#include <uchar.h>
#include <errno.h>

size_t mbrtoc16(
    char16_t   *restrict pc16,
    char const *restrict s,
    size_t               n,
    mbstate_t  *restrict ps
) {
    if(s == NULL) {
        return 0;
    }
    size_t nbytes;
    char16_t parsed_char;
    char16_t next_char;
    // First check leftovers
    if(ps->leftover == 0) {
        // Decode the first byte of UTF-8 sequence
        unsigned byte0 = *s;
        if     (0x00 <= byte0 && byte0 < 0x80) nbytes = 1;
        else if(0xc0 <= byte0 && byte0 < 0xe0) nbytes = 2;
        else if(0xe0 <= byte0 && byte0 < 0xf0) nbytes = 3;
        else if(0xf0 <= byte0 && byte0 < 0xf8) nbytes = 4;
        else goto encoding_error;
        unsigned nbytesreq = nbytes;
        if(n < nbytesreq) {
            return (size_t)(-2);
        }
        char32_t cp = byte0;
        switch(nbytesreq) {
            case 2: cp &= 0x1f; break;
            case 3: cp &= 0x0f; break;
            case 4: cp &= 0x07; break;
        }
        while(--nbytesreq)
            cp |= (cp << 6) | ((*++s) & 0x3f);
        if(0xdc00 <= cp && cp <= 0xe000)
            goto encoding_error;
        // Overloing seqs
        if(cp < 0x80    && nbytes > 1) goto encoding_error;
        if(cp < 0x800   && nbytes > 2) goto encoding_error;
        if(cp < 0x10000 && nbytes > 3) goto encoding_error;
        if(cp > 0x10ffff) goto encoding_error;
        // Now convert this char shit to UTF-16
        if(cp < 0x10000) {
            parsed_char = cp;
            next_char   = 0; // no next
        }
        else {
            cp -= 0x10000;
            parsed_char = 0xd800 | (cp & 0x3ff);
            next_char   = 0xdc00 | (cp >> 10);
        }
    }
    else {
        if(pc16 != NULL) *pc16 = ps->leftover;
        ps->leftover = 0;
        return (size_t)(-3);
    }
    if(pc16 != NULL) *pc16 = parsed_char;
    ps->leftover = next_char;
    if(parsed_char == 0)
        return 0;
    else
        return nbytes;
encoding_error:
    errno = EILSEQ;
    return (size_t)(-1);
}

size_t c16rtomb(
    char *restrict      s,
    char16_t            c16,
    mbstate_t *restrict ps
);

size_t mbrtoc32(
    char32_t   *restrict pc32,
    char const *restrict s,
    size_t               n,
    mbstate_t  *restrict ps
);

size_t c32rtomb(
    char *restrict      s,
    char32_t            c32,
    mbstate_t *restrict ps
);