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The Checker Framework enhances Java's type system to make it more powerful and useful. This lets software developers detect and prevent errors in their Java programs. The Checker Framework includes compiler plug-ins ("checkers") that find bugs or verify their absence. It also permits you to write your own compiler plug-ins.

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package org.checkerframework.dataflow.reachingdef;

import org.checkerframework.checker.nullness.qual.Nullable;
import org.checkerframework.dataflow.analysis.ForwardTransferFunction;
import org.checkerframework.dataflow.analysis.RegularTransferResult;
import org.checkerframework.dataflow.analysis.TransferInput;
import org.checkerframework.dataflow.analysis.TransferResult;
import org.checkerframework.dataflow.analysis.UnusedAbstractValue;
import org.checkerframework.dataflow.cfg.UnderlyingAST;
import org.checkerframework.dataflow.cfg.node.AbstractNodeVisitor;
import org.checkerframework.dataflow.cfg.node.AssignmentNode;
import org.checkerframework.dataflow.cfg.node.LocalVariableNode;
import org.checkerframework.dataflow.cfg.node.Node;

import java.util.List;

/**
 * The reaching definition transfer function. The transfer function processes the
 * ReachingDefinitionNode in ReachingDefinitionStore, killing the node with same LHS and putting new
 * generated node into the store. See dataflow manual for more details.
 */
public class ReachingDefinitionTransfer
        extends AbstractNodeVisitor<
                TransferResult,
                TransferInput>
        implements ForwardTransferFunction {

    /** Create a new ReachingDefinitionTransfer. */
    public ReachingDefinitionTransfer() {}

    @Override
    public ReachingDefinitionStore initialStore(
            UnderlyingAST underlyingAST, @Nullable List parameters) {
        return new ReachingDefinitionStore();
    }

    @Override
    public RegularTransferResult visitNode(
            Node n, TransferInput p) {
        return new RegularTransferResult<>(null, p.getRegularStore());
    }

    @Override
    public RegularTransferResult visitAssignment(
            AssignmentNode n, TransferInput p) {
        RegularTransferResult transferResult =
                (RegularTransferResult)
                        super.visitAssignment(n, p);
        processDefinition(n, transferResult.getRegularStore());
        return transferResult;
    }

    /**
     * Update a reaching definition node in the store from an assignment statement.
     *
     * @param def the definition that should be put into the store
     * @param store the reaching definition store
     */
    private void processDefinition(AssignmentNode def, ReachingDefinitionStore store) {
        store.killDef(def.getTarget());
        store.putDef(new ReachingDefinitionNode(def));
    }
}