CRC中的CRC32算法/实现没有查找表和公共许可证

Question

我试图在C中实现一个不使用查找表的CRC32算法（我需要在没有足够内存的引导加载程序中使用它）。是否有可获得公共许可证的解决方案？

Answer 1

快速搜索收获this webpage。我无法找到这些代码段的许可证。

以下应该做的工作：

// ----------------------------- crc32b --------------------------------

/* This is the basic CRC-32 calculation with some optimization but no
table lookup. The the byte reversal is avoided by shifting the crc reg
right instead of left and by using a reversed 32-bit word to represent
the polynomial.
   When compiled to Cyclops with GCC, this function executes in 8 + 72n
instructions, where n is the number of bytes in the input message. It
should be doable in 4 + 61n instructions.
   If the inner loop is strung out (approx. 5*8 = 40 instructions),
it would take about 6 + 46n instructions. */

unsigned int crc32b(unsigned char *message) {
   int i, j;
   unsigned int byte, crc, mask;

   i = 0;
   crc = 0xFFFFFFFF;
   while (message[i] != 0) {
      byte = message[i];            // Get next byte.
      crc = crc ^ byte;
      for (j = 7; j >= 0; j--) {    // Do eight times.
         mask = -(crc & 1);
         crc = (crc >> 1) ^ (0xEDB88320 & mask);
      }
      i = i + 1;
   }
   return ~crc;
}