(a = 0)  (c = 0)  (f = 0)  (  = 0)  = (a + b)(¬b + c + d) (¬b + e)(¬d + ¬e + f)  Clause Recording During backtrack search, for each conflict create.

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Presentation transcript:

(a = 0)  (c = 0)  (f = 0)  (  = 0)  = (a + b)(¬b + c + d) (¬b + e)(¬d + ¬e + f)  Clause Recording During backtrack search, for each conflict create clause that explains and prevents recurrence of same conflict Assume (decisions) c = 0 and f = 0 Assign a = 0 and imply assignments A conflict is reached: (¬d + ¬e + f) is unsat (  = 1)  (a = 1)  (c = 1)  (f = 1)  create new clause: (a + c + f) Thanks Ines Lynce! + stands for 

Clause Recording Clauses derived from conflicts can also be viewed as the result of applying selective resolution  = (a + b)(¬b + c + d) (¬b + e)(¬d + ¬e + f)  (a + c + d) resolution (a + c + ¬e + f) (a + c + f) (a + e) Unit clause: prevents conflict and implies assignment a = 1 Clause (a + c + f) would have prevented the conflict !

More on Clause Recording Clause recording can be made polynomial –For each conflict 1 clause is recorded –Keep clauses of size  K –Larger clauses get deleted when (become) unresolved –Growth in the number of clauses is polynomial in K Relevance-based learning –Delete large unresolved clauses with  M free literals

(a = 1)  (c = 0)  (f = 0)  (  = 0) Non-Chronological Backtracking During backtrack search, in the presence of conflicts, backtrack to one of the causes of the conflict  =(a + b)(¬b + c + d) (¬b + e)(¬d + ¬e + f) (a + c + f)(¬a + g)(¬g + b)(¬h + j)(¬i + k)  Assume (decisions) c = 0, f = 0, h = 0 and i = 0 Assignment a = 0 caused conflict  clause (a + c + f) created (a + c + f) implies a = 1  =(a + b)(¬b + c + d) (¬b + e)(¬d + ¬e + f) (a + c + f)(¬a + g)(¬g + b)(¬h + j)(¬i + k)   =(a + b)(¬b + c + d) (¬b + e)(¬d + ¬e + f) (a + c + f)(¬a + g)(¬g + b)(¬h + j)(¬i + k)   =(a + b)(¬b + c + d) (¬b + e)(¬d + ¬e + f) (a + c + f)(¬a + g)(¬g + b)(¬h + j)(¬i + k)  A conflict is again reached: (¬d + ¬e + f) is unsat (  = 1)  (a = 0)  (c = 1)  (f = 1)  create new clause: (¬a + c + f)

Non-Chronological Backtracking Created clauses: (a + c + f) and (¬a + c + f)  backtrack to most recent decision: f = 0  created clauses/implicates: (a + c + f), (¬a + c + f), and (c + f) Apply resolution: new unsat clause (c + f) 0 0 c f i h 0 0 a 0 1 (c + f)

 =(a + b)(¬b + c + ¬g) (¬b + ¬h)(g + h + i) (¬i + d) (¬i + e)(¬d + ¬e + f)   =(a + b)(¬b + c + ¬g) (¬b + ¬h)(g + h + i) (¬i + d) (¬i + e)(¬d + ¬e + f)   =(a + b)(¬b + c + ¬g) (¬b + ¬h)(g + h + i) (¬i + d) (¬i + e)(¬d + ¬e + f)  Conflict-Induced Assignments Exploit structure of conflicting implication sequences for identifying more necessary assignments Assume (decisions) c = 0, f = 0, and a = 0, and imply assignments ab g h i d e f c  recorded clause: (a + c + f) implied assignment: a = 1 clauses: ( ¬ i + f) and (a + c + i) assignments: i = 0 and a = 1