/*
 * Copyright 2009-2010 by The Regents of the University of California
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * you may obtain a copy of the License from
 * 
 *     http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package edu.uci.ics.hyracks.algebricks.rewriter.rules;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import edu.uci.ics.hyracks.algebricks.common.exceptions.AlgebricksException;
import edu.uci.ics.hyracks.algebricks.core.algebra.base.ILogicalOperator;
import edu.uci.ics.hyracks.algebricks.core.algebra.base.IOptimizationContext;
import edu.uci.ics.hyracks.algebricks.core.algebra.base.LogicalVariable;
import edu.uci.ics.hyracks.algebricks.core.algebra.operators.logical.AbstractLogicalOperator;
import edu.uci.ics.hyracks.algebricks.core.algebra.operators.logical.visitors.VariableUtilities;

/**
 * Inlines variables that are referenced exactly once.
 * 
 * Preconditions/Assumptions:
 * Assumes no projects are in the plan.
 * 
 * Example assuming variable $$3 is referenced exactly once.
 * 
 * Before plan:
 * select (funcA($$3))
 *   ...
 *     assign [$$3] <- [field-access($$0, 1)]
 * 
 * After plan:
 * select (funcA(field-access($$0, 1))
 *   ...
 *     assign [] <- []
 */
public class InlineSingleReferenceVariablesRule extends InlineVariablesRule {

    // Maps from variable to a list of operators using that variable.
    protected Map<LogicalVariable, List<ILogicalOperator>> usedVarsMap = new HashMap<LogicalVariable, List<ILogicalOperator>>();
    protected List<LogicalVariable> usedVars = new ArrayList<LogicalVariable>();
    
    @Override
    protected void prepare(IOptimizationContext context) {
        super.prepare(context);
        usedVarsMap.clear();
        usedVars.clear();
    }
    
    @Override
    protected boolean performFinalAction() throws AlgebricksException {
        boolean modified = false;
        for (Map.Entry<LogicalVariable, List<ILogicalOperator>> entry : usedVarsMap.entrySet()) {
            // Perform replacement only if variable is referenced a single time.
            if (entry.getValue().size() == 1) {
                ILogicalOperator op = entry.getValue().get(0);
                if (!op.requiresVariableReferenceExpressions()) {
                    inlineVisitor.setOperator(op);
                    inlineVisitor.setTargetVariable(entry.getKey());
                    if (op.acceptExpressionTransform(inlineVisitor)) {
                        modified = true;
                    }
                    inlineVisitor.setTargetVariable(null);
                }         
            }
        }
        return modified;
    }

    @Override
    protected boolean performBottomUpAction(AbstractLogicalOperator op) throws AlgebricksException {
        usedVars.clear();
        VariableUtilities.getUsedVariables(op, usedVars);
        for (LogicalVariable var : usedVars) {
            List<ILogicalOperator> opsUsingVar = usedVarsMap.get(var);
            if (opsUsingVar == null) {
                opsUsingVar = new ArrayList<ILogicalOperator>();
                usedVarsMap.put(var, opsUsingVar);
            }
            opsUsingVar.add(op);
        }
        return false;
    }
}