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<divclass="title">sat_decision.h</div></div>
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<ahref="sat__decision_8h.html">Go to the documentation of this file.</a><divclass="fragment"><divclass="line"><aname="l00001"></a><spanclass="lineno"> 1</span> <spanclass="comment">// Copyright 2010-2018 Google LLC</span></div>
<divclass="line"><aname="l00002"></a><spanclass="lineno"> 2</span> <spanclass="comment">// Licensed under the Apache License, Version 2.0 (the "License");</span></div>
<divclass="line"><aname="l00003"></a><spanclass="lineno"> 3</span> <spanclass="comment">// you may not use this file except in compliance with the License.</span></div>
<divclass="line"><aname="l00004"></a><spanclass="lineno"> 4</span> <spanclass="comment">// You may obtain a copy of the License at</span></div>
<divclass="line"><aname="l00008"></a><spanclass="lineno"> 8</span> <spanclass="comment">// Unless required by applicable law or agreed to in writing, software</span></div>
<divclass="line"><aname="l00009"></a><spanclass="lineno"> 9</span> <spanclass="comment">// distributed under the License is distributed on an "AS IS" BASIS,</span></div>
<divclass="line"><aname="l00010"></a><spanclass="lineno"> 10</span> <spanclass="comment">// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.</span></div>
<divclass="line"><aname="l00011"></a><spanclass="lineno"> 11</span> <spanclass="comment">// See the License for the specific language governing permissions and</span></div>
<divclass="line"><aname="l00012"></a><spanclass="lineno"> 12</span> <spanclass="comment">// limitations under the License.</span></div>
<divclass="line"><aname="l00032"></a><spanclass="lineno"> 32</span> <spanclass="comment">// Implement the SAT branching policy responsible for deciding the next Boolean</span></div>
<divclass="line"><aname="l00033"></a><spanclass="lineno"> 33</span> <spanclass="comment">// variable to branch on, and its polarity (true or false).</span></div>
<divclass="line"><aname="l00038"></a><spanclass="lineno"> 38</span> <spanclass="comment">// Notifies that more variables are now present. Note that currently this may</span></div>
<divclass="line"><aname="l00039"></a><spanclass="lineno"> 39</span> <spanclass="comment">// change the current variable order because the priority queue need to be</span></div>
<divclass="line"><aname="l00043"></a><spanclass="lineno"> 43</span> <spanclass="comment">// Reinitializes the decision heuristics (which variables to choose with which</span></div>
<divclass="line"><aname="l00044"></a><spanclass="lineno"> 44</span> <spanclass="comment">// polarity) according to the current parameters. Note that this also resets</span></div>
<divclass="line"><aname="l00045"></a><spanclass="lineno"> 45</span> <spanclass="comment">// the activity of the variables to 0. Note that this function is lazy, and</span></div>
<divclass="line"><aname="l00046"></a><spanclass="lineno"> 46</span> <spanclass="comment">// the work will only happen on the first NextBranch() to cover the cases when</span></div>
<divclass="line"><aname="l00047"></a><spanclass="lineno"> 47</span> <spanclass="comment">// this policy is not used at all.</span></div>
<divclass="line"><aname="l00050"></a><spanclass="lineno"> 50</span> <spanclass="comment">// Returns next decision to branch upon. This shouldn't be called if all the</span></div>
<divclass="line"><aname="l00051"></a><spanclass="lineno"> 51</span> <spanclass="comment">// variables are assigned.</span></div>
<divclass="line"><aname="l00054"></a><spanclass="lineno"> 54</span> <spanclass="comment">// Updates statistics about literal occurences in constraints.</span></div>
<divclass="line"><aname="l00055"></a><spanclass="lineno"> 55</span> <spanclass="comment">// Input is a canonical linear constraint of the form (terms <= rhs).</span></div>
<divclass="line"><aname="l00059"></a><spanclass="lineno"> 59</span> <spanclass="comment">// Bumps the activity of all variables appearing in the conflict. All literals</span></div>
<divclass="line"><aname="l00060"></a><spanclass="lineno"> 60</span> <spanclass="comment">// must be currently assigned. See VSIDS decision heuristic: Chaff:</span></div>
<divclass="line"><aname="l00061"></a><spanclass="lineno"> 61</span> <spanclass="comment">// Engineering an Efficient SAT Solver. M.W. Moskewicz et al. ANNUAL ACM IEEE</span></div>
<divclass="line"><aname="l00065"></a><spanclass="lineno"> 65</span> <spanclass="comment">// Updates the increment used for activity bumps. This is basically the same</span></div>
<divclass="line"><aname="l00066"></a><spanclass="lineno"> 66</span> <spanclass="comment">// as decaying all the variable activities, but it is a lot more efficient.</span></div>
<divclass="line"><aname="l00069"></a><spanclass="lineno"> 69</span> <spanclass="comment">// Called on Untrail() so that we can update the set of possible decisions.</span></div>
<divclass="line"><aname="l00072"></a><spanclass="lineno"> 72</span> <spanclass="comment">// Called on a new conflict before Untrail(). The trail before the given index</span></div>
<divclass="line"><aname="l00073"></a><spanclass="lineno"> 73</span> <spanclass="comment">// is used in the phase saving heuristic as a partial assignment.</span></div>
<divclass="line"><aname="l00076"></a><spanclass="lineno"> 76</span> <spanclass="comment">// By default, we alternate between a stable phase (better suited for finding</span></div>
<divclass="line"><aname="l00077"></a><spanclass="lineno"> 77</span> <spanclass="comment">// SAT solution) and a more restart heavy phase more suited for proving UNSAT.</span></div>
<divclass="line"><aname="l00078"></a><spanclass="lineno"> 78</span> <spanclass="comment">// This changes a bit the polarity heuristics and is controlled from within</span></div>
<divclass="line"><aname="l00083"></a><spanclass="lineno"> 83</span> <spanclass="comment">// This is used to temporarily disable phase_saving when we do some probing</span></div>
<divclass="line"><aname="l00084"></a><spanclass="lineno"> 84</span> <spanclass="comment">// during search for instance.</span></div>
<divclass="line"><aname="l00089"></a><spanclass="lineno"> 89</span> <spanclass="comment">// Gives a hint so the solver tries to find a solution with the given literal</span></div>
<divclass="line"><aname="l00090"></a><spanclass="lineno"> 90</span> <spanclass="comment">// set to true. Currently this take precedence over the phase saving heuristic</span></div>
<divclass="line"><aname="l00091"></a><spanclass="lineno"> 91</span> <spanclass="comment">// and a variable with a preference will always be branched on according to</span></div>
<divclass="line"><aname="l00092"></a><spanclass="lineno"> 92</span> <spanclass="comment">// this preference.</span></div>
<divclass="line"><aname="l00094"></a><spanclass="lineno"> 94</span> <spanclass="comment">// The weight is used as a tie-breaker between variable with the same</span></div>
<divclass="line"><aname="l00095"></a><spanclass="lineno"> 95</span> <spanclass="comment">// activities. Larger weight will be selected first. A weight of zero is the</span></div>
<divclass="line"><aname="l00096"></a><spanclass="lineno"> 96</span> <spanclass="comment">// default value for the other variables.</span></div>
<divclass="line"><aname="l00098"></a><spanclass="lineno"> 98</span> <spanclass="comment">// Note(user): Having a lot of different weights may slow down the priority</span></div>
<divclass="line"><aname="l00099"></a><spanclass="lineno"> 99</span> <spanclass="comment">// queue operations if there is millions of variables.</span></div>
<divclass="line"><aname="l00102"></a><spanclass="lineno"> 102</span> <spanclass="comment">// Returns the vector of the current assignment preferences.</span></div>
<divclass="line"><aname="l00109"></a><spanclass="lineno"> 109</span> <spanclass="comment">// Rescales activity value of all variables when one of them reached the max.</span></div>
<divclass="line"><aname="l00112"></a><spanclass="lineno"> 112</span> <spanclass="comment">// Reinitializes the inital polarity of all the variables with an index</span></div>
<divclass="line"><aname="l00113"></a><spanclass="lineno"> 113</span> <spanclass="comment">// greater than or equal to the given one.</span></div>
<divclass="line"><aname="l00116"></a><spanclass="lineno"> 116</span> <spanclass="comment">// Code used for resetting the initial polarity at the beginning of each</span></div>
<divclass="line"><aname="l00123"></a><spanclass="lineno"> 123</span> <spanclass="comment">// Adds the given variable to var_ordering_ or updates its priority if it is</span></div>
<divclass="line"><aname="l00132"></a><spanclass="lineno"> 132</span> <spanclass="comment">// Variable ordering (priority will be adjusted dynamically). queue_elements_</span></div>
<divclass="line"><aname="l00133"></a><spanclass="lineno"> 133</span> <spanclass="comment">// holds the elements used by var_ordering_ (it uses pointers).</span></div>
<divclass="line"><aname="l00135"></a><spanclass="lineno"> 135</span> <spanclass="comment">// Note that we recover the variable that a WeightedVarQueueElement refers to</span></div>
<divclass="line"><aname="l00136"></a><spanclass="lineno"> 136</span> <spanclass="comment">// by its position in the queue_elements_ vector, and we can recover the later</span></div>
<divclass="line"><aname="l00137"></a><spanclass="lineno"> 137</span> <spanclass="comment">// using (pointer - &queue_elements_[0]).</span></div>
<divclass="line"><aname="l00142"></a><spanclass="lineno"> 142</span> <spanclass="comment">// Priority order. The IntegerPriorityQueue returns the largest element</span></div>
<divclass="line"><aname="l00145"></a><spanclass="lineno"> 145</span> <spanclass="comment">// Note(user): We used to also break ties using the variable index, however</span></div>
<divclass="line"><aname="l00146"></a><spanclass="lineno"> 146</span> <spanclass="comment">// this has two drawbacks:</span></div>
<divclass="line"><aname="l00147"></a><spanclass="lineno"> 147</span> <spanclass="comment">// - On problem with many variables, this slow down quite a lot the priority</span></div>
<divclass="line"><aname="l00148"></a><spanclass="lineno"> 148</span> <spanclass="comment">// queue operations (which do as little work as possible and hence benefit</span></div>
<divclass="line"><aname="l00149"></a><spanclass="lineno"> 149</span> <spanclass="comment">// from having the majority of elements with a priority of 0).</span></div>
<divclass="line"><aname="l00150"></a><spanclass="lineno"> 150</span> <spanclass="comment">// - It seems to be a bad heuristics. One reason could be that the priority</span></div>
<divclass="line"><aname="l00151"></a><spanclass="lineno"> 151</span> <spanclass="comment">// queue will automatically diversify the choice of the top variables</span></div>
<divclass="line"><aname="l00152"></a><spanclass="lineno"> 152</span> <spanclass="comment">// amongst the ones with the same priority.</span></div>
<divclass="line"><aname="l00154"></a><spanclass="lineno"> 154</span> <spanclass="comment">// Note(user): For the same reason as explained above, it is probably a good</span></div>
<divclass="line"><aname="l00155"></a><spanclass="lineno"> 155</span> <spanclass="comment">// idea not to have too many different values for the tie_breaker field. I</span></div>
<divclass="line"><aname="l00156"></a><spanclass="lineno"> 156</span> <spanclass="comment">// am not even sure we should have such a field...</span></div>
<divclass="line"><aname="l00165"></a><spanclass="lineno"> 165</span> <spanclass="comment">// TODO(user): Experiment with float. In the rest of the code, we use</span></div>
<divclass="line"><aname="l00166"></a><spanclass="lineno"> 166</span> <spanclass="comment">// double, but maybe we don't need that much precision. Using float here may</span></div>
<divclass="line"><aname="l00167"></a><spanclass="lineno"> 167</span> <spanclass="comment">// save memory and make the PQ operations faster.</span></div>
<divclass="line"><aname="l00176"></a><spanclass="lineno"> 176</span> <spanclass="comment">// This is used for the branching heuristic described in "Learning Rate Based</span></div>
<divclass="line"><aname="l00177"></a><spanclass="lineno"> 177</span> <spanclass="comment">// Branching Heuristic for SAT solvers", J.H.Liang, V. Ganesh, P. Poupart,</span></div>
<divclass="line"><aname="l00178"></a><spanclass="lineno"> 178</span> <spanclass="comment">// K.Czarnecki, SAT 2016.</span></div>
<divclass="line"><aname="l00180"></a><spanclass="lineno"> 180</span> <spanclass="comment">// The entries are sorted by trail index, and one can get the number of</span></div>
<divclass="line"><aname="l00181"></a><spanclass="lineno"> 181</span> <spanclass="comment">// conflicts during which a variable at a given trail index i was assigned by</span></div>
<divclass="line"><aname="l00182"></a><spanclass="lineno"> 182</span> <spanclass="comment">// summing the entry.count for all entries with a trail index greater than i.</span></div>
<divclass="line"><aname="l00190"></a><spanclass="lineno"> 190</span> <spanclass="comment">// Whether the priority of the given variable needs to be updated in</span></div>
<divclass="line"><aname="l00191"></a><spanclass="lineno"> 191</span> <spanclass="comment">// var_ordering_. Note that this is only accessed for assigned variables and</span></div>
<divclass="line"><aname="l00192"></a><spanclass="lineno"> 192</span> <spanclass="comment">// that for efficiency it is indexed by trail indices. If</span></div>
<divclass="line"><aname="l00194"></a><spanclass="lineno"> 194</span> <spanclass="comment">// true when we untrail var, then either var need to be inserted in the queue,</span></div>
<divclass="line"><aname="l00195"></a><spanclass="lineno"> 195</span> <spanclass="comment">// or we need to notify that its priority has changed.</span></div>
<divclass="line"><aname="l00198"></a><spanclass="lineno"> 198</span> <spanclass="comment">// Increment used to bump the variable activities.</span></div>
<divclass="line"><aname="l00201"></a><spanclass="lineno"> 201</span> <spanclass="comment">// Stores variable activity and the number of time each variable was "bumped".</span></div>
<divclass="line"><aname="l00202"></a><spanclass="lineno"> 202</span> <spanclass="comment">// The later is only used with the ERWA heuristic.</span></div>
<divclass="line"><aname="l00207"></a><spanclass="lineno"> 207</span> <spanclass="comment">// If the polarity if forced (externally) we alway use this first.</span></div>
<divclass="line"><aname="l00211"></a><spanclass="lineno"> 211</span> <spanclass="comment">// If we are in a stable phase, we follow the current target.</span></div>
<divclass="line"><aname="l00217"></a><spanclass="lineno"> 217</span> <spanclass="comment">// Otherwise we follow var_polarity_ which is reset at the beginning of</span></div>
<divclass="line"><aname="l00218"></a><spanclass="lineno"> 218</span> <spanclass="comment">// each new polarity phase. This is also overwritten by phase saving.</span></div>
<divclass="line"><aname="l00219"></a><spanclass="lineno"> 219</span> <spanclass="comment">// Each phase last for an arithmetically increasing number of conflicts.</span></div>
<divclass="line"><aname="l00225"></a><spanclass="lineno"> 225</span> <spanclass="comment">// The longest partial assignment since the last reset.</span></div>
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