Improve index_of with KMP algorithm

[davidgaspardev]

Description

Replace the current naive O(n×m) search in index_of with the Knuth-Morris-Pratt (KMP) algorithm, which runs in O(n+m) time by preprocessing the pattern to avoid redundant comparisons.

Current complexity

Naive: O(n × m)  — n = target length, m = fragment length

Target complexity

KMP:   O(n + m)  — linear time

How KMP works

1. Build a failure function (partial match table) from the pattern — O(m)
2. Use it to skip positions in the target that are guaranteed not to match — O(n)

References

• Algorithm: Knuth-Morris-Pratt string search
• Authors: Donald E. Knuth, James H. Morris, Vaughan R. Pratt (1977)
• Paper: Fast Pattern Matching in Strings, SIAM Journal on Computing, 1977
• Wikipedia: https://en.wikipedia.org/wiki/Knuth%E2%80%93Morris%E2%80%93Pratt_algorithm