PHP中用hash实现的数组_php技巧

PHP中使用最多的非Array莫属了，那Array是如何实现的？在PHP内部Array通过一个hashtable来实现，其中使用链接法解决hash冲突的问题，这样最坏情况下，查找Array元素的复杂度为O(N)，最好则为1.
而其计算字符串hash值的方法如下，将源码摘出来以供查备：

 <BR>static inline ulong zend_inline_hash_func(const char *arKey, uint nKeyLength) <BR>{ <BR>register ulong hash = 5381;                                                   //此处初始值的设置有什么玄机么？ <BR>/* variant with the hash unrolled eight times */ <BR>for (; nKeyLength >= 8; nKeyLength -= 8) {                         //这种step=8的方式是为何？ <BR>hash = ((hash << 5) + hash) + *arKey++; <BR>hash = ((hash << 5) + hash) + *arKey++; <BR>hash = ((hash << 5) + hash) + *arKey++; <BR>hash = ((hash << 5) + hash) + *arKey++;                         //比直接*33要快 <BR>hash = ((hash << 5) + hash) + *arKey++; <BR>hash = ((hash << 5) + hash) + *arKey++; <BR>hash = ((hash << 5) + hash) + *arKey++; <BR>hash = ((hash << 5) + hash) + *arKey++; <BR>} <BR>switch (nKeyLength) { <BR>case 7: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */                             //此处是将剩余的字符hash <BR>case 6: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */ <BR>case 5: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */ <BR>case 4: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */ <BR>case 3: hash = ((hash << 5) + hash) + *arKey+<p>+本文来源gao!%daima.com搞$代*!码9网(</p><strong>搞gaodaima代码</strong>+; /* fallthrough... */ <BR>case 2: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough... */                     <BR>case 1: hash = ((hash << 5) + hash) + *arKey++; break; <BR>case 0: break; <BR>EMPTY_SWITCH_DEFAULT_CASE() <BR>} <BR>return hash;//返回hash值 <BR>} <BR>

ps：对于以下函数，仍有两点不明：
hash = 5381设置的理由？
这种step=8的循环方式是为了效率么？

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