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ProGuardCORE is a free library to read, analyze, modify, and write Java class files.
There is a newer version: 9.1.6
/*
 * ProGuardCORE -- library to process Java bytecode.
 *
 * Copyright (c) 2002-2020 Guardsquare NV
 *
 * 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 at
 *
 *      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 proguard.classfile.editor;

import java.util.Arrays;
import proguard.classfile.*;
import proguard.classfile.attribute.*;
import proguard.classfile.attribute.visitor.AttributeVisitor;

/**
 * This {@link AttributeVisitor} accumulates specified changes to local variables, and then applies
 * these accumulated changes to the code attributes that it visits.
 *
 * @author Eric Lafortune
 */
public class VariableEditor implements AttributeVisitor {
  private boolean modified;

  private boolean[] deleted = new boolean[ClassEstimates.TYPICAL_VARIABLES_SIZE];
  private int[] variableMap = new int[ClassEstimates.TYPICAL_VARIABLES_SIZE];

  private final VariableRemapper variableRemapper = new VariableRemapper();

  /**
   * Resets the accumulated code changes.
   *
   * @param maxLocals the length of the local variable frame that will be edited next.
   */
  public void reset(int maxLocals) {
    // Try to reuse the previous array.
    if (deleted.length < maxLocals) {
      // Create a new array.
      deleted = new boolean[maxLocals];
    } else {
      // Reset the array.
      Arrays.fill(deleted, 0, maxLocals, false);
    }

    modified = false;
  }

  /**
   * Remembers to delete the given variable.
   *
   * @param variableIndex the index of the variable to be deleted.
   */
  public void deleteVariable(int variableIndex) {
    deleted[variableIndex] = true;

    modified = true;
  }

  /** Returns whether the given variable at the given offset will be deleted. */
  public boolean isDeleted(int instructionOffset) {
    return deleted[instructionOffset];
  }

  // Implementations for AttributeVisitor.

  public void visitAnyAttribute(Clazz clazz, Attribute attribute) {}

  public void visitCodeAttribute(Clazz clazz, Method method, CodeAttribute codeAttribute) {
    // Avoid doing any work if nothing is changing anyway.
    if (!modified) {
      return;
    }

    int oldMaxLocals = codeAttribute.u2maxLocals;

    // Make sure there is a sufficiently large variable map.
    if (variableMap.length < oldMaxLocals) {
      variableMap = new int[oldMaxLocals];
    }

    // Fill out the variable map.
    int newVariableIndex = 0;
    for (int oldVariableIndex = 0; oldVariableIndex < oldMaxLocals; oldVariableIndex++) {
      variableMap[oldVariableIndex] = deleted[oldVariableIndex] ? -1 : newVariableIndex++;
    }

    // Set the map.
    variableRemapper.setVariableMap(variableMap);

    // Remap the variables.
    variableRemapper.visitCodeAttribute(clazz, method, codeAttribute);

    // Update the length of local variable frame.
    codeAttribute.u2maxLocals = newVariableIndex;
  }
}