if self | b: | |
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216 raise ValueError(b"concurrent pvecs") |
16249 | 217 return self._depth - b._depth |
218 | |
219 def distance(self, b): | |
220 d = abs(b._depth - self._depth) | |
221 h = _hamming(self._vec, b._vec) | |
222 return max(d, h) | |
223 | |
224 def near(self, b): | |
225 dist = abs(b.depth - self._depth) | |
226 if dist > _radius or _hamming(self._vec, b._vec) > _radius: | |
227 return False |