[fpc-pascal] Best integer type to be used for string indexes

[Vladimir Zhirov]

Hi,

Reviewing a bunch of my string processing functions 
made me wonder what integer type should I use for
position / length parameters. 

I used to plain "Integer" for this task, but looking through
RTL and LCL code I noticed that sometimes SizeInt and
even PtrInt are also used:
- Length, Copy use Integer;
- Pos, Delete, Insert use SizeInt;
- UTF8Length, UTF8Pos, UTF8Copy, etc. use PtrInt.

PtrInt use seems to be generally discouraged by FPC docs 
but does it apply to this particular case?

The only benefit I see in SizeInt/PtrInt is larger size on 64-bit
targets, that should theoretically make it possible to index
larger strings. But is AnsiString type capable of holding more
than High(Integer) bytes, or will it be so in the future?

Thus, the question basically is: what integer type 
is recommended to be used as index/length 
for future-proof string processing functions?

--
Vladimir