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PHP中的Hash算法

Hash Table是PHP的核心,这话一点都不过分.
PHP的数组,关联数组,对象属性,函数表,符号表,等等都是用HashTable来做为容器的.
PHP的HashTable采用的拉链法来解决冲突, 这个自不用多说, 我今天主要关注的就是PHP的Hash算法, 和这个算法本身透露出来的一些思想.
PHP的Hash采用的是目前最为普遍的DJBX33A (Daniel J. Bernstein, Times 33 with Addition), 这个算法被广泛运用与多个软件项目,Apache, Perl和Berkeley DB等. 对于字符串而言这是目前所知道的最好的哈希算法,原因在于该算法的速度非常快,而且分类非常好(冲突小,分布均匀).
算法的核心思想就是:

	hash(i) = hash(i-1) * 33 + str[i]

在zend_hash.h中,我们可以找到在PHP中的这个算法:

static inline ulong zend_inline_hash_func(char *arKey, uint nKeyLength)
{
    register ulong hash = 5381;
    /* variant with the hash unrolled eight times */
    for (; nKeyLength >= 8; nKeyLength -= 8) {
        hash = ((hash << 5) + hash) + *arKey++;
        hash = ((hash << 5) + hash) + *arKey++;
        hash = ((hash << 5) + hash) + *arKey++;
        hash = ((hash << 5) + hash) + *arKey++;
        hash = ((hash << 5) + hash) + *arKey++;
        hash = ((hash << 5) + hash) + *arKey++;
        hash = ((hash << 5) + hash) + *arKey++;
        hash = ((hash << 5) + hash) + *arKey++;
    }
    switch (nKeyLength) {
        case 7: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */
        case 6: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */
        case 5: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */
        case 4: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */
        case 3: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */
        case 2: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */
        case 1: hash = ((hash << 5) + hash) + *arKey++; break;
        case 0: break;
EMPTY_SWITCH_DEFAULT_CASE()
    }
    return hash;
}

相比在Apache和Perl中直接采用的经典Times 33算法:

hashing function used in Perl 5.005:
  # Return the hashed value of a string: $hash = perlhash("key")
  # (Defined by the PERL_HASH macro in hv.h)
  sub perlhash
  {
      $hash = 0;
      foreach (split //, shift) {
          $hash = $hash*33 + ord($_);
      }
      return $hash;
  }

在PHP的hash算法中, 我们可以看出很处细致的不同.
首先, 最不一样的就是, PHP中并没有使用直接乘33, 而是采用了:

  hash << 5 + hash

这样当然会比用乘快了.
然后, 特别要主意的就是使用的unrolled, 我前几天看过一片文章讲Discuz的缓存机制, 其中就有一条说是Discuz会根据帖子的热度不同采用不同的缓存策略, 根据用户习惯,而只缓存帖子的第一页(因为很少有人会翻帖子).
于此类似的思想, PHP鼓励8位一下的字符索引, 他以8为单位使用unrolled来提高效率, 这不得不说也是个很细节的,很细致的地方.
另外还有inline, register变量 ... 可以看出PHP的开发者在hash的优化上也是煞费苦心
最后就是, hash的初始值设置成了5381, 相比在Apache中的times算法和Perl中的Hash算法(都采用初始hash为0), 为什么选5381呢? 具体的原因我也不知道, 但是我发现了5381的一些特性:

Magic Constant 5381:
  1. odd number
  2. prime number
  3. deficient number
  4. 001/010/100/000/101 b

看了这些, 我有理由相信这个初始值的选定能提供更好的分类.
至于说, 为什么是Times 33而不是Times 其他数字, 在PHP Hash算法的注释中也有一些说明, 希望对有兴趣的同学有用:

  DJBX33A (Daniel J. Bernstein, Times 33 with Addition)
  This is Daniel J. Bernstein's popular `times 33' hash function as
  posted by him years ago on comp.lang.c. It basically uses a function
  like ``hash(i) = hash(i-1) * 33 + str[i]''. This is one of the best
  known hash functions for strings. Because it is both computed very
  fast and distributes very well.
  The magic of number 33, i.e. why it works better than many other
  constants, prime or not, has never been adequately explained by
  anyone. So I try an explanation: if one experimentally tests all
  multipliers between 1 and 256 (as RSE did now) one detects that even
  numbers are not useable at all. The remaining 128 odd numbers
  (except for the number 1) work more or less all equally well. They
  all distribute in an acceptable way and this way fill a hash table
  with an average percent of approx. 86%.
  If one compares the Chi^2 values of the variants, the number 33 not
  even has the best value. But the number 33 and a few other equally
  good numbers like 17, 31, 63, 127 and 129 have nevertheless a great
  advantage to the remaining numbers in the large set of possible
  multipliers: their multiply operation can be replaced by a faster
  operation based on just one shift plus either a single addition
  or subtraction operation. And because a hash function has to both
  distribute good _and_ has to be very fast to compute, those few
  numbers should be preferred and seems to be the reason why Daniel J.
  Bernstein also preferred it.
                   -- Ralf S. Engelschall <rse@engelschall.com>

33 Comments

  1. 1
    1 August 2, 2016

    8)

  2. […]        写这篇文章的时候,发现鸟哥已经写了一篇《PHP中的hash算法》,里边对hash的算法、思想等都做了比较详细的解答,这里就不在做过多的解释,只说一点:unrolled。unrolled本身是展开的意思,对于nKeyLength长度的key, PHP的hash算法会以8为单位做unrolled,也就是这样的形式: […]

  3. […]        写这篇文章的时候,发现鸟哥已经写了一篇《PHP中的hash算法》,里边对hash的算法、思想等都做了比较详细的解答,这里就不在做过多的解释,只说一点:unrolled。unrolled本身是展开的意思,对于nKeyLength长度的key, PHP的hash算法会以8为单位做unrolled,也就是这样的形式: […]

  4. 啊啊
    啊啊 April 28, 2014

    厉害,学习了

  5. Anonymous
    Anonymous March 5, 2012

    能把代码里的那个笑脸给修正下吗

  6. Sapphire
    Sapphire March 25, 2011

    for (; nKeyLength >= 8; nKeyLength -= 8 ) {
    鸟哥这里修正一下啦

  7. 竹纤维家纺
    竹纤维家纺 October 27, 2010

    现代都市生活忙碌而紧张,一个竹纤维家纺温暖的家能带给每个人最放松的休闲时光,如果说 的发展顺应人们热爱健康、
    巴黎诗碧曼注重环保的迫切需求;就是这样的一个品牌,看好 ,源于它 ,其推出的竹纤维品牌家纺产品绝对是符合市场消费需求的产品,深大玉妹符合追求健康时尚产品和生活品味的人们所要求的。2.自动割管机机械设备防寒过冬所需各种油料的储备工作,自动割料机及时更换各类机械发动机的润滑油和防冻液,检查调整蓄电池电解液的比重等;带式过滤机对轨道车等进行彻底检查,严格按照要求做好橡胶带式过滤机制动试验,确保制动性能良好。

  8. MURRAY28ZELMA
    MURRAY28ZELMA July 28, 2010

    Houses and cars are quite expensive and not everyone is able to buy it. However, loans was created to support people in such cases.

  9. 小蔡
    小蔡 October 24, 2009

    使用hash方式组织数组,有没有比较过当数组元素个数多大的时候会比较大的影响效率呀?

  10. lovecs0079
    lovecs0079 October 5, 2009

    请问这个函数可以直接在PHP中调用吗?

  11. panjinww
    panjinww July 29, 2009

    受教了

  12. sychen
    sychen July 24, 2009

    博主对PHP的研究,很深层!

  13. 雪候鸟
    雪候鸟 July 23, 2009

    @xi2008wang 哦~~,对的.

  14. xi2008wang
    xi2008wang July 23, 2009

    就是54chen的《Discuz! BBS缓存整体方案》,呵呵

Comments are closed.