Optimize String.count and String.countn by avoiding repeated reallocations.

[Ivorforce]

This is a fairly straight-forward change.
I'm guessing the author was not aware that find and find have a p_from parameter (or they did not when the function was written).

Benchmark

I tested the following benchmark:

	String s = "Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.";

	auto t0 = std::chrono::high_resolution_clock::now();
	for (int i = 0; i < 100000; i ++) {
		// Test
		int x = s.count("it");
	}
	auto t1 = std::chrono::high_resolution_clock::now();
	std::cout << std::chrono::duration_cast<std::chrono::milliseconds>(t1 - t0).count() << "ms\n";

This yielded:

• 191ms on master (a40fc23)
• 66ms on this PR (ef3eecd)

So, we can expect an up to 3x speedup (specifically if many instances of the string are found).

[RandomShaper]

Looks good. I'm wondering if the one-time copies in the lines above could also be replaced by some additional awareness of the start and end indices, while you're at it.

[Ivorforce]

Looks good. I'm wondering if the one-time copies in the lines above could also be replaced by some additional awareness of the start and end indices, while you're at it.

I did consider that too, but it was impossible because find does not take a p_to parameter. It will be possible once #100293 is merged though, by simply clamping the view.

[Chaosus]

I'm guessing the author was not aware that find and find have a p_from parameter (or they did not when the function was written).

That's something I definitely would do at a similar scenario now. Thanks for this optimization!

[akien-mga]

Seems good to me.

[akien-mga]

Thanks!