Hash Table是PHP的核心,這話一點都不過度.算法
PHP的數組,關聯數組,對象屬性,函數表,符號表,等等都是用HashTable來作爲容器的.數組
PHP的HashTable採用的拉鍊法來解決衝突, 這個自不用多說, 我今天主要關注的就是PHP的Hash算法, 和這個算法自己透露出來的一些思想.緩存
PHP的Hash採用的是目前最爲廣泛的DJBX33A (Daniel J. Bernstein, Times 33 with Addition), 這個算法被普遍運用與多個軟件項目,Apache, Perl和Berkeley DB等. 對於字符串而言這是目前所知道的最好的哈希算法,緣由在於該算法的速度很是快,並且分類很是好(衝突小,分佈均勻).app
算法的核心思想就是:less
- hash(i) = hash(i-1) * 33 + str[i]
在zend_hash.h中,咱們能夠找到在PHP中的這個算法:函數
- 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 -=
{
- 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中直接採用的經典Times 33算法:post
- 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, 咱們能夠看出很處細緻的不一樣.優化
首先, 最不同的就是, PHP中並無使用直接乘33, 而是採用了:ui
- hash << 5 + hash
這樣固然會比用乘快了.this
而後, 特別要主意的就是使用的unrolled, 我前幾天看過一片文章講Discuz的緩存機制, 其中就有一條說是Discuz會根據帖子的熱度不一樣採用不一樣的緩存策略, 根據用戶習慣,而只緩存帖子的第一頁(由於不多有人會翻帖子).
於此相似的思想, PHP鼓勵8位一下的字符索引, 他以8爲單位使用unrolled來提升效率, 這不得不說也是個很細節的,很細緻的地方.
另外還有inline, register變量 … 能夠看出PHP的開發者在hash的優化上也是煞費苦心
最後就是, hash的初始值設置成了5381, 相比在Apache中的times算法和Perl中的Hash算法(都採用初始hash爲0), 爲何選5381呢? 具體的緣由我也不知道, 可是我發現了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
看了這些, 我有理由相信這個初始值的選定能提供更好的分類.
至於說, 爲何是Times 33而不是Times 其餘數字, 在PHP Hash算法的註釋中也有一些說明, 但願對有興趣的同窗有用:
- 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>