Normal Forms for Boolean Expressions

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Transcription:

Normal Forms for Boolean Expressions A NORMAL FORM defines a class expressions s.t. a. Satisfy certain structural properties b. Are usually universal: able to express every boolean function 1. Disjunctive Normal Form (DNF) - Sum Of Products of literals, i.e., a variable or its negation Example: xy'z + yz + w 2. Conjunctive Normal Form (CNF) - Product of CLAUSES, i.e., sum of literals Example: (z+w).(x+y+z'+w), (x+y'+z).(y+z).w 3. Negation Normal Form (NNF): Negation appears only at leav Example: (x+yz).y Counter Example: (a.b) +c

Propositional Logic Decidability Complexity Theorem: Satisfiability of CNF formulas is NP-complete Theorem: Validity of DNF formulas is NP-complete Theorem: Satisfiability and Validity of arbitrary boolean formulas is NP-complete Intuition behind NP-completeness: Transformation b/w normal forms can have exponential blow-u

2SAT Satisfiability is Polynomial Time Implication Graph Notes: 1. Each clause is an implication e.g., x +y = x y 2. Vertex for each literal in claus 3. One edge for each implication For each variable Check if there is a path from X to X as well as from X to X Path checking on graph is Poly!!

Reduction of 3SAT CNF to Clique Problem on Grap Theorem: 3SAT and above is NP-comple Note: Clique is NP-comple

Are we doomed then? No, there are efficient methods that work VERY well for large classes of formulas We study two techniques that are the basis for widely used tools in practice ROBDD: A compact cannonical form for arbitrary boolean functions SAT solving: An efficient heuristic-based algorithm to check satisfiablity of CNF formulas

SAT Solver Handling Capacity Progress

Techniques underlying state-of-art SAT Solvers Motivation for SAT BDD is an overkill, especially if just want SAT (e.g., you don't want to do equivalence checking) BDDs often explode without good ordering Revolutionary heuristic-based improvements on CNF-based resolution/sat methods Isn't conversion to CNF itself a problem?? Tseitin Transformation: Can be done with linear increase in size provided you also allow for linear increase in variables Acknowledgements: Sharad Malik, Princeton, Daniel Kroening, Oxford University

Some Easy Situations for CNF SAT Every literal occurs with the same polarity e.g., (a+b )(c +d) Every clause has at least one literal that occurs with same polarity everywhere e.g., (a+b )(b+c ) Nontrivial cases: Every clause has at least one literal that occurs with both polarity everywhere e.g., (a+b )(c+d)(b+c +a )d

Resolution Rule Resolution of a pair of clauses with incompatible variables Pick EXACTLY one such pivot variable Resolvent, is union of rets of literals in the premise clauses a + b + c + f g + h + c + f a + b + g + h + f Completeness: It is complete or checking SAT/UNSAT, given a set of clauses Soundness: Resolvent EQUISAT Premise CNF a. Resolvent is true whenever premise CNF is true i.e., Resolvent is SAT iff premise CNF is SAT b. If premise CNF is UNSAT Resolvent is UNSAT e.g., {a} {a'} --> resolvent is empty

The Timeline 196: Davis Putnam Resolution Based 1 variables

Davis Putnam Algorithm M.Davis, H. Putnam, A computing procedure for quantification theory", J. of ACM, Vol. 7, pp. 21-214, 196 Existential abstraction using resolution Iteratively select a variable for resolution till no more variables are left. F = (a + b + c) (b + c + f ) (b + e) F = (a + b) (a + b ) (a + c) (a + c ) b F = (a + c + e) (c + e + f) b F = (a) (a + c) (a + c ) bc F = (a + e + f) ba F = (c) (c ) bcaef F = 1 bac F = ( ) SAT UNSAT Potential memory explosion problem!

The Timeline 1962 Davis Logemann Loveland Depth First Search 1 var 196 DP 1 var 1952 Quine 1 var

DLL Algorithm Davis, Logemann and Loveland M. Davis, G. Logemann and D. Loveland, A Machine Program for Theorem-Proving", Communications of ACM, Vol. 5, No. 7, pp. 394-397, 1962 Also known as DPLL for historical reasons Basic framework for many modern SAT solvers

What s the big deal? x 1 Conflict clause: x1 +x3+x5 x 2 x 3 x 3 x 5 x 4 x 5 x 5 x 4 x 5 Significantly prune the search space learned clause is useful forever! Useful in generating future conflict clauses.

Implications and Boolean Constraint Propagation Implication A variable is forced to be assigned to be True or False based on previous assignments. Unit clause rule (rule for elimination of one literal clauses) An unsatisfied clause is a unit clause if it has exactly one unassigned literal. (a +b + c)(b + c )(a + c ) Satisfied Literal Unsatisfied Literal a = T, b = T, c is unassigned Unassigned Literal The unassigned literal is implied because of the unit clause. Boolean Constraint Propagation (BCP) Iteratively apply the unit clause rule until there is no unit clause available. a.k.a. Unit Propagation Workhorse of DLL based algorithms.

Basic DLL Procedure - DFS (a + b + c) (a + c + d) (a + c + d ) (a + c + d) (a + c + d ) (b + c + d) (a + b + c ) (a + b + c)

Basic DPLL Procedure - DFS (a + b + c) (a + c + d) (a + c + d ) (a + c + d) (a + c + d ) (b + c + d) (a + b + c ) (a + b + c) a

Basic DPLL Procedure - DFS (a + b + c) (a + c + d) (a + c + d ) (a + c + d) (a + c + d ) (b + c + d) (a + b + c ) (a + b + c) a Decision

Basic DPLL Procedure - DFS (a + b + c) (a + c + d) (a + c + d ) (a + c + d) (a + c + d ) (b + c + d) (a + b + c ) (a + b + c) a b Decision

Basic DLL Procedure - DFS (a + b + c) (a + c + d) (a + c + d ) (a + c + d) (a + c + d ) (b + c + d) (a + b + c ) (a + b + c) a b c Decision

Basic DLL Procedure - DFS (a + b + c) (a + c + d) (a + c + d ) (a + c + d) (a + c + d ) (b + c + d) (a + b + c ) (a + b + c) c b a Implication Graph a @1 c @3 b@2 (a + c + d) d@3 (a + c + d ) (a + c + d ) Conflict!

Basic DLL Procedure - DFS (a + b + c) (a + c + d) (a + c + d ) (a + c + d) (a + c + d ) (b + c + d) (a + b + c ) (a + b + c) c b a a @ (a + c + d) d@3 Implication Graph (a + c + d) c @3 (a + c + d ) Conflict! b@2

Basic DPLL Procedure - DFS (a + b + c) (a + c + d) (a + c + d ) (a + c + d) (a + c + d ) (b + c + d) (a + b + c ) (a + b + c) c a b Backtrack

Basic DLL Procedure - DFS (a + b + c) (a + c + d) (a + c + d ) (a + c + d) (a + c + d ) (b + c + d) (a + b + c ) (a + b + c) a b c 1 Forced Decision a @1 (a + c + d) d@3 Conflict! c@3 (a + c + d ) b@2

Basic DPLL Procedure - DFS (a + b + c) (a + c + d) (a + c + d ) (a + c + d) (a + c + d ) (b + c + d) (a + b + c ) (a + b + c) a b c Backtrack 1

Basic DPLL Procedure - DFS (a + b + c) (a + c + d) (a + c + d ) (a + c + d) (a + c + d ) (b + c + d) (a + b + c ) (a + b + c) c 1 b a 1 Forced Decision

Basic DLL Procedure - DFS (a + b + c) (a + c + d) (a + c + d ) (a + c + d) (a + c + d ) (b + c + d) (a + b + c ) (a + b + c) c 1 b 1 c a Decision a @1 (a + c + d) d@3 Conflict! c @3 (a + c + d ) b@2

Basic DPLL Procedure - DFS (a + b + c) (a + c + d) (a + c + d ) (a + c + d) (a + c + d ) (b + c + d) (a + b + c ) (a + b + c) c 1 b 1 c a Backtrack

Basic DLL Procedure - DFS (a + b + c) (a + c + d) (a + c + d ) (a + c + d) (a + c + d ) (b + c + d) (a + b + c ) (a + b + c) c 1 b a 1 c 1 Forced Decision a @1 (a + c + d) d@3 Conflict! c@3 (a + c + d ) b@2

Basic PProcedure - DFS (a + b + c) (a + c + d) (a + c + d ) (a + c + d) (a + c + d ) (b + c + d) (a + b + c ) (a + b + c) c 1 b a 1 c 1 Backtrack

Basic DPLL Procedure - DFS (a + b + c) (a + c + d) (a + c + d ) (a + c + d) (a + c + d ) (b + c + d) (a + b + c ) (a + b + c) c 1 b a 1 c 1 1 Forced Decision

Basic DPLL Procedure - DFS (a + b + c) (a + c + d) (a + c + d ) (a + c + d) (a + c + d ) (b + c + d) (a + b + c ) (a + b + c) c 1 b a 1 c 1 1 b Decision

Basic DLL Procedure - DFS (a + b + c) (a + c + d) (a + c + d ) (a + c + d) (a + c + d ) (b + c + d) (a + b + c ) (a + b + c) c 1 b a 1 c 1 1 b a@1 (a + b + c) c@2 Conflict! b @2 (a + b + c )

Basic DPLL Procedure - DFS (a + b + c) (a + c + d) (a + c + d ) (a + c + d) (a + c + d ) (b + c + d) (a + b + c ) (a + b + c) c 1 b a 1 c 1 1 b Backtrack

Basic DLL Procedure - DFS (a + b + c) (a + c + d) (a + c + d ) (a + c + d) (a + c + d ) (b + c + d) (a + b + c ) (a + b + c) c 1 b a 1 c 1 1 b 1 Forced Decision a@1 (a + b + c) c@2 b@2

Basic DLL Procedure - DFS (a + b + c) (a + c + d) (a + c + d ) (a + c + d) (a + c + d ) (b + c + d) (a + b + c ) (a + b + c) c 1 b a 1 c 1 1 b 1 a@1 (a + b + c) (b + c + d) c@2 d@2 b@2

Basic DLL Procedure - DFS (a + b + c) (a + c + d) (a + c + d ) (a + c + d) (a + c + d ) (b + c + d) (a + b + c ) (a + b + c) c 1 b a 1 c 1 1 b 1 SAT a@1 (a + b + c) (b + c + d) c@2 d@2 b@2

Features of DPLL Eliminates the exponential memory requirements of DP Exponential time is still a problem Limited practical applicability largest use seen in automatic theorem proving Very limited size of problems are allowed 32K word memory Problem size limited by total size of clauses (13 clauses)

The Timeline 1986 Binary Decision Diagrams (BDDs) 1 var 196 DP 1 var 1952 Quine 1 var 1962 DLL 1 var

Using BDDs to Solve SAT R. Bryant. Graph-based algorithms for Boolean function manipulation. IEEE Trans. on Computers, C-35, 8:677-691, 1986. Store the function in a Directed Acyclic Graph (DAG) representation. Compacted form of the function decision tree. Reduction rules guarantee canonicity under fixed variable order. Provides for efficient Boolean function manipulation. Overkill for SAT.

The Timeline 196 DP 1 var 1996 GRASP Conflict Driven Learning, Non-chornological Backtracking 1k var 1988 SOCRATES 3k var 1994 Hannibal 3k var 1952 Quine 1 var 1962 DLL 1 var 1986 BDDs 1 var 1992 GSAT 3 var 1996 Stålmarck 1k var

The Timeline 196 DP 1 var 1996 GRASP Conflict Driven Learning, Non-chornological Backtracking 1k var 1988 SOCRATES 3k var 1994 Hannibal 3k var 1952 Quine 1 var 1962 DLL 1 var 1986 BDDs 1 var 1992 GSAT 3 var 1996 Stålmarck 1k var

GRASP Marques-Silva and Sakallah [SS96,SS99] J. P. Marques-Silva and K. A. Sakallah, "GRASP -- A New Search Algorithm for Satisfiability, Proc. ICCAD 1996. J. P. Marques-Silva and Karem A. Sakallah, GRASP: A Search Algorithm for Propositional Satisfiability, IEEE Trans. Computers, C-48, 5:56-521, 1999. Incorporates conflict driven learning and non-chronological backtracking Practical SAT instances can be solved in reasonable time Bayardo and Schrag s RelSAT also proposed conflict driven learning [BS97] R. J. Bayardo Jr. and R. C. Schrag Using CSP look-back techniques to solve real world SAT instances. Proc. AAAI, pp. 23-28, 1997(144 citations)

What can we learn from the implication graph? The decisions causing conflicts are the roots. i.e., the sinks None of the other decisions, e.g., at levels 4,5, matter Learn you can t have assignment: (x1. x9. x1. x11 )

backtrack to the largest decision level in the conflict Clause!!

What constitutes a sufficient condition for conflict: 1. Make a cut to separate conflict side from reason side 2. Conjunction (C) of literals labeling the set of nodes on the reason side that have at least one edge to the conflict side Conflict Clause: Negation of C

What is a good conflict clause? Must be new!! An asserting clause that flips decision made at current level Backtracks to the lowest level? Shorter clauses

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