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关于libmemcached中的crc的实现

缘起

libmemcached备受青睐，号称错误信息是很详细的，比所谓的memcache模块强多了；但是，就是因为他的名气大，还是被坑了；所谓的错误信息详细在get操作时完全不存在了，不管是连接失败还是各种超时，得到的错误信息都是NOTFOUND；于是乎，我想自己用PHP写一个memcached的client，起初，我可以之实现我需要的几个方法，鉴于memcache的协议很简单，应该是比较容易完成的。

问题

其实，我1天时间已经完成了memcache的几个简单的协议，出于一些不想说的考虑，当add多个server时，我需要把根据key的hash结果和libmemcached中的完全一样；看了libmemcached中几种hash和distribute的策略，我只想选择一种最简单的hash和distribute策略来实现，我选择了 crc32 + modular；简而言之，算法就是： crc32(key) % count(server) 就Ok了；但是，问题是PHP的crc32结果和libmemcached中的crc32结果是不同的，鉴于我的C语言水平有限，不想看和看不懂使得我咨询了一下牛人，没有结果，于是还是硬着头皮看吧。

分析

后面分别贴一下libmemcached和php中crc的实现，这里先说结论了：

二者差别不大，主要差别在于：

PHP中的crc32返回的是一个32位的有符号数，而libmemecached中返回的是前者32位有符号数种的高16位，并且忽略了符号位；

如果用PHP实现一个和libmemcached相同的crc32将是：

<?php
echo (crc32('test') >> 16) & 0x7fff;

或者如下实现：

本来只需要简单的位移操作、异或操作就可以处理的事情，用PHP来实现却需要大量的pack、unpack，对‘test’进行crc32大约需要0.1ms(很慢的)，如果用上面实现，大约比该实现快100倍

<?php

function _crc32($str) {
       $arr = array(
           0x00000000, 0x77073096, 0xee0e612c, 0x990951ba,
           0x076dc419, 0x706af48f, 0xe963a535, 0x9e6495a3,
           0x0edb8832, 0x79dcb8a4, 0xe0d5e91e, 0x97d2d988,
           0x09b64c2b, 0x7eb17cbd, 0xe7b82d07, 0x90bf1d91,
           0x1db71064, 0x6ab020f2, 0xf3b97148, 0x84be41de,
           0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7,
           0x136c9856, 0x646ba8c0, 0xfd62f97a, 0x8a65c9ec,
           0x14015c4f, 0x63066cd9, 0xfa0f3d63, 0x8d080df5,
           0x3b6e20c8, 0x4c69105e, 0xd56041e4, 0xa2677172,
           0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b,
           0x35b5a8fa, 0x42b2986c, 0xdbbbc9d6, 0xacbcf940,
           0x32d86ce3, 0x45df5c75, 0xdcd60dcf, 0xabd13d59,
           0x26d930ac, 0x51de003a, 0xc8d75180, 0xbfd06116,
           0x21b4f4b5, 0x56b3c423, 0xcfba9599, 0xb8bda50f,
           0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924,
           0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d,
           0x76dc4190, 0x01db7106, 0x98d220bc, 0xefd5102a,
           0x71b18589, 0x06b6b51f, 0x9fbfe4a5, 0xe8b8d433,
           0x7807c9a2, 0x0f00f934, 0x9609a88e, 0xe10e9818,
           0x7f6a0dbb, 0x086d3d2d, 0x91646c97, 0xe6635c01,
           0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e,
           0x6c0695ed, 0x1b01a57b, 0x8208f4c1, 0xf50fc457,
           0x65b0d9c6, 0x12b7e950, 0x8bbeb8ea, 0xfcb9887c,
           0x62dd1ddf, 0x15da2d49, 0x8cd37cf3, 0xfbd44c65,
           0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2,
           0x4adfa541, 0x3dd895d7, 0xa4d1c46d, 0xd3d6f4fb,
           0x4369e96a, 0x346ed9fc, 0xad678846, 0xda60b8d0,
           0x44042d73, 0x33031de5, 0xaa0a4c5f, 0xdd0d7cc9,
           0x5005713c, 0x270241aa, 0xbe0b1010, 0xc90c2086,
           0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f,
           0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4,
           0x59b33d17, 0x2eb40d81, 0xb7bd5c3b, 0xc0ba6cad,
           0xedb88320, 0x9abfb3b6, 0x03b6e20c, 0x74b1d29a,
           0xead54739, 0x9dd277af, 0x04db2615, 0x73dc1683,
           0xe3630b12, 0x94643b84, 0x0d6d6a3e, 0x7a6a5aa8,
           0xe40ecf0b, 0x9309ff9d, 0x0a00ae27, 0x7d079eb1,
           0xf00f9344, 0x8708a3d2, 0x1e01f268, 0x6906c2fe,
           0xf762575d, 0x806567cb, 0x196c3671, 0x6e6b06e7,
           0xfed41b76, 0x89d32be0, 0x10da7a5a, 0x67dd4acc,
           0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5,
           0xd6d6a3e8, 0xa1d1937e, 0x38d8c2c4, 0x4fdff252,
           0xd1bb67f1, 0xa6bc5767, 0x3fb506dd, 0x48b2364b,
           0xd80d2bda, 0xaf0a1b4c, 0x36034af6, 0x41047a60,
           0xdf60efc3, 0xa867df55, 0x316e8eef, 0x4669be79,
           0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236,
           0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f,
           0xc5ba3bbe, 0xb2bd0b28, 0x2bb45a92, 0x5cb36a04,
           0xc2d7ffa7, 0xb5d0cf31, 0x2cd99e8b, 0x5bdeae1d,
           0x9b64c2b0, 0xec63f226, 0x756aa39c, 0x026d930a,
           0x9c0906a9, 0xeb0e363f, 0x72076785, 0x05005713,
           0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38,
           0x92d28e9b, 0xe5d5be0d, 0x7cdcefb7, 0x0bdbdf21,
           0x86d3d2d4, 0xf1d4e242, 0x68ddb3f8, 0x1fda836e,
           0x81be16cd, 0xf6b9265b, 0x6fb077e1, 0x18b74777,
           0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c,
           0x8f659eff, 0xf862ae69, 0x616bffd3, 0x166ccf45,
           0xa00ae278, 0xd70dd2ee, 0x4e048354, 0x3903b3c2,
           0xa7672661, 0xd06016f7, 0x4969474d, 0x3e6e77db,
           0xaed16a4a, 0xd9d65adc, 0x40df0b66, 0x37d83bf0,
           0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9,
           0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6,
           0xbad03605, 0xcdd70693, 0x54de5729, 0x23d967bf,
           0xb3667a2e, 0xc4614ab8, 0x5d681b02, 0x2a6f2b94,
           0xb40bbe37, 0xc30c8ea1, 0x5a05df1b, 0x2d02ef8d,
       );
       $table = '';
       foreach($arr as $int) {
               $table.= pack('N', $int);
       }

       $crc = pack('N', 4294967295);
       for ($i = 0; $i < strlen($str); $i++) {
               $m = $crc ^ (chr(0).chr(0).chr(0).$str{$i});
               $n = $m & (chr(0).chr(0).chr(0).chr(0xff));
               $arr = unpack('N', $n);
               $n = $arr[1];
               $arr = unpack('N', $crc);
               $crc = (pack('N', $arr[1] >> 8)) ^ substr($table, $n*4, 4);
       }
       $arr = unpack('N', ~$crc);
       $res = (pack('N', $arr[1] >> 16)) & (chr(0).chr(0).chr(0x7f).chr(0xff));
       $arr = unpack('N', $res);
       return $arr[1];
}

libmemcached：

static const uint32_tcrc32tab[256] = {
   0x00000000, 0x77073096, 0xee0e612c, 0x990951ba,
   0x076dc419, 0x706af48f, 0xe963a535, 0x9e6495a3,
   0x0edb8832, 0x79dcb8a4, 0xe0d5e91e, 0x97d2d988,
   0x09b64c2b, 0x7eb17cbd, 0xe7b82d07, 0x90bf1d91,
   0x1db71064, 0x6ab020f2, 0xf3b97148, 0x84be41de,
   0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7,
   0x136c9856, 0x646ba8c0, 0xfd62f97a, 0x8a65c9ec,
   0x14015c4f, 0x63066cd9, 0xfa0f3d63, 0x8d080df5,
   0x3b6e20c8, 0x4c69105e, 0xd56041e4, 0xa2677172,
   0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b,
   0x35b5a8fa, 0x42b2986c, 0xdbbbc9d6, 0xacbcf940,
   0x32d86ce3, 0x45df5c75, 0xdcd60dcf, 0xabd13d59,
   0x26d930ac, 0x51de003a, 0xc8d75180, 0xbfd06116,
   0x21b4f4b5, 0x56b3c423, 0xcfba9599, 0xb8bda50f,
   0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924,
   0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d,
   0x76dc4190, 0x01db7106, 0x98d220bc, 0xefd5102a,
   0x71b18589, 0x06b6b51f, 0x9fbfe4a5, 0xe8b8d433,
   0x7807c9a2, 0x0f00f934, 0x9609a88e, 0xe10e9818,
   0x7f6a0dbb, 0x086d3d2d, 0x91646c97, 0xe6635c01,
   0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e,
   0x6c0695ed, 0x1b01a57b, 0x8208f4c1, 0xf50fc457,
   0x65b0d9c6, 0x12b7e950, 0x8bbeb8ea, 0xfcb9887c,
   0x62dd1ddf, 0x15da2d49, 0x8cd37cf3, 0xfbd44c65,
   0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2,
   0x4adfa541, 0x3dd895d7, 0xa4d1c46d, 0xd3d6f4fb,
   0x4369e96a, 0x346ed9fc, 0xad678846, 0xda60b8d0,
   0x44042d73, 0x33031de5, 0xaa0a4c5f, 0xdd0d7cc9,
   0x5005713c, 0x270241aa, 0xbe0b1010, 0xc90c2086,
   0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f,
   0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4,
   0x59b33d17, 0x2eb40d81, 0xb7bd5c3b, 0xc0ba6cad,
   0xedb88320, 0x9abfb3b6, 0x03b6e20c, 0x74b1d29a,
   0xead54739, 0x9dd277af, 0x04db2615, 0x73dc1683,
   0xe3630b12, 0x94643b84, 0x0d6d6a3e, 0x7a6a5aa8,
   0xe40ecf0b, 0x9309ff9d, 0x0a00ae27, 0x7d079eb1,
   0xf00f9344, 0x8708a3d2, 0x1e01f268, 0x6906c2fe,
   0xf762575d, 0x806567cb, 0x196c3671, 0x6e6b06e7,
   0xfed41b76, 0x89d32be0, 0x10da7a5a, 0x67dd4acc,
   0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5,
   0xd6d6a3e8, 0xa1d1937e, 0x38d8c2c4, 0x4fdff252,
   0xd1bb67f1, 0xa6bc5767, 0x3fb506dd, 0x48b2364b,
   0xd80d2bda, 0xaf0a1b4c, 0x36034af6, 0x41047a60,
   0xdf60efc3, 0xa867df55, 0x316e8eef, 0x4669be79,
   0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236,
   0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f,
   0xc5ba3bbe, 0xb2bd0b28, 0x2bb45a92, 0x5cb36a04,
   0xc2d7ffa7, 0xb5d0cf31, 0x2cd99e8b, 0x5bdeae1d,
   0x9b64c2b0, 0xec63f226, 0x756aa39c, 0x026d930a,
   0x9c0906a9, 0xeb0e363f, 0x72076785, 0x05005713,
   0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38,
   0x92d28e9b, 0xe5d5be0d, 0x7cdcefb7, 0x0bdbdf21,
   0x86d3d2d4, 0xf1d4e242, 0x68ddb3f8, 0x1fda836e,
   0x81be16cd, 0xf6b9265b, 0x6fb077e1, 0x18b74777,
   0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c,
   0x8f659eff, 0xf862ae69, 0x616bffd3, 0x166ccf45,
   0xa00ae278, 0xd70dd2ee, 0x4e048354, 0x3903b3c2,
   0xa7672661, 0xd06016f7, 0x4969474d, 0x3e6e77db,
   0xaed16a4a, 0xd9d65adc, 0x40df0b66, 0x37d83bf0,
   0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9,
   0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6,
   0xbad03605, 0xcdd70693, 0x54de5729, 0x23d967bf,
   0xb3667a2e, 0xc4614ab8, 0x5d681b02, 0x2a6f2b94,
   0xb40bbe37, 0xc30c8ea1, 0x5a05df1b, 0x2d02ef8d,
};

uint32_t
hash_crc32(const char *key, size_tkey_length)
{
   uint64_t x;
   uint32_tcrc = UINT32_MAX;

   for (x= 0; x < key_length; x++) {
       crc = (crc >> 8) ^ crc32tab[(crc ^ (uint64_t)key[x]) & 0xff];
   }

   return ((~crc) >> 16) & 0x7fff;
}

php中的实现(说明一下：这里的crc32tab和上面的是一样的)：

/* {{{ proto string crc32(string str)
  Calculate the crc32 polynomial of a string */
PHP_NAMED_FUNCTION(php_if_crc32)
{
   char *p;
   int len, nr;
   php_uint32crcinit = 0;
   register php_uint32crc;

   if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "s", &p, &nr) == FAILURE) {
       return;
   }
   crc = crcinit^0xFFFFFFFF;

   for (len =+nr; nr--; ++p) {
       crc = ((crc >> 8) & 0x00FFFFFF) ^ crc32tab[(crc ^ (*p)) & 0xFF ];
   }
   RETVAL_LONG(crc^0xFFFFFFFF);
}
/* }}} */

关于crc32_table 的由来，crc32_table是出于提高效率的目的而诞生的，来源于一个神奇的数字 0x04c11db7：

#include <stdio.h>
unsigned long int crc32_table[256]; 
unsigned long int ulPolynomial = 0x04c11db7; 
unsigned long int Reflect(unsigned long int ref, char ch) {
       unsigned long int value = 0; 
       // 交换bit0 和bit7，bit1 和bit6，类推
       int i;
       for(i = 1; i < (ch + 1); i++) {
               if(ref & 1) 
                       value |= 1 << (ch - i); 
               ref >>= 1;
       } 
       return value; 
} 
void init_crc32_table() {
       unsigned long int crc,temp; 
       // 256 个值
       int i, j;
       for(i = 0; i <= 0xFF; i++) {
               temp=Reflect(i, 8); 
               crc32_table[i]= temp<< 24; 
               for (j = 0; j < 8; j++){ 
                       unsigned long int t1,t2; 
                       unsigned long int flag=crc32_table[i]&0x80000000; 
                       t1=(crc32_table[i] << 1); 
                       if(flag==0) 
                               t2=0; 
                       else 
                               t2=ulPolynomial; 
                       crc32_table[i] =t1^t2 ; } 
                       crc=crc32_table[i]; 
                       crc32_table[i] = Reflect(crc32_table[i], 32); 
       } 
}
int main(int ac, char ** av) {
       init_crc32_table();
       int i;
       for(i = 0; i < 256; i++) {
               printf("0x%08x ", crc32_table[i]);
               if (i % 4 == 3) printf(" ");
       }

       return 0;
}

参考资料：

http://baike.baidu.com/view/410181.htm?fr=aladdin

http://hi.baidu.com/qyiyunso/item/c06719c5872ed5d5964452ac

http://zh.wikipedia.org/wiki/CRC32

原文 http://blogread.cn/it/article/7003?f=hot1

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