decoding 代码贴一下。_bart bitmask-优快云博客

本文链接：https://blog.youkuaiyun.com/anycom/article/details/6287575

本文介绍了一种名为CosetIDA的解码算法的具体实现细节。该算法用于处理特定格式的数据片段，通过复杂的数学运算恢复原始消息。文章深入探讨了算法的工作原理，包括初始化字段参数、生成解码表等步骤，并提供了详细的伪代码示例。

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public static byte[] decode(
              byte[] rec_fragments,
             final int Nrec,
             final CosetIDAParameters p) //throws Exception
       {


          byte[] rec_message = new byte[p.getMessageLength()];//p.getMessageLength()

          int ExpFE;


          final int numSegments = p.getSegmentsPerSlice();
          final int logOfField = p.getLogOfFieldLength();

          final int[] COLBIT = InitField.getCOLBIT(logOfField);
          final int[] BIT = InitField.getBIT(logOfField);
          final int[] ExptoFE = InitField.getExptoFE(logOfField);
          final int[] FEtoExp = InitField.getFEtoExp(logOfField);

          final int numDataSlices = p.getNumDataSlices();
          final int numCodeSlices = p.getNumCodeSlices();

          Integer key = new Integer(p.getNumDataSlices() + numSegments * p.getNumCodeSlices()
                * logOfField);
          FiniteStack stack = (FiniteStack) _decodeTable.get(key);

          if (stack == null)
          {
             synchronized (_decodeTable)
             {
                if ((stack = (FiniteStack) _decodeTable.get(key)) == null)
                {
                   Object bucket = null;
                   stack = new FiniteStack(STACK_SIZE);
                   if ((bucket = _decodeTable.put(key, stack)) != null)
                      System.err.println("CosetDecode.decode: bucket=" + bucket
                            + " for key=" + key + " stack=" + stack);
                }
             }
          }

          ArrayObj arrayObj = (ArrayObj) stack.pop();
          if (arrayObj == null)
             arrayObj = new ArrayObj(p.getNumDataSlices(), p.getNumCodeSlices(), numSegments, logOfField);

          int[] Rec_index = arrayObj.Rec_index;
          int[] Col_Ind = arrayObj.Col_Ind;
          int[] Row_Ind = arrayObj.Row_Ind;
          byte[] M = arrayObj.M;
          int[] C = arrayObj.C;
          int[] D = arrayObj.D;
          int[] E = arrayObj.E;
          int[] F = arrayObj.F;

          if (Nrec < p.getNumDataSlices()+p.getNumCodeSlices())
          {
             //throw new Exception("Decode error!!!!");
             System.out.println("Decode error!!!!=========="+Nrec+"tot:"+(p.getNumDataSlices()+p.getNumCodeSlices()));
          }

        // coset ==================================================coset begin===============
          // For Loop Iterators
          int itr, row, col, rowEquation, columnEquation;

          // Miscellaneous variables used in code slice calculation
          int sliceArrayPosition, messageArrayPosition;
          int ExpFEplusRow;
          int arrayPositionDifference, sliceArrayPositionPlusNumSegments;
          int sliceArrayIterator;

          final int[] equationBitMask = InitField.getBIT(p.getLogOfFieldLength());
          final int[] exponentToFiniteFieldElement = InitField.getExptoFE(p.getLogOfFieldLength());
          final int[] finiteFieldElementToExponent = InitField.getFEtoExp(p.getLogOfFieldLength());
          final int multFieldSize = p.getMultiplicationFieldSize();

          InitField.generateG(logOfField, numDataSlices, numCodeSlices);
          final int[][] G= InitField.getG();
          for ( row = 0; row <numDataSlices; row++)
          {
             /**
              * Compute values of equations applied to message and fill into
              * fragment(row+DataSlices).
              */
             final int rowOffset = ((row) * p.getSliceLength()); // p.getTotalSliceLength()

             /**
              * Second, fill in contents relevant portions of fragment
              */
             for ( col = 0; col < numDataSlices+numCodeSlices ; col++)
             {
                //message->code
                messageArrayPosition = col * (p.getSliceLength()+1)+1;
                //if(messageArrayPosition==0)messageArrayPosition=1;
            // System.out.println("messageArrayPosition:"+messageArrayPosition);
              /* ExpFE = (multFieldSize - finiteFieldElementToExponent[row ^ col ^ COLBIT[0]])
                      % multFieldSize;*/
               // System.out.println("row:"+row+"col:"+col);
              // System.out.println("G[row][col]:"+G[row][col]);
                ExpFE = finiteFieldElementToExponent[G[row][col]];

                for (rowEquation = 0; rowEquation < logOfField; rowEquation++)
                {
                   ExpFEplusRow = ExpFE + rowEquation;
                   sliceArrayPosition = rowOffset + (rowEquation * numSegments);

                     // System.out.println("rowOffset:"+rowOffset);
                     // System.out.println("sliceArrayPosition:"+sliceArrayPosition);
                   for (columnEquation = 0; columnEquation < logOfField; columnEquation++)
                   {
                    // System.out.print((exponentToFiniteFieldElement[ExpFE + columnEquation] & equationBitMask[rowEquation])>0?1:0);
                    // System.out.println("columnEquation"+columnEquation+"ExpFEplusRow");
                      if ((exponentToFiniteFieldElement[ExpFE + columnEquation] & equationBitMask[rowEquation]) > 0)
                      {

                         /*
                          * Warning: the following code is heavily optimized and difficult to read
                          * The following loop is the result of much optimization
                          * and is an attempt to use as few instructions as possible
                          * within this very deeply nested loop.
                          */
                         arrayPositionDifference = ((columnEquation * numSegments + messageArrayPosition) - sliceArrayPosition);
                         sliceArrayPositionPlusNumSegments = sliceArrayPosition + numSegments;
                         sliceArrayIterator = sliceArrayPosition;
                  //    System.out.println("numSegments"+numSegments+"sliceArrayPosition"+sliceArrayPosition+"sliceArrayPositionPlusNumSegments"+sliceArrayPositionPlusNumSegments+"sliceArrayIterator + arrayPositionDifference:"+(sliceArrayIterator + arrayPositionDifference));
                         assert(sliceArrayPosition<rec_message.length);
                         while (sliceArrayIterator < sliceArrayPositionPlusNumSegments)
                         {
                      //        System.out.println("sliceArrayIterator"+sliceArrayIterator+" arrayPositionDifference:"+( arrayPositionDifference));
                            //if(sliceArrayIterator==4144) sliceArrayIterator=4143;
                            rec_message[sliceArrayIterator] ^= rec_fragments[sliceArrayIterator + arrayPositionDifference];
                             sliceArrayIterator++;
                         }

                      }
                   }
                 // System.out.print("/n");
                }
             }
          }

/*          System.out.println("=======================rec_message=====================:"+rec_message.length);
          printHexString(rec_message);
          System.out.println("=======================rec_message=====================");*/
          return rec_message;




       }