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1 # pvec.py - probabilistic vector clocks for Mercurial
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2 #
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3 # Copyright 2012 Matt Mackall <mpm@selenic.com>
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4 #
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5 # This software may be used and distributed according to the terms of the
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6 # GNU General Public License version 2 or any later version.
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7
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8 '''
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9 A "pvec" is a changeset property based on the theory of vector clocks
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10 that can be compared to discover relatedness without consulting a
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11 graph. This can be useful for tasks like determining how a
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12 disconnected patch relates to a repository.
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13
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14 Currently a pvec consist of 448 bits, of which 24 are 'depth' and the
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15 remainder are a bit vector. It is represented as a 70-character base85
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16 string.
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17
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18 Construction:
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19
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20 - a root changeset has a depth of 0 and a bit vector based on its hash
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21 - a normal commit has a changeset where depth is increased by one and
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22 one bit vector bit is flipped based on its hash
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23 - a merge changeset pvec is constructed by copying changes from one pvec into
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24 the other to balance its depth
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25
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26 Properties:
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27
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28 - for linear changes, difference in depth is always <= hamming distance
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29 - otherwise, changes are probably divergent
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30 - when hamming distance is < 200, we can reliably detect when pvecs are near
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31
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32 Issues:
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33
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34 - hamming distance ceases to work over distances of ~ 200
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35 - detecting divergence is less accurate when the common ancestor is very close
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36 to either revision or total distance is high
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37 - this could probably be improved by modeling the relation between
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38 delta and hdist
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39
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40 Uses:
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41
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42 - a patch pvec can be used to locate the nearest available common ancestor for
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43 resolving conflicts
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44 - ordering of patches can be established without a DAG
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45 - two head pvecs can be compared to determine whether push/pull/merge is needed
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46 and approximately how many changesets are involved
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47 - can be used to find a heuristic divergence measure between changesets on
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48 different branches
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49 '''
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50
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51 import base85, util
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52 from node import nullrev
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53
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54 _size = 448 # 70 chars b85-encoded
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55 _bytes = _size / 8
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56 _depthbits = 24
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57 _depthbytes = _depthbits / 8
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58 _vecbytes = _bytes - _depthbytes
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59 _vecbits = _vecbytes * 8
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60 _radius = (_vecbits - 30) / 2 # high probability vecs are related
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61
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62 def _bin(bs):
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63 '''convert a bytestring to a long'''
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64 v = 0
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65 for b in bs:
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66 v = v * 256 + ord(b)
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67 return v
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68
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69 def _str(v, l):
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70 bs = ""
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71 for p in xrange(l):
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72 bs = chr(v & 255) + bs
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73 v >>= 8
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74 return bs
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75
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76 def _split(b):
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77 '''depth and bitvec'''
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78 return _bin(b[:_depthbytes]), _bin(b[_depthbytes:])
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79
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80 def _join(depth, bitvec):
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81 return _str(depth, _depthbytes) + _str(bitvec, _vecbytes)
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82
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83 def _hweight(x):
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84 c = 0
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85 while x:
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86 if x & 1:
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87 c += 1
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88 x >>= 1
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89 return c
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90 _htab = [_hweight(x) for x in xrange(256)]
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91
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92 def _hamming(a, b):
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93 '''find the hamming distance between two longs'''
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94 d = a ^ b
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95 c = 0
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96 while d:
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97 c += _htab[d & 0xff]
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98 d >>= 8
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99 return c
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100
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101 def _mergevec(x, y, c):
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102 # Ideally, this function would be x ^ y ^ ancestor, but finding
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103 # ancestors is a nuisance. So instead we find the minimal number
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104 # of changes to balance the depth and hamming distance
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105
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106 d1, v1 = x
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107 d2, v2 = y
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108 if d1 < d2:
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109 d1, d2, v1, v2 = d2, d1, v2, v1
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110
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111 hdist = _hamming(v1, v2)
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112 ddist = d1 - d2
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113 v = v1
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114 m = v1 ^ v2 # mask of different bits
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115 i = 1
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116
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117 if hdist > ddist:
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118 # if delta = 10 and hdist = 100, then we need to go up 55 steps
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119 # to the ancestor and down 45
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120 changes = (hdist - ddist + 1) / 2
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121 else:
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122 # must make at least one change
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123 changes = 1
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124 depth = d1 + changes
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125
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126 # copy changes from v2
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127 if m:
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128 while changes:
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129 if m & i:
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130 v ^= i
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131 changes -= 1
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132 i <<= 1
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133 else:
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134 v = _flipbit(v, c)
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135
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136 return depth, v
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137
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138 def _flipbit(v, node):
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139 # converting bit strings to longs is slow
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140 bit = (hash(node) & 0xffffffff) % _vecbits
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141 return v ^ (1<<bit)
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142
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143 def ctxpvec(ctx):
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144 '''construct a pvec for ctx while filling in the cache'''
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145 r = ctx._repo
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146 if not util.safehasattr(r, "_pveccache"):
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147 r._pveccache = {}
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148 pvc = r._pveccache
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149 if ctx.rev() not in pvc:
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150 cl = r.changelog
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151 for n in xrange(ctx.rev() + 1):
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152 if n not in pvc:
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153 node = cl.node(n)
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154 p1, p2 = cl.parentrevs(n)
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155 if p1 == nullrev:
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156 # start with a 'random' vector at root
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157 pvc[n] = (0, _bin((node * 3)[:_vecbytes]))
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158 elif p2 == nullrev:
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159 d, v = pvc[p1]
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160 pvc[n] = (d + 1, _flipbit(v, node))
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161 else:
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162 pvc[n] = _mergevec(pvc[p1], pvc[p2], node)
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163 bs = _join(*pvc[ctx.rev()])
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164 return pvec(base85.b85encode(bs))
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165
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166 class pvec(object):
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167 def __init__(self, hashorctx):
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168 if isinstance(hashorctx, str):
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169 self._bs = hashorctx
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170 self._depth, self._vec = _split(base85.b85decode(hashorctx))
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171 else:
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172 self._vec = ctxpvec(ctx)
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173
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174 def __str__(self):
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175 return self._bs
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176
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177 def __eq__(self, b):
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178 return self._vec == b._vec and self._depth == b._depth
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179
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180 def __lt__(self, b):
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181 delta = b._depth - self._depth
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182 if delta < 0:
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183 return False # always correct
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184 if _hamming(self._vec, b._vec) > delta:
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185 return False
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186 return True
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187
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188 def __gt__(self, b):
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189 return b < self
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190
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191 def __or__(self, b):
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192 delta = abs(b._depth - self._depth)
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193 if _hamming(self._vec, b._vec) <= delta:
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194 return False
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195 return True
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196
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197 def __sub__(self, b):
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198 if self | b:
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199 raise ValueError("concurrent pvecs")
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200 return self._depth - b._depth
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201
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202 def distance(self, b):
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203 d = abs(b._depth - self._depth)
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204 h = _hamming(self._vec, b._vec)
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205 return max(d, h)
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206
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207 def near(self, b):
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208 dist = abs(b.depth - self._depth)
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209 if dist > _radius or _hamming(self._vec, b._vec) > _radius:
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210 return False
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