Implementation of array hash function in PHP

PHP The most used non Array It must be , that Array How is it realized ？

stay PHP Inside Array Through one hashtable To achieve , Among them, the link method is used to solve hash The question of conflict , In the worst case , lookup Array The complexity of the element is O(N), The best is 1.

And its calculation string hash The method of calculating the value is as follows , Pick out the source code for reference ：

ps： For the following functions , Two points remain unclear ：

1. hash = 5381 The reason for setting ？

2. such step=8 Is the circulation mode of for efficiency ？

Php Code

1. static inline ulong zend_inline_hash_func(const char *arKey, uint nKeyLength)
2. {
3. register ulong hash = 5381; // Is there any mystery about the setting of the initial value here ？
5. /* variant with the hash unrolled eight times */
6. for (; nKeyLength >= 8; nKeyLength -= 8) { // such step=8 The way is ？
7. hash = ((hash << 5) + hash) + *arKey++;
8. hash = ((hash << 5) + hash) + *arKey++;
9. hash = ((hash << 5) + hash) + *arKey++;
10. hash = ((hash << 5) + hash) + *arKey++; // Than directly *33 Be quick
11. hash = ((hash << 5) + hash) + *arKey++;
12. hash = ((hash << 5) + hash) + *arKey++;
13. hash = ((hash << 5) + hash) + *arKey++;
14. hash = ((hash << 5) + hash) + *arKey++;
15. }
16. switch (nKeyLength) {
17. case 7: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough… */ // Here are the remaining characters hash
18. case 6: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough… */
19. case 5: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough… */
20. case 4: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough… */
21. case 3: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough… */
22. case 2: hash = ((hash << 5) + hash) + *arKey++; /* fallthrough… */
23. case 1: hash = ((hash << 5) + hash) + *arKey++; break;
24. case 0: break;
25. EMPTY_SWITCH_DEFAULT_CASE()
26. }
27. return hash; // return hash value
28. }

Publisher ： Full stack programmer stack length , Reprint please indicate the source ：https://javaforall.cn/109530.html Link to the original text ：https://javaforall.cn