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//               The Next Generation Logic Library                       //
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//  Copyright 2015-20xx Christoph Zengler                                //
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package org.logicng.knowledgecompilation.bdds.orderings;

import org.logicng.formulas.BinaryOperator;
import org.logicng.formulas.Formula;
import org.logicng.formulas.Literal;
import org.logicng.formulas.Not;
import org.logicng.formulas.PBConstraint;
import org.logicng.formulas.Variable;

import java.util.ArrayList;
import java.util.LinkedHashSet;
import java.util.List;

/**
 * A depth-first-search BDD variable ordering.  Traverses the formula in a DFS manner
 * and gathers all variables in the occurrence.
 * @version 2.0.0
 * @since 1.4.0
 */
public final class DFSOrdering implements VariableOrderingProvider {

    @Override
    public List getOrder(final Formula formula) {
        final LinkedHashSet order = new LinkedHashSet<>(formula.variables().size());
        dfs(formula, order);
        return new ArrayList<>(order);
    }

    private void dfs(final Formula formula, final LinkedHashSet variables) {
        switch (formula.type()) {
            case LITERAL:
                variables.add(((Literal) formula).variable());
                break;
            case NOT:
                dfs(((Not) formula).operand(), variables);
                break;
            case IMPL:
            case EQUIV:
                final BinaryOperator op = (BinaryOperator) formula;
                dfs(op.left(), variables);
                dfs(op.right(), variables);
                break;
            case AND:
            case OR:
                for (final Formula o : formula) {
                    dfs(o, variables);
                }
                break;
            case PBC:
                final PBConstraint pbc = (PBConstraint) formula;
                for (final Literal lit : pbc.operands()) {
                    variables.add(lit.variable());
                }
                break;
        }
    }
}