Python字符串 'in'算子实现算法和时间复杂度

Question

我在想 in 运算符是如何实现的，例如

>>> s1 = 'abcdef'
>>> s2 = 'bcd'
>>> s2 in s1
True

在CPython中，用什么算法来实现字符串匹配，时间复杂度是多少？有没有关于这个的官方文档或维基？

Answer 1

它是 Boyer-Moore 的组合和 Horspool .

可以查看C代码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: https://web.archive.org/web/20201107074620/http://effbot.org/zone/stringlib.htm.

从上面的链接:

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