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

    Copyright(C) Jonas 'Sortie' Termansen 2012, 2014.

    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/mbrtowc.cpp
    Convert a multibyte sequence to a wide character.

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

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

static
size_t utf8_mbrtowc(wchar_t* restrict pwc,
                    const char* restrict s,
                    size_t n,
                    mbstate_t* restrict ps)
{
	size_t i;
	for ( i = 0; !(i && ps->count == 0); i++ )
	{
		// Handle the case where we were not able to fully decode a character,
		// but it is still possible to finish decoding given more bytes.
		if ( n <= i )
			return (size_t) -2;

		char c = s[i];
		unsigned char uc = (unsigned char) c;

		// The initial state is that we expect a leading byte that informs us of
		// the length of this character sequence. The number of consecutive high
		// order bits tells us how many bytes make up this character (one
		// leading byte followed by zero or more continuation bytes).
		if ( ps->count == 0 )
		{
			if ( (uc & 0b10000000) == 0b00000000 ) /* 0xxxxxxx */
			{
				ps->length = (ps->count = 0) + 1;
				ps->wch = (wchar_t) uc & 0b1111111;
			}
			else if ( (uc & 0b11100000) == 0b11000000 ) /* 110xxxxx */
			{
				ps->length = (ps->count = 1) + 1;
				ps->wch = (wchar_t) uc & 0b11111;
			}
			else if ( (uc & 0b11110000) == 0b11100000 ) /* 1110xxxx */
			{
				ps->length = (ps->count = 2) + 1;
				ps->wch = (wchar_t) uc & 0b1111;
			}
			else if ( (uc & 0b11111000) == 0b11110000 ) /* 11110xxx */
			{
				ps->length = (ps->count = 3) + 1;
				ps->wch = (wchar_t) uc & 0b111;
			}
#if 0 /* 5-byte and 6-byte sequences are forbidden by RFC 3629 */
			else if ( (uc & 0b11111100) == 0b11111000 ) /* 111110xx */
			{
				ps->length = (ps->count = 4) + 1) + 1;
				ps->wch = (wchar_t) uc & 0b11;
			}
			else if ( (uc & 0b11111110) == 0b11111100 ) /* 1111110x */
			{
				ps->length = (ps->count = 5) + 1) + 1;
				ps->wch = (wchar_t) uc & 0b1;
			}
#endif
			else
				return errno = EILSEQ, (size_t) -1;
		}

		// The secondary state is that following a leading byte, we are
		// expecting a non-zero number of continuation byte bytes.
		else
		{
			// Verify this is a continuation byte.
			if ( (uc & 0b11000000) != 0b10000000 )
				return errno = EILSEQ, (size_t) -1;
			ps->wch = ps->wch << 6 | (uc & 0b00111111);
			ps->count--;
		}
	}

	// Reject the character if it was produced with an overly long sequence.
	if ( ps->length == 1 && 1 << 7 <= ps->wch )
		return errno = EILSEQ, (size_t) -1;
	if ( ps->length == 2 && 1 << (5 + 1 * 6) <= ps->wch )
		return errno = EILSEQ, (size_t) -1;
	if ( ps->length == 3 && 1 << (4 + 2 * 6) <= ps->wch )
		return errno = EILSEQ, (size_t) -1;
	if ( ps->length == 4 && 1 << (3 + 3 * 6) <= ps->wch )
		return errno = EILSEQ, (size_t) -1;
#if 0 /* 5-byte and 6-byte sequences are forbidden by RFC 3629 */
	if ( ps->length == 5 && 1 << (2 + 4 * 6) <= ps->wch )
		return errno = EILSEQ, (size_t) -1;
	if ( ps->length == 6 && 1 << (1 + 5 * 6) <= ps->wch )
		return errno = EILSEQ, (size_t) -1;
#endif

	// RFC 3629 limits UTF-8 to 0x0 through 0x10FFFF.
	if ( 0x10FFFF <= ps->wch )
		return errno = EILSEQ, (size_t) -1;

	wchar_t result = ps->wch;

	if ( pwc )
		*pwc = result;

	ps->length = 0;
	ps->wch = 0;

	return result != L'\0' ? i : 0;
}

extern "C"
size_t mbrtowc(wchar_t* restrict pwc,
               const char* restrict s,
               size_t n,
               mbstate_t* restrict ps)
{
	static mbstate_t static_ps;
	if ( !ps )
		ps = &static_ps;
	if ( !s )
		s = "", n = 1;

	// TODO: Verify whether the current locale is UTF-8.
	return utf8_mbrtowc(pwc, s, n, ps);
}