Character Conversions Handling

Hi,

I'm trying to understand the schema laying behind
backend/utils/adt/like.c to downcase letters [1]. When I look at the
other tolower() implementations, there're lots of them spread around.
(In interfaces/libpq, backend/regex, backend/utils/adt/like and etc.)
For example, despite having pg_wc_tolower() function in regc_locale.c,
achieving same with manually in iwchareq() of like.c.

I'd so appreciated if somebody can point me the places where I should
start to look at to understand the character handling with different
encodings. Also, I wonder why didn't we use any btow/mbsrtowc/wctomb
like functions. Is this for portability with other compilers?

[1] iwchareq() is using pg_mb2wchar_with_len() which decides the right
mb2wchar function from pg_wchar_table. When I look at
backend/mb/wchar.c there're some other specific to locale mblen and
mb2wchar routines. For example, EUC_KR is handled with
pg_euc2wchar_with_len() function, but LATIN5 is handled with
pg_latin12wchar_with_len() function. Will we write a new function for
latin5 like pg_latin52wchar_with_len() if we'd encounter with a new
problem with latin5?

Regards.