sortix--sortix/libc/wchar/wcrtomb.cpp

/*******************************************************************************

    Copyright(C) Jonas 'Sortie' Termansen 2012.

    This file is part of the Sortix C Library.

    The Sortix C Library is free software: you can redistribute it and/or modify
    it under the terms of the GNU Lesser General Public License as published by
    the Free Software Foundation, either version 3 of the License, or (at your
    option) any later version.

    The Sortix C Library is distributed in the hope that it will be useful, but
    WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
    or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
    License for more details.

    You should have received a copy of the GNU Lesser General Public License
    along with the Sortix C Library. If not, see <http://www.gnu.org/licenses/>.

    wchar/wcrtomb.cpp
    Convert a wide character to a multibyte sequence.

*******************************************************************************/

#include <errno.h>
#include <stdint.h>
#include <wchar.h>

extern "C"
size_t wcrtomb(char* restrict s, wchar_t wc, mbstate_t* restrict /*ps*/)
{
	if ( !wc )
	{
		if ( s )
			*s = '\0';
		return 1;
	}

	uint32_t unicode = wc;
	uint8_t* buf = (uint8_t*) s;
	unsigned bytes = 1;
	unsigned bits = 7;
	if ( (1U<<7U) <= unicode ) { bytes = 2; bits = 11; }
	if ( (1U<<11U) <= unicode ) { bytes = 3; bits = 16; }
	if ( (1U<<16U) <= unicode ) { bytes = 4; bits = 21; }
	if ( (1U<<21U) <= unicode ) { bytes = 5; bits = 26; }
	if ( (1U<<26U) <= unicode ) { bytes = 6; bits = 31; }
	if ( (1U<<31U) <= unicode ) { errno = EILSEQ; return (size_t) -1; }

	if ( !s )
		return bytes;

	uint8_t prefix;
	unsigned prefixavai;
	switch ( bytes )
	{
	case 1: prefixavai = 7; prefix = 0b0U << prefixavai; break;
	case 2: prefixavai = 5; prefix = 0b110U << prefixavai; break;
	case 3: prefixavai = 4; prefix = 0b1110U << prefixavai; break;
	case 4: prefixavai = 3; prefix = 0b11110U << prefixavai; break;
	case 5: prefixavai = 2; prefix = 0b111110U << prefixavai; break;
	case 6: prefixavai = 1; prefix = 0b1111110U << prefixavai; break;
	default: __builtin_unreachable();
	}

	// Put the first bits in the unused area of the prefix.
	prefix |= unicode >> (bits - prefixavai);
	*buf++ = prefix;
	unsigned bitsleft = bits - prefixavai;

	while ( bitsleft )
	{
		bitsleft -= 6;
		uint8_t elembits = (unicode>>bitsleft) & ((1U<<6U)-1U);
		uint8_t elem = (0b10U<<6U) | elembits;
		*buf++ = elem;
	}

	return bytes;
}