无需查找表并具有公共许可证的C中CRC32算法/实现 [英] CRC32 algorithm/implementation in C without a look up table and with a public license

问题描述

我正在尝试在不使用查找表的C语言中实现CRC32算法（我需要在没有足够可用内存的引导加载程序中使用该算法）。

I am trying to implement a CRC32 algorithm in C that does not use a look up table (I need to use it in a boot loader that doesn't have enough memory available to have one). Is there an available solution to this that has a public license?

推荐答案

快速搜索得到的此网页。我找不到这些代码段的许可证。

A quick search harvested this webpage. I wasn't able to find the license for these code snippets.

以下应完成此工作：

// ----------------------------- 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;
}

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