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;

1 2	<?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];
}

<?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_t crc32tab[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_t key_length)
{
    uint64_t x;
    uint32_t crc = UINT32_MAX;

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

    return ((~crc) >> 16) & 0x7fff;
}

static const uint32_t crc32tab[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_t key_length)

{

uint64_t x;

uint32_t crc = 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_uint32 crcinit = 0;
    register php_uint32 crc;

    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);
}
/* }}} */

/* {{{ proto string crc32(string str)

Calculate the crc32 polynomial of a string */

PHP_NAMED_FUNCTION(php_if_crc32)

{

char *p;

int len, nr;

php_uint32 crcinit = 0;

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\t", crc32_table[i]);
                if (i % 4 == 3) printf("\n");
        }

        return 0;
}

#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\t", crc32_table[i]);

if (i % 4 == 3) printf("\n");

}

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

关于libmemcached中的crc的实现

缘起

问题

分析

留下评论

取消回复