[shu-chen-dot-ca]

This Pull Request fixes/closes #5651.

JsStr::index_of (StringIndexOf) does self.windows(n).skip(from_index). Our Windows wrapper only implemented next, so Skip fell back to the default Iterator::nth, which calls next once per skipped window. Every search therefore cost O(from_index) before its first comparison, which made forward indexOf loops, includes with a position, and split / replaceAll with a string pattern quadratic in the string length.

It changes the following:

• Windows now forwards nth and size_hint to the inner std::slice::Windows, so skipping is constant time (and size_hint is exact, matching the existing ExactSizeIterator impl).
• Adds windows_nth_and_size_hint (checks nth, size_hint and the following next against std::slice::Windows for Latin-1 and UTF-16, every window size and skip count, including out of range ones).
• Adds index_of_from_index (compares index_of against a naive search for every from_index up to len + 2, for one- and two-unit needles and the empty needle, on Latin-1 and UTF-16 strings, plus a match right at a large from_index).

Timings with the release boa CLI on a string of "abcdefghi@".repeat(40000) (400,000 units, 40,000 matches):

	before	after
indexOf loop from the previous match	3425 ms	14 ms
split("@")	3692 ms	2 ms
replaceAll("@", "#")	3053 ms	4 ms
indexOf loop, UTF-16 string	3661 ms	13 ms
lastIndexOf loop (unchanged, for comparison)	33 ms	24 ms

All results are identical before and after.

Tested with:

• cargo fmt --all --check
• cargo clippy -p boa_string --all-features --all-targets -- -D warnings
• cargo test -p boa_string
• cargo test -p boa_engine -- builtins::string builtins::regexp

Screenshots

Before	After