将 CRC 算法从 C 转换为 Java

Question

我有以下代码，我正在尝试将其转换为 Java。

WORD ComputeCRC16(BYTE *data, DWORD data_length)
{
   BYTE *ptr;
   BYTEWORD retval;

    /* Initialize the CRC */
    retval.w = 0xFFFF;

   /* Iterate through the data */
   for (ptr=data; ptr<data+data_length; ptr++)
   {
      // retval.w = IterateCRC16(ptr, retval);
      retval.w = retval.b.hi ^ (ccittrev_tbl[retval.b.lo ^ *ptr]);

   }

   /* Finalize the CRC */
   retval.w = ~retval.w;

   /* Done. */
   return retval.w;
}

下面一行是什么意思？

retval.w = retval.b.hi ^ (ccittrev_tbl[retval.b.lo ^ *ptr]);

如果我转换为 Java，retval 应该是一个 int 对吗？如果那样的话，它怎么会有一个名为“w”的成员？请建议我如何将上述行转换为 Java？

我正在编辑问题以发布我错过的部分。

typedef union
{
  WORD  w;
  struct
  {
     BYTE  lo,
        hi;
   } b;
} BYTEWORD;         

建议后编辑：

static int calculate_crc(byte[] data) {
     int retval_w = 0x0000;
     int ccittrev_tbl[] = {
                0x0000,  0xC0C1,  0xC181,  0x0140,  0xC301,  0x03C0,  0x0280,  0xC241,
                0xC601,  0x06C0,  0x0780,  0xC741,  0x0500,  0xC5C1,  0xC481,  0x0440,
                0xCC01,  0x0CC0,  0x0D80,  0xCD41,  0x0F00,  0xCFC1,  0xCE81,  0x0E40,
                0x0A00,  0xCAC1,  0xCB81,  0x0B40,  0xC901,  0x09C0,  0x0880,  0xC841,
                0xD801,  0x18C0,  0x1980,  0xD941,  0x1B00,  0xDBC1,  0xDA81,  0x1A40,
                0x1E00,  0xDEC1,  0xDF81,  0x1F40,  0xDD01,  0x1DC0,  0x1C80,  0xDC41,
                0x1400,  0xD4C1,  0xD581,  0x1540,  0xD701,  0x17C0,  0x1680,  0xD641,
                0xD201,  0x12C0,  0x1380,  0xD341,  0x1100,  0xD1C1,  0xD081,  0x1040,
                0xF001,  0x30C0,  0x3180,  0xF141,  0x3300,  0xF3C1,  0xF281,  0x3240,
                0x3600,  0xF6C1,  0xF781,  0x3740,  0xF501,  0x35C0,  0x3480,  0xF441,
                0x3C00,  0xFCC1,  0xFD81,  0x3D40,  0xFF01,  0x3FC0,  0x3E80,  0xFE41,
                0xFA01,  0x3AC0,  0x3B80,  0xFB41,  0x3900,  0xF9C1,  0xF881,  0x3840,
                0x2800,  0xE8C1,  0xE981,  0x2940,  0xEB01,  0x2BC0,  0x2A80,  0xEA41,
                0xEE01,  0x2EC0,  0x2F80,  0xEF41,  0x2D00,  0xEDC1,  0xEC81,  0x2C40,
                0xE401,  0x24C0,  0x2580,  0xE541,  0x2700,  0xE7C1,  0xE681,  0x2640,
                0x2200,  0xE2C1,  0xE381,  0x2340,  0xE101,  0x21C0,  0x2080,  0xE041,
                0xA001,  0x60C0,  0x6180,  0xA141,  0x6300,  0xA3C1,  0xA281,  0x6240,
                0x6600,  0xA6C1,  0xA781,  0x6740,  0xA501,  0x65C0,  0x6480,  0xA441,
                0x6C00,  0xACC1,  0xAD81,  0x6D40,  0xAF01,  0x6FC0,  0x6E80,  0xAE41,
                0xAA01,  0x6AC0,  0x6B80,  0xAB41,  0x6900,  0xA9C1,  0xA881,  0x6840,
                0x7800,  0xB8C1,  0xB981,  0x7940,  0xBB01,  0x7BC0,  0x7A80,  0xBA41,
                0xBE01,  0x7EC0,  0x7F80,  0xBF41,  0x7D00,  0xBDC1,  0xBC81,  0x7C40,
                0xB401,  0x74C0,  0x7580,  0xB541,  0x7700,  0xB7C1,  0xB681,  0x7640,
                0x7200,  0xB2C1,  0xB381,  0x7340,  0xB101,  0x71C0,  0x7080,  0xB041,
                0x5000,  0x90C1,  0x9181,  0x5140,  0x9301,  0x53C0,  0x5280,  0x9241,
                0x9601,  0x56C0,  0x5780,  0x9741,  0x5500,  0x95C1,  0x9481,  0x5440,
                0x9C01,  0x5CC0,  0x5D80,  0x9D41,  0x5F00,  0x9FC1,  0x9E81,  0x5E40,
                0x5A00,  0x9AC1,  0x9B81,  0x5B40,  0x9901,  0x59C0,  0x5880,  0x9841,
                0x8801,  0x48C0,  0x4980,  0x8941,  0x4B00,  0x8BC1,  0x8A81,  0x4A40,
                0x4E00,  0x8EC1,  0x8F81,  0x4F40,  0x8D01,  0x4DC0,  0x4C80,  0x8C41,
                0x4400,  0x84C1,  0x8581,  0x4540,  0x8701,  0x47C0,  0x4680,  0x8641,
                0x8201,  0x42C0,  0x4380,  0x8341,  0x4100,  0x81C1,  0x8081,  0x4040
            };

        for (byte b : data) {
            retval_w = ((retval_w>>8)&0xff) ^ (ccittrev_tbl[(retval_w&0xff) ^ b]);

        }
}

Answer 1

根据代码我猜BYTEWORD是unsigned short和两个字节结构的联合，所以作者可以很容易地访问这个short的高低字节，即：

typedef union BYTEWORD
{
     short w;
     struct {
         char lo, hi;
     } b;
} BYTEWORD;

由于java不支持联合（正如讨论中提到的，上面有字节序缺陷），你将不得不使用>>、|和&运算符来访问高8和低8短片：

  short retval_w = -1;
  ...
  retval_w = ((retval_w>>8)&0xff) ^ (ccittrev_tbl[(retval_w&0xff) ^ (data[i]&0xff)]);

Answer 2

^ 是二元运算符，表示Exclusive OR (XOR)。