while not a requirement, it's common convention in other iconv
implementations to accept "CHAR" as an alias for nl_langinfo(CODESET),
meaning the encoding used for char[] strings in the current locale,
and also "" as an alternate form. supporting this is not costly and
improves compatibility.
with these changes, the character set implemented as "big5" in musl is
a pure superset of cp950, the canonical "big5", and agrees with the
normative parts of Unicode. this means it has minor differences from
both hkscs and big5-2003:
- the range A2CC-A2CE maps to CJK ideographs rather than numerals,
contrary to changes made in big5-2003.
- C6CD maps to a CJK ideograph rather than its corresponding Kangxi
radical character, contrary to changes made in hkscs.
- F9FE maps to U+2593 rather than U+FFED.
of these differences, none but the last are visually distinct, and the
last is a character used purely for text-based graphics, not to convey
linguistic content.
should there be future demand for strict conformance to big5-2003 or
hkscs mappings, the present charset aliases can be replaced with
distinct variants.
reportedly there are other non-standard big5 extensions in common use
in Taiwan and perhaps elsewhere, which could also be added as layers
on top of the existing big5 support.
there may be additional characters which should be added to the hkscs
table: the whatwg standard for big5 defines what appears to be a
superset of hkscs.
at this point, it is just the common base charset equivalent to
Windows CP 950, with no further extensions. HKSCS and possibly other
supersets will be added later. other aliases may need to be added too.
like for other character sets, stateful iso-2022 form is not supported
yet but everything else should work. all charset aliases are treated
the same, as Windows codepage 949, because reportedly the EUC-KR
charset name is in widespread (mis?)usage in email and on the web for
data which actually uses the extended characters outside the standard
93x94 grid. this could easily be changed if desired.
the principle of this converter for handling the giant bulk of rare
Hangul syllables outside of the standard KS X 1001 93x94 grid is the
same as the GB18030 converter's treatment of non-explicitly-coded
Unicode codepoints: sequences in the extension range are mapped to an
integer index N, and the converter explicitly computes the Nth Hangul
syllable not explicitly encoded in the character map. empirically,
this requires at most 7 passes over the grid. this approach reduces
the table size required for Korean legacy encodings from roughly 44k
to 17k and should have minimal performance impact on real-world text
conversions since the "slow" characters are rare. where it does have
impact, the cost is merely a large constant time factor.
to deal with the fact that the public headers may be used with pre-c99
compilers, __restrict is used in place of restrict, and defined
appropriately for any supported compiler. we also avoid the form
[restrict] since older versions of gcc rejected it due to a bug in the
original c99 standard, and instead use the form *restrict.
sadly the C language does not specify any such implicit conversion, so
this is not a matter of just fixing warnings (as gcc treats it) but
actual errors. i would like to revisit a number of these changes and
possibly revise the types used to reduce the number of casts required.
Rich Felker