1 
/* 
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* Function for computing CRC32 for the purpose of adding to Ethernet packets. 
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* 
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*/ 
5 

6 
#include "crc32.h" 
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static const uint32 crc32table[0x100] = { 
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0x00000000L, 0x77073096L, 0xee0e612cL, 0x990951baL, 
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0x076dc419L, 0x706af48fL, 0xe963a535L, 0x9e6495a3L, 
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0x0edb8832L, 0x79dcb8a4L, 0xe0d5e91eL, 0x97d2d988L, 
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0x09b64c2bL, 0x7eb17cbdL, 0xe7b82d07L, 0x90bf1d91L, 
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0x1db71064L, 0x6ab020f2L, 0xf3b97148L, 0x84be41deL, 
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0x1adad47dL, 0x6ddde4ebL, 0xf4d4b551L, 0x83d385c7L, 
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0x136c9856L, 0x646ba8c0L, 0xfd62f97aL, 0x8a65c9ecL, 
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0x14015c4fL, 0x63066cd9L, 0xfa0f3d63L, 0x8d080df5L, 
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0x3b6e20c8L, 0x4c69105eL, 0xd56041e4L, 0xa2677172L, 
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0x3c03e4d1L, 0x4b04d447L, 0xd20d85fdL, 0xa50ab56bL, 
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0x35b5a8faL, 0x42b2986cL, 0xdbbbc9d6L, 0xacbcf940L, 
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0x32d86ce3L, 0x45df5c75L, 0xdcd60dcfL, 0xabd13d59L, 
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0x26d930acL, 0x51de003aL, 0xc8d75180L, 0xbfd06116L, 
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0x21b4f4b5L, 0x56b3c423L, 0xcfba9599L, 0xb8bda50fL, 
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0x2802b89eL, 0x5f058808L, 0xc60cd9b2L, 0xb10be924L, 
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0x2f6f7c87L, 0x58684c11L, 0xc1611dabL, 0xb6662d3dL, 
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0x76dc4190L, 0x01db7106L, 0x98d220bcL, 0xefd5102aL, 
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0x71b18589L, 0x06b6b51fL, 0x9fbfe4a5L, 0xe8b8d433L, 
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0x7807c9a2L, 0x0f00f934L, 0x9609a88eL, 0xe10e9818L, 
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0x7f6a0dbbL, 0x086d3d2dL, 0x91646c97L, 0xe6635c01L, 
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0x6b6b51f4L, 0x1c6c6162L, 0x856530d8L, 0xf262004eL, 
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0x6c0695edL, 0x1b01a57bL, 0x8208f4c1L, 0xf50fc457L, 
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0x65b0d9c6L, 0x12b7e950L, 0x8bbeb8eaL, 0xfcb9887cL, 
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0x62dd1ddfL, 0x15da2d49L, 0x8cd37cf3L, 0xfbd44c65L, 
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0x4db26158L, 0x3ab551ceL, 0xa3bc0074L, 0xd4bb30e2L, 
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0x4adfa541L, 0x3dd895d7L, 0xa4d1c46dL, 0xd3d6f4fbL, 
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0x4369e96aL, 0x346ed9fcL, 0xad678846L, 0xda60b8d0L, 
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0x44042d73L, 0x33031de5L, 0xaa0a4c5fL, 0xdd0d7cc9L, 
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0x5005713cL, 0x270241aaL, 0xbe0b1010L, 0xc90c2086L, 
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0x5768b525L, 0x206f85b3L, 0xb966d409L, 0xce61e49fL, 
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0x5edef90eL, 0x29d9c998L, 0xb0d09822L, 0xc7d7a8b4L, 
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0x59b33d17L, 0x2eb40d81L, 0xb7bd5c3bL, 0xc0ba6cadL, 
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0xedb88320L, 0x9abfb3b6L, 0x03b6e20cL, 0x74b1d29aL, 
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0xead54739L, 0x9dd277afL, 0x04db2615L, 0x73dc1683L, 
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0xe3630b12L, 0x94643b84L, 0x0d6d6a3eL, 0x7a6a5aa8L, 
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0xe40ecf0bL, 0x9309ff9dL, 0x0a00ae27L, 0x7d079eb1L, 
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0xf00f9344L, 0x8708a3d2L, 0x1e01f268L, 0x6906c2feL, 
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0xf762575dL, 0x806567cbL, 0x196c3671L, 0x6e6b06e7L, 
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0xfed41b76L, 0x89d32be0L, 0x10da7a5aL, 0x67dd4accL, 
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0xf9b9df6fL, 0x8ebeeff9L, 0x17b7be43L, 0x60b08ed5L, 
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0xd6d6a3e8L, 0xa1d1937eL, 0x38d8c2c4L, 0x4fdff252L, 
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0xd1bb67f1L, 0xa6bc5767L, 0x3fb506ddL, 0x48b2364bL, 
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0xd80d2bdaL, 0xaf0a1b4cL, 0x36034af6L, 0x41047a60L, 
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0xdf60efc3L, 0xa867df55L, 0x316e8eefL, 0x4669be79L, 
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0xcb61b38cL, 0xbc66831aL, 0x256fd2a0L, 0x5268e236L, 
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0xcc0c7795L, 0xbb0b4703L, 0x220216b9L, 0x5505262fL, 
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0xc5ba3bbeL, 0xb2bd0b28L, 0x2bb45a92L, 0x5cb36a04L, 
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0xc2d7ffa7L, 0xb5d0cf31L, 0x2cd99e8bL, 0x5bdeae1dL, 
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0x9b64c2b0L, 0xec63f226L, 0x756aa39cL, 0x026d930aL, 
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0x9c0906a9L, 0xeb0e363fL, 0x72076785L, 0x05005713L, 
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0x95bf4a82L, 0xe2b87a14L, 0x7bb12baeL, 0x0cb61b38L, 
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0x92d28e9bL, 0xe5d5be0dL, 0x7cdcefb7L, 0x0bdbdf21L, 
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0x86d3d2d4L, 0xf1d4e242L, 0x68ddb3f8L, 0x1fda836eL, 
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0x81be16cdL, 0xf6b9265bL, 0x6fb077e1L, 0x18b74777L, 
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0x88085ae6L, 0xff0f6a70L, 0x66063bcaL, 0x11010b5cL, 
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0x8f659effL, 0xf862ae69L, 0x616bffd3L, 0x166ccf45L, 
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0xa00ae278L, 0xd70dd2eeL, 0x4e048354L, 0x3903b3c2L, 
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0xa7672661L, 0xd06016f7L, 0x4969474dL, 0x3e6e77dbL, 
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0xaed16a4aL, 0xd9d65adcL, 0x40df0b66L, 0x37d83bf0L, 
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0xa9bcae53L, 0xdebb9ec5L, 0x47b2cf7fL, 0x30b5ffe9L, 
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0xbdbdf21cL, 0xcabac28aL, 0x53b39330L, 0x24b4a3a6L, 
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0xbad03605L, 0xcdd70693L, 0x54de5729L, 0x23d967bfL, 
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0xb3667a2eL, 0xc4614ab8L, 0x5d681b02L, 0x2a6f2b94L, 
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0xb40bbe37L, 0xc30c8ea1L, 0x5a05df1bL, 0x2d02ef8dL 
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}; 
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// The previous table could have been built using the following function : 
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/* 
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#define CRC32_POLY 0xedb88320; // this is a 0x04c11db7 reflection 
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void init_crc32() 
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{ 
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int i, j, b; 
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uint32 c; 
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for (i = 0; i < 0x100; i++) { 
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for (c = i, j = 0; j < 8; j++) { 
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b = c & 1; 
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c >>= 1; 
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if (b) 
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c ^= CRC32_POLY; 
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} 
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crc32table[i] = c; 
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} 
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} 
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*/ 
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// With this macro defined, the function runs about 35% faster, but the code is about 3 times bigger : 
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#define RUN_FASTER 
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#define DO_CRC(b) crc = (crc >> 8) ^ crc32table[(crc & 0xff) ^ (b)] 
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uint32 ether_crc(size_t len, const byte *p) 
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{ 
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uint32 crc = 0xffffffff; // preload shift register, per CRC32 spec 
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for (; len>0; len) { 
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DO_CRC(*p++); 
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} 
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return ~crc; // transmit complement, per CRC32 spec 
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} 
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/* 
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* A brief CRC tutorial. 
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* 
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* A CRC is a longdivision remainder. You add the CRC to the message, 
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* and the whole thing (message+CRC) is a multiple of the given 
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* CRC polynomial. To check the CRC, you can either check that the 
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* CRC matches the recomputed value, *or* you can check that the 
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* remainder computed on the message+CRC is 0. This latter approach 
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* is used by a lot of hardware implementations, and is why so many 
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* protocols put the endofframe flag after the CRC. 
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* 
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* It's actually the same long division you learned in school, except that 
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*  We're working in binary, so the digits are only 0 and 1, and 
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*  When dividing polynomials, there are no carries. Rather than add and 
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* subtract, we just xor. Thus, we tend to get a bit sloppy about 
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* the difference between adding and subtracting. 
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* 
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* A 32bit CRC polynomial is actually 33 bits long. But since it's 
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* 33 bits long, bit 32 is always going to be set, so usually the CRC 
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* is written in hex with the most significant bit omitted. (If you're 
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* familiar with the IEEE 754 floatingpoint format, it's the same idea.) 
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* 
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* Note that a CRC is computed over a string of *bits*, so you have 
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* to decide on the endianness of the bits within each byte. To get 
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* the best errordetecting properties, this should correspond to the 
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* order they're actually sent. For example, standard RS232 serial is 
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* littleendian; the most significant bit (sometimes used for parity) 
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* is sent last. And when appending a CRC word to a message, you should 
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* do it in the right order, matching the endianness. 
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* 
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* Just like with ordinary division, the remainder is always smaller than 
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* the divisor (the CRC polynomial) you're dividing by. Each step of the 
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* division, you take one more digit (bit) of the dividend and append it 
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* to the current remainder. Then you figure out the appropriate multiple 
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* of the divisor to subtract to being the remainder back into range. 
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* In binary, it's easy  it has to be either 0 or 1, and to make the 
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* XOR cancel, it's just a copy of bit 32 of the remainder. 
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* 
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* When computing a CRC, we don't care about the quotient, so we can 
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* throw the quotient bit away, but subtract the appropriate multiple of 
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* the polynomial from the remainder and we're back to where we started, 
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* ready to process the next bit. 
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* 
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* A bigendian CRC written this way would be coded like: 
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* for (i = 0; i < input_bits; i++) { 
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* multiple = remainder & 0x80000000 ? CRCPOLY : 0; 
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* remainder = (remainder << 1  next_input_bit()) ^ multiple; 
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* } 
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* Notice how, to get at bit 32 of the shifted remainder, we look 
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* at bit 31 of the remainder *before* shifting it. 
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* 
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* But also notice how the next_input_bit() bits we're shifting into 
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* the remainder don't actually affect any decisionmaking until 
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* 32 bits later. Thus, the first 32 cycles of this are pretty boring. 
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* Also, to add the CRC to a message, we need a 32bitlong hole for it at 
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* the end, so we have to add 32 extra cycles shifting in zeros at the 
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* end of every message, 
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* 
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* So the standard trick is to rearrage merging in the next_input_bit() 
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* until the moment it's needed. Then the first 32 cycles can be precomputed, 
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* and merging in the final 32 zero bits to make room for the CRC can be 
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* skipped entirely. 
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* This changes the code to: 
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* for (i = 0; i < input_bits; i++) { 
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* remainder ^= next_input_bit() << 31; 
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* multiple = (remainder & 0x80000000) ? CRCPOLY : 0; 
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* remainder = (remainder << 1) ^ multiple; 
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* } 
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* With this optimization, the littleendian code is simpler: 
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* for (i = 0; i < input_bits; i++) { 
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* remainder ^= next_input_bit(); 
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* multiple = (remainder & 1) ? CRCPOLY : 0; 
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* remainder = (remainder >> 1) ^ multiple; 
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* } 
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* 
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* Note that the other details of endianness have been hidden in CRCPOLY 
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* (which must be bitreversed) and next_input_bit(). 
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* 
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* However, as long as next_input_bit is returning the bits in a sensible 
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* order, we can actually do the merging 8 or more bits at a time rather 
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* than one bit at a time: 
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* for (i = 0; i < input_bytes; i++) { 
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* remainder ^= next_input_byte() << 24; 
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* for (j = 0; j < 8; j++) { 
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* multiple = (remainder & 0x80000000) ? CRCPOLY : 0; 
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* remainder = (remainder << 1) ^ multiple; 
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* } 
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* } 
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* Or in littleendian: 
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* for (i = 0; i < input_bytes; i++) { 
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* remainder ^= next_input_byte(); 
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* for (j = 0; j < 8; j++) { 
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* multiple = (remainder & 1) ? CRCPOLY : 0; 
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* remainder = (remainder >> 1) ^ multiple; 
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* } 
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* } 
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* If the input is a multiple of 32 bits, you can even XOR in a 32bit 
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* word at a time and increase the inner loop count to 32. 
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* 
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* You can also mix and match the two loop styles, for example doing the 
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* bulk of a message byteatatime and adding bitatatime processing 
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* for any fractional bytes at the end. 
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* 
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* The only remaining optimization is to the byteatatime table method. 
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* Here, rather than just shifting one bit of the remainder to decide 
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* in the correct multiple to subtract, we can shift a byte at a time. 
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* This produces a 40bit (rather than a 33bit) intermediate remainder, 
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* but again the multiple of the polynomial to subtract depends only on 
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* the high bits, the high 8 bits in this case. 
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* 
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* The multile we need in that case is the low 32 bits of a 40bit 
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* value whose high 8 bits are given, and which is a multiple of the 
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* generator polynomial. This is simply the CRC32 of the given 
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* onebyte message. 
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* 
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* Two more details: normally, appending zero bits to a message which 
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* is already a multiple of a polynomial produces a larger multiple of that 
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* polynomial. To enable a CRC to detect this condition, it's common to 
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* invert the CRC before appending it. This makes the remainder of the 
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* message+crc come out not as zero, but some fixed nonzero value. 
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* 
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* The same problem applies to zero bits prepended to the message, and 
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* a similar solution is used. Instead of starting with a remainder of 
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* 0, an initial remainder of all ones is used. As long as you start 
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* the same way on decoding, it doesn't make a difference. 
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*/ 