/*

  * Licensed to the Apache Software Foundation (ASF) under one or more

  * contributor license agreements.  See the NOTICE file distributed with

  * this work for additional information regarding copyright ownership.

  * The ASF licenses this file to You 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 org.apache.commons.nabla.automatic.trimming;

 import org.objectweb.asm.Opcodes;

 import org.objectweb.asm.tree.AbstractInsnNode;

 import org.objectweb.asm.tree.InsnList;

 import org.objectweb.asm.tree.VarInsnNode;

 /** Trimmer replacing (DUP2_X2, POP2, DSTORE i, DSTORE j) with (DSTORE j, DSTORE i).

  */

 public class SwappedDstoreTrimmer extends BytecodeTrimmer {

     /** Holder for the singleton instance.*/

     private static class LazyHolder  {

         /** The singleton instance. */

         private static final BytecodeTrimmer INSTANCE = new SwappedDstoreTrimmer();

     }

     /** Hidden constructor.

      */

     private SwappedDstoreTrimmer() {

         super(4);

     }

     /** Get the singleton instance.

      * <p>We use here the Initialization on Demand Holder idiom.</p>

      * @return the singleton instance

      */

     public static BytecodeTrimmer getInstance() {

         return LazyHolder.INSTANCE;

     }

     /** {@inheritDoc} */

     @Override

     protected boolean trimWindow(final InsnList instructions,

                                  final AbstractInsnNode[] window) {

         if ((window[0].getOpcode() == Opcodes.DUP2_X2) &&

             (window[1].getOpcode() == Opcodes.POP2) &&

             (window[2].getOpcode() == Opcodes.DSTORE) &&

             (window[3].getOpcode() == Opcodes.DSTORE)) {

             // reverse the DSTORE orders

             final int tmp = ((VarInsnNode) window[2]).var;

             ((VarInsnNode) window[2]).var = ((VarInsnNode) window[3]).var;

             ((VarInsnNode) window[3]).var = tmp;

             // remove the operand stack swap instructions

             instructions.remove(window[0]);

             instructions.remove(window[1]);

             // update lookahead instructions

             window[0] = window[2];

             window[1] = window[3];

             window[2] = window[1].getNext();

             window[3] = (window[2] == null) ? null : window[2].getNext();

             return true;

         }

         // nothing have been done

         return false;

     }

 }