/* CRC32 implementation by Gary S. Brown. See license claim below. */

/* ============================================================= */
/*  COPYRIGHT (C) 1986 Gary S. Brown.  You may use this program, or       */
/*  code or tables extracted from it, as desired without restriction.     */
/*                                                                        */
/*  First, the polynomial itself and its table of feedback terms.  The    */
/*  polynomial is                                                         */
/*  X^32+X^26+X^23+X^22+X^16+X^12+X^11+X^10+X^8+X^7+X^5+X^4+X^2+X^1+X^0   */
/*                                                                        */
/*  Note that we take it "backwards" and put the highest-order term in    */
/*  the lowest-order bit.  The X^32 term is "implied"; the LSB is the     */
/*  X^31 term, etc.  The X^0 term (usually shown as "+1") results in      */
/*  the MSB being 1.                                                      */
/*                                                                        */
/*  Note that the usual hardware shift register implementation, which     */
/*  is what we're using (we're merely optimizing it by doing eight-bit    */
/*  chunks at a time) shifts bits into the lowest-order term.  In our     */
/*  implementation, that means shifting towards the right.  Why do we     */
/*  do it this way?  Because the calculated CRC must be transmitted in    */
/*  order from highest-order term to lowest-order term.  UARTs transmit   */
/*  characters in order from LSB to MSB.  By storing the CRC this way,    */
/*  we hand it to the UART in the order low-byte to high-byte; the UART   */
/*  sends each low-bit to hight-bit; and the result is transmission bit   */
/*  by bit from highest- to lowest-order term without requiring any bit   */
/*  shuffling on our part.  Reception works similarly.                    */
/*                                                                        */
/*  The feedback terms table consists of 256, 32-bit entries.  Notes:     */
/*                                                                        */
/*      The table can be generated at runtime if desired; code to do so   */
/*      is shown later.  It might not be obvious, but the feedback        */
/*      terms simply represent the results of eight shift/xor opera-      */
/*      tions for all combinations of data and CRC register values.       */
/*                                                                        */
/*      The values must be right-shifted by eight bits by the "updcrc"    */
/*      logic; the shift must be unsigned (bring in zeroes).  On some     */
/*      hardware you could probably optimize the shift in assembler by    */
/*      using byte-swap instructions.                                     */
/*      polynomial $edb88320                                              */
/*                                                                        */
/*  --------------------------------------------------------------------  */
#include "crc32.h"

static uint32_t crc32_tab[] = {
  0x00000000U, 0x77073096U, 0xee0e612cU, 0x990951baU, 0x076dc419U,
  0x706af48fU, 0xe963a535U, 0x9e6495a3U, 0x0edb8832U, 0x79dcb8a4U,
  0xe0d5e91eU, 0x97d2d988U, 0x09b64c2bU, 0x7eb17cbdU, 0xe7b82d07U,
  0x90bf1d91U, 0x1db71064U, 0x6ab020f2U, 0xf3b97148U, 0x84be41deU,
  0x1adad47dU, 0x6ddde4ebU, 0xf4d4b551U, 0x83d385c7U, 0x136c9856U,
  0x646ba8c0U, 0xfd62f97aU, 0x8a65c9ecU, 0x14015c4fU, 0x63066cd9U,
  0xfa0f3d63U, 0x8d080df5U, 0x3b6e20c8U, 0x4c69105eU, 0xd56041e4U,
  0xa2677172U, 0x3c03e4d1U, 0x4b04d447U, 0xd20d85fdU, 0xa50ab56bU,
  0x35b5a8faU, 0x42b2986cU, 0xdbbbc9d6U, 0xacbcf940U, 0x32d86ce3U,
  0x45df5c75U, 0xdcd60dcfU, 0xabd13d59U, 0x26d930acU, 0x51de003aU,
  0xc8d75180U, 0xbfd06116U, 0x21b4f4b5U, 0x56b3c423U, 0xcfba9599U,
  0xb8bda50fU, 0x2802b89eU, 0x5f058808U, 0xc60cd9b2U, 0xb10be924U,
  0x2f6f7c87U, 0x58684c11U, 0xc1611dabU, 0xb6662d3dU, 0x76dc4190U,
  0x01db7106U, 0x98d220bcU, 0xefd5102aU, 0x71b18589U, 0x06b6b51fU,
  0x9fbfe4a5U, 0xe8b8d433U, 0x7807c9a2U, 0x0f00f934U, 0x9609a88eU,
  0xe10e9818U, 0x7f6a0dbbU, 0x086d3d2dU, 0x91646c97U, 0xe6635c01U,
  0x6b6b51f4U, 0x1c6c6162U, 0x856530d8U, 0xf262004eU, 0x6c0695edU,
  0x1b01a57bU, 0x8208f4c1U, 0xf50fc457U, 0x65b0d9c6U, 0x12b7e950U,
  0x8bbeb8eaU, 0xfcb9887cU, 0x62dd1ddfU, 0x15da2d49U, 0x8cd37cf3U,
  0xfbd44c65U, 0x4db26158U, 0x3ab551ceU, 0xa3bc0074U, 0xd4bb30e2U,
  0x4adfa541U, 0x3dd895d7U, 0xa4d1c46dU, 0xd3d6f4fbU, 0x4369e96aU,
  0x346ed9fcU, 0xad678846U, 0xda60b8d0U, 0x44042d73U, 0x33031de5U,
  0xaa0a4c5fU, 0xdd0d7cc9U, 0x5005713cU, 0x270241aaU, 0xbe0b1010U,
  0xc90c2086U, 0x5768b525U, 0x206f85b3U, 0xb966d409U, 0xce61e49fU,
  0x5edef90eU, 0x29d9c998U, 0xb0d09822U, 0xc7d7a8b4U, 0x59b33d17U,
  0x2eb40d81U, 0xb7bd5c3bU, 0xc0ba6cadU, 0xedb88320U, 0x9abfb3b6U,
  0x03b6e20cU, 0x74b1d29aU, 0xead54739U, 0x9dd277afU, 0x04db2615U,
  0x73dc1683U, 0xe3630b12U, 0x94643b84U, 0x0d6d6a3eU, 0x7a6a5aa8U,
  0xe40ecf0bU, 0x9309ff9dU, 0x0a00ae27U, 0x7d079eb1U, 0xf00f9344U,
  0x8708a3d2U, 0x1e01f268U, 0x6906c2feU, 0xf762575dU, 0x806567cbU,
  0x196c3671U, 0x6e6b06e7U, 0xfed41b76U, 0x89d32be0U, 0x10da7a5aU,
  0x67dd4accU, 0xf9b9df6fU, 0x8ebeeff9U, 0x17b7be43U, 0x60b08ed5U,
  0xd6d6a3e8U, 0xa1d1937eU, 0x38d8c2c4U, 0x4fdff252U, 0xd1bb67f1U,
  0xa6bc5767U, 0x3fb506ddU, 0x48b2364bU, 0xd80d2bdaU, 0xaf0a1b4cU,
  0x36034af6U, 0x41047a60U, 0xdf60efc3U, 0xa867df55U, 0x316e8eefU,
  0x4669be79U, 0xcb61b38cU, 0xbc66831aU, 0x256fd2a0U, 0x5268e236U,
  0xcc0c7795U, 0xbb0b4703U, 0x220216b9U, 0x5505262fU, 0xc5ba3bbeU,
  0xb2bd0b28U, 0x2bb45a92U, 0x5cb36a04U, 0xc2d7ffa7U, 0xb5d0cf31U,
  0x2cd99e8bU, 0x5bdeae1dU, 0x9b64c2b0U, 0xec63f226U, 0x756aa39cU,
  0x026d930aU, 0x9c0906a9U, 0xeb0e363fU, 0x72076785U, 0x05005713U,
  0x95bf4a82U, 0xe2b87a14U, 0x7bb12baeU, 0x0cb61b38U, 0x92d28e9bU,
  0xe5d5be0dU, 0x7cdcefb7U, 0x0bdbdf21U, 0x86d3d2d4U, 0xf1d4e242U,
  0x68ddb3f8U, 0x1fda836eU, 0x81be16cdU, 0xf6b9265bU, 0x6fb077e1U,
  0x18b74777U, 0x88085ae6U, 0xff0f6a70U, 0x66063bcaU, 0x11010b5cU,
  0x8f659effU, 0xf862ae69U, 0x616bffd3U, 0x166ccf45U, 0xa00ae278U,
  0xd70dd2eeU, 0x4e048354U, 0x3903b3c2U, 0xa7672661U, 0xd06016f7U,
  0x4969474dU, 0x3e6e77dbU, 0xaed16a4aU, 0xd9d65adcU, 0x40df0b66U,
  0x37d83bf0U, 0xa9bcae53U, 0xdebb9ec5U, 0x47b2cf7fU, 0x30b5ffe9U,
  0xbdbdf21cU, 0xcabac28aU, 0x53b39330U, 0x24b4a3a6U, 0xbad03605U,
  0xcdd70693U, 0x54de5729U, 0x23d967bfU, 0xb3667a2eU, 0xc4614ab8U,
  0x5d681b02U, 0x2a6f2b94U, 0xb40bbe37U, 0xc30c8ea1U, 0x5a05df1bU,
  0x2d02ef8dU
};

/* Returns a 32-bit CRC of the contents of the buffer. */
uint32_t Crc32(const void *buffer, uint32_t len) {
  uint8_t *byte = (uint8_t*)buffer;
  uint32_t i;
  uint32_t value = ~0U;

  for (i = 0; i < len; ++i)
    value = crc32_tab[(value ^ byte[i]) & 0xff] ^ (value >> 8);
  return value ^ ~0U;
}