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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.livevariable;

import java.util.List;
import org.checkerframework.dataflow.analysis.BackwardTransferFunction;
import org.checkerframework.dataflow.analysis.RegularTransferResult;
import org.checkerframework.dataflow.analysis.TransferInput;
import org.checkerframework.dataflow.analysis.TransferResult;
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.MethodInvocationNode;
import org.checkerframework.dataflow.cfg.node.Node;
import org.checkerframework.dataflow.cfg.node.ObjectCreationNode;
import org.checkerframework.dataflow.cfg.node.ReturnNode;
import org.checkerframework.dataflow.cfg.node.StringConcatenateAssignmentNode;

/** A live variable transfer function. */
public class LiveVarTransfer
    extends AbstractNodeVisitor<
        TransferResult, TransferInput>
    implements BackwardTransferFunction {

  @Override
  public LiveVarStore initialNormalExitStore(
      UnderlyingAST underlyingAST, List returnNodes) {
    return new LiveVarStore();
  }

  @Override
  public LiveVarStore initialExceptionalExitStore(UnderlyingAST underlyingAST) {
    return new LiveVarStore();
  }

  @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);
    processLiveVarInAssignment(n.getTarget(), n.getExpression(), transferResult.getRegularStore());
    return transferResult;
  }

  @Override
  public RegularTransferResult visitStringConcatenateAssignment(
      StringConcatenateAssignmentNode n, TransferInput p) {
    RegularTransferResult transferResult =
        (RegularTransferResult)
            super.visitStringConcatenateAssignment(n, p);
    processLiveVarInAssignment(
        n.getLeftOperand(), n.getRightOperand(), transferResult.getRegularStore());
    return transferResult;
  }

  @Override
  public RegularTransferResult visitMethodInvocation(
      MethodInvocationNode n, TransferInput p) {
    RegularTransferResult transferResult =
        (RegularTransferResult) super.visitMethodInvocation(n, p);
    LiveVarStore store = transferResult.getRegularStore();
    for (Node arg : n.getArguments()) {
      store.addUseInExpression(arg);
    }
    return transferResult;
  }

  @Override
  public RegularTransferResult visitObjectCreation(
      ObjectCreationNode n, TransferInput p) {
    RegularTransferResult transferResult =
        (RegularTransferResult) super.visitObjectCreation(n, p);
    LiveVarStore store = transferResult.getRegularStore();
    for (Node arg : n.getArguments()) {
      store.addUseInExpression(arg);
    }
    return transferResult;
  }

  @Override
  public RegularTransferResult visitReturn(
      ReturnNode n, TransferInput p) {
    RegularTransferResult transferResult =
        (RegularTransferResult) super.visitReturn(n, p);
    Node result = n.getResult();
    if (result != null) {
      LiveVarStore store = transferResult.getRegularStore();
      store.addUseInExpression(result);
    }
    return transferResult;
  }

  /**
   * Update the information of live variables from an assignment statement.
   *
   * @param variable the variable that should be killed
   * @param expression the expression in which the variables should be added
   * @param store the live variable store
   */
  private void processLiveVarInAssignment(Node variable, Node expression, LiveVarStore store) {
    store.killLiveVar(new LiveVarValue(variable));
    store.addUseInExpression(expression);
  }
}