GRASP: A Search Algorithm for Propositional Satisfiability

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GRASP A Search Algorithm for Propositional
Satisfiability
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Sat in a Nutshell

• Given a Boolean formula, find a variable
  assignment such that the formula evaluates to 1,
  or prove that no such assignment exists.
• For n variable,s there are 2n possible truth
  assignments to be checked.
• NP-Complete problem.

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Problem Representation

• Conjunctive Normal Form
• F (ab)(abc)
• Simple representation (more efficient data
  structures)
• Logic circuit representation
• Circuits have structural and direction
  information
• Circuit -gt CNF conversion is straightforward

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DLL Algorithm
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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
• Basic framework for many modern SAT solvers
• Also known as DPLL for historical reasons
• DFS depth first search

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Implications and BooleanConstraint Propagation
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Implications and BooleanConstraint 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.
• 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
• Workhorse of DLL based algorithms.

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GRASP

1. Marques-Silva and Sakallah SS96,SS99
2. J.P.Marques-Silva and K.A.Sakallah,GRASP A New
   Search Algorithm for Satisfiability, Proc. ICCAD
   1996. (49 citations)
3. Incorporates conflict driven learning and
   non-chronological backtracking
4. Practical SAT instances can be solved in
   reasonable time
5. Bayardo and Schrags RelSAT also proposed
   conflict driven learning BS97

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Conflict Driven Learning and Non-Chronological
Backtracking
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Conflict Clause

• Significantly prune the search speace
• Learned clause is useful forever
• Useful in generating future conflict clauses

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GRASP A Search Algorithm for Propositional
Satisfiability

• EE878C Homework 2
• 2002/11/1
• KAIST, EECS ICS Lab
• Lee, Dongsoo