Use StringView and Span in Player

[mateofio]

Depends on: EasyRPG/liblcf#384

This adds StringView in many places to make code generic to different string types.

Utils::ReplacePlaceholders has been greatly optimizing by removing all unnecessary copies from it's interface.

[mateofio]

Here are performance numbers for Utils::ReplacePlaceholders before and after the stringview/span conversion

with std::string and std::vector

-----------------------------------------------------------------
Benchmark                       Time             CPU   Iterations
-----------------------------------------------------------------
BM_ReplacePlaceholders        136 ns          135 ns      4977778

with StringView and Span

-----------------------------------------------------------------
Benchmark                       Time             CPU   Iterations
-----------------------------------------------------------------
BM_ReplacePlaceholders       94.1 ns         94.2 ns      7466667

So 30% of the time in this function was being spent just allocating and freeing temporary arrays.

[mateofio]

Played some rm2k games with the battle system testing the changes to wordwrap and replaceplaceholders. Everything seem fine.

[Ghabry]

Also have to learn about span, never used them before.

[Ghabry]

Question: So, a string view is always used when the target function does not take ownership on the string?

[mateofio]

I wish I could tell you yes, replace all const std::string& with StringView. Unfortunately this is C++ so there is always one big caveat.

StringView is not null terminated, so if at the end of your call stack you end up calling a C library which requires null terminated const char*, you'll end up having to make a copy so you can get a new string with a null byte at the end.

Aside from that caveat, yes use StringView where-ever you need a non-owning reference. Not only is it a generic adapter but the calling convention used is faster (see below).

Span does not have this caveat, fortunately so you can use it anywhere you'd pass a vector by reference.

Performance Note

const string& if you think about it in C terms, is essentially a double pointer const char**. The reference is a pointer to the string object, which contains another pointer to the bytes.

When you pass a StringView you're passing a const char*, removing one level of indirection. In some cases this can help the optimizer with it's aliasing analysis to produce faster code.

The same rule applies to vector and Span.

[Ghabry]

Question: Couldn't you also use ToString() here instead of std::string(). Does it make a difference?

[mateofio]

Ahh this was from the other PR before I added ToString. Will fix.

They do the same thing in this case, but ToString is easier to grep for.

[Ghabry]

Question: What's the advantage of using Make*()?

[mateofio]

So I don't have to say std::array<char,2>('V', 'U')

This was actually supposed to go in the standard library, but they dropped it after adding deduction guides in C++17.

https://en.cppreference.com/w/cpp/experimental/make_array