问题
I am thinking of how the in operator implement, for instance
>>> s1 = \'abcdef\'
>>> s2 = \'bcd\'
>>> s2 in s1
True
In CPython, which algorithm is used to implement the string match, and what is the time complexity? Is there any official document or wiki about this?
回答1:
It's a combination of Boyer-Moore and Horspool.
You can view the C code here:
Fast search/count implementation, based on a mix between Boyer-Moore and Horspool, with a few more bells and whistles on the top. For some more background, see: http://effbot.org/zone/stringlib.htm.
From the link above:
When designing the new algorithm, I used the following constraints:
- should be faster than the current brute-force algorithm for all test cases (based on real-life code), including Jim Hugunin’s worst-case test
- small setup overhead; no dynamic allocation in the fast path (O(m) for speed, O(1) for storage)
- sublinear search behaviour in good cases (O(n/m))
- no worse than the current algorithm in worst case (O(nm))
- should work well for both 8-bit strings and 16-bit or 32-bit Unicode strings (no O(σ) dependencies)
- many real-life searches should be good, very few should be worst case reasonably simple implementation
来源：https://stackoverflow.com/questions/18139660/python-string-in-operator-implementation-algorithm-and-time-complexity