快捷導(dǎo)航

PHP中用hash實(shí)現(xiàn)的數(shù)組

更新時(shí)間：2011年07月17日 12:50:25 作者：

今天回顧學(xué)習(xí)了PHP中變量實(shí)現(xiàn)的方法，在瀏覽其源碼是發(fā)現(xiàn)在PHP中所有的數(shù)據(jù)類型通過(guò)一個(gè)union存儲(chǔ)。php語(yǔ)言是弱類型語(yǔ)言，其實(shí)現(xiàn)中通過(guò)記錄變量的類型和值來(lái)實(shí)現(xiàn)其管理。

PHP中使用最多的非Array莫屬了，那Array是如何實(shí)現(xiàn)的？在PHP內(nèi)部Array通過(guò)一個(gè)hashtable來(lái)實(shí)現(xiàn)，其中使用鏈接法解決hash沖突的問(wèn)題，這樣最壞情況下，查找Array元素的復(fù)雜度為O(N)，最好則為1.
而其計(jì)算字符串hash值的方法如下，將源碼摘出來(lái)以供查備：

復(fù)制代碼代碼如下:

 
static inline ulong zend_inline_hash_func(const char *arKey, uint nKeyLength) 
{ 
register ulong hash = 5381; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; //此處初始值的設(shè)置有什么玄機(jī)么？ 
/* variant with the hash unrolled eight times */ 
for (; nKeyLength >= 8; nKeyLength -= 8) { &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; //這種step=8的方式是為何？ 
hash = ((hash << 5) + hash) + *arKey++; 
hash = ((hash << 5) + hash) + *arKey++; 
hash = ((hash << 5) + hash) + *arKey++; 
hash = ((hash << 5) + hash) + *arKey++; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; //比直接*33要快 
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... */ &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; //此處是將剩余的字符hash 
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... */ &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; 
case 1: hash = ((hash << 5) + hash) + *arKey++; break; 
case 0: break; 
EMPTY_SWITCH_DEFAULT_CASE() 
} 
return hash;//返回hash值 
} 