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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.constantpropagation;

import java.util.List;
import org.checkerframework.dataflow.analysis.ConditionalTransferResult;
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.cfg.UnderlyingAST;
import org.checkerframework.dataflow.cfg.node.AbstractNodeVisitor;
import org.checkerframework.dataflow.cfg.node.AssignmentNode;
import org.checkerframework.dataflow.cfg.node.EqualToNode;
import org.checkerframework.dataflow.cfg.node.IntegerLiteralNode;
import org.checkerframework.dataflow.cfg.node.LocalVariableNode;
import org.checkerframework.dataflow.cfg.node.Node;

public class ConstantPropagationTransfer
    extends AbstractNodeVisitor<
        TransferResult,
        TransferInput>
    implements ForwardTransferFunction {

  @Override
  public ConstantPropagationStore initialStore(
      UnderlyingAST underlyingAST, List parameters) {
    ConstantPropagationStore store = new ConstantPropagationStore();
    return store;
  }

  @Override
  public TransferResult visitLocalVariable(
      LocalVariableNode node, TransferInput before) {
    ConstantPropagationStore store = before.getRegularStore();
    Constant value = store.getInformation(node);
    return new RegularTransferResult<>(value, store);
  }

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

  @Override
  public TransferResult visitAssignment(
      AssignmentNode n, TransferInput pi) {
    ConstantPropagationStore p = pi.getRegularStore();
    Node target = n.getTarget();
    Constant info = null;
    if (target instanceof LocalVariableNode) {
      LocalVariableNode t = (LocalVariableNode) target;
      info = p.getInformation(n.getExpression());
      p.setInformation(t, info);
    }
    return new RegularTransferResult<>(info, p);
  }

  @Override
  public TransferResult visitIntegerLiteral(
      IntegerLiteralNode n, TransferInput pi) {
    ConstantPropagationStore p = pi.getRegularStore();
    Constant c = new Constant(n.getValue());
    p.setInformation(n, c);
    return new RegularTransferResult<>(c, p);
  }

  @Override
  public TransferResult visitEqualTo(
      EqualToNode n, TransferInput pi) {
    ConstantPropagationStore p = pi.getRegularStore();
    ConstantPropagationStore old = p.copy();
    Node left = n.getLeftOperand();
    Node right = n.getRightOperand();
    process(p, left, right);
    process(p, right, left);
    return new ConditionalTransferResult<>(null, p, old);
  }

  protected void process(ConstantPropagationStore p, Node a, Node b) {
    Constant val = p.getInformation(a);
    if (b instanceof LocalVariableNode && val.isConstant()) {
      p.setInformation(b, val);
    }
  }
}