Replace usage of range-checked 'at' method when vector/string has already been size checked

[eduar-hte]

what

Replace places in the codebase where the at method of a std::vector or std::string instance is used to access an element instead of the subscript operator ([]), and the index of the item is guaranteed to be valid.

Additionally, simplify some of these uses where an old-style C loop is used to iterate over all elements and replace it with a range-based loops (which also implicitly guarantees no out-of-bound access).

why

The at method is not equivalent to the subscript operator ([]) operator, as it's range-checked and can thus throw a std::out_of_range exception which makes the code slower not only because of the additional check but also due to the generation of exception handling code at these call sites.

Because the updated instances are guaranteed to access valid indexes in the container, we can replace the usage of at to improve performance.

And where possible, simplify the code by adopting ranged-based loops.

misc

This is part of a series of PRs to improve performance of the library (7/n). Previous: #3253

[eduar-hte]

This is part of a series of PRs to improve performance of the library (7/n). Previous: #3253

These changes provide minor performance improvements to the library.

These results were obtained running the benchmark test with 100'000 iterations with the OWASP CRS v4 and are presented as a reference (average of three runs):

• Mac mini (1st generation)
  • v3/master: 80.74 secs
  • This PR: 80.39 secs (0.44% improvement)
• Debian (bullseye) container running on WSL on Windows Dev Kit 2023
  • v3/master: 119.01 secs
  • This PR: 114.87 secs (3.5% improvement)

[sonarcloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[airween]

LGTM

[airween]

Thanks @eduar-hte!