/* 
 * Kodkod -- Copyright (c) 2005-present, Emina Torlak
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */
package kodkod.engine.satlab;

/**
 * Provides an interface to a SAT solver that can generate
 * proofs of unsatisfiability.
 * 
 * @specfield variables: set [1..)
 * @specfield clauses: set Clause
 * @specfield resolvents: set Clause  
 * @invariant all i: [2..) | i in variables => i-1 in variables
 * @invariant all c: clauses + resolvents | all lit: c.lits | lit in variables || -lit in variables
 * @invariant all c: clauses + resolvents | all disj i,j: c.lits | abs(i) != abs(j)
 * @author Emina Torlak
 */
public interface SATProver extends SATSolver {
	
	/**
	 * Returns a resolution-based proof of  unsatisfiability of this.clauses.
	 * @requires {@link SATSolver#solve()} has been called, and it returned false
	 * @return { t: ResolutionTrace | t.prover = this }
	 * @throws IllegalStateException  {@link SATSolver#solve()} has not been called, 
	 * or the last call to {@link SATSolver#solve()} returned true
	 */
	public ResolutionTrace proof();

	/**
	 * Uses the given reduction strategy to remove irrelevant clauses from 
	 * the set of unsatisfiable clauses stored in this prover.  
	 * A clause c is irrelevant iff this.clauses - c is unsatisfiable.
	 * The removal algorithm works as follows:
	 * <pre>
	 * for (IntSet next = strategy.next(this.proof()); !next.isEmpty(); next = strategy.next(this.proof())) {
	 *  let oldClauses = this.clauses, oldResolvents = this.resolvents
	 *  clear this.clauses
	 *  clear this.resolvents
	 *  for(Clause c : this.proof().elts[next]) {
	 *    if (no c.antecedents)
	 *      add c to this.clauses
	 *    else
	 *      add c to this.resolvents
	 *  }
	 *  if (this.solve()) {
	 *   this.clauses = oldClauses 
	 *   this.resolvents = oldResolvents
	 *  }
	 * }
	 * </pre>
	 * @requires {@link SATSolver#solve()} has been called, and it returned false
	 * @ensures modifies this.clauses and this.resolvents according to the algorithm described above 
	 * @throws IllegalStateException  {@link SATSolver#solve()} has not been called, 
	 * or the last call to {@link SATSolver#solve()} returned true
	 * @see ReductionStrategy
	 */
	public void reduce(ReductionStrategy strategy);
	
}