// size_t mbrtoc16(
//     char16_t   *restrict pc16,
//     char const *restrict s,
//     size_t               n,
//     mbstate_t  *restrict ps
// ) {
//     // Figure out the conversion state
//     static mbstate_t static_mbstate = {0};
//     if(ps == NULL) ps = &static_mbstate;
//     if(s == NULL) {
//         *ps = (mbstate_t) {0xd800};
//         return 0;
//     }
//     // Check leftovers, using 0xd800 as "no leftover" marker because it
//     // doesn't encode a valid character.
//     if(ps->leftover == 0xd800) {
//         // Decode the UTF-8 encoded codepoint
//         char32_t code_point;
//         int mblen = utf8_chdec((char8_t *)s, n, &code_point);
//         if(mblen == UNI_ESTRLN) return (size_t)(-2);
//         if(mblen <= 0) goto invalid_seq;
//         // Encode the codepoint into UTF-16 string
//         char16_t str[2];
//         int c16len = utf16_chenc(str, 2, code_point);
//         if(c16len <= 0) goto invalid_seq;
//         // Assign the decoded UTF-16 character, decide leftover
//         if(pc16 != NULL) *pc16 = str[0];
//         ps->leftover = (c16len == 2? str[1] : 0xd800);
//         return (size_t)mblen;
//     }
//     else {
//         // Otherwise use and reset the leftover
//         if(pc16 != NULL) *pc16 = ps->leftover;
//         ps->leftover = 0xd800;
//         return (size_t)(-3);
//     }
// invalid_seq:
//     errno = EILSEQ;
//     return (size_t)(-1);
// }



// size_t c16rtomb(
//     char *restrict      s,
//     char16_t            c16,
//     mbstate_t *restrict ps
// ) {
//     // Figure out conversion state
//     static mbstate_t static_mbstate = {0};
//     if(ps == NULL) ps = &static_mbstate;
//     if(s == NULL) {
//         *ps = (mbstate_t) {0xd800};
//         return 0;
//     }
//     char32_t codepoint_to_write;
//     // Check whether a high surrogate was detected in a previous call to the
//     // function. If not, the high_surrogate value is 0xd800
//     if(ps->high_surrogate == 0xd800) {
//         // If c16 is a surrogate record it, or throw an error
//         if(uni_is_hsur(c16)) {
//             ps->high_surrogate = c16;
//             return 0;
//         }
//         else if(uni_is_lsur(c16)) {
//             goto invalid_char;
//         }
//         // We'll just write c16
//         codepoint_to_write = c16;
//     }
//     // If high surrogate exists, the next character must be a low surrogate
//     // so we'll write a codepoint made out of high and low surrogates
//     else if(uni_is_lsur(c16)) {
//         codepoint_to_write = uni_surtoc(ps->high_surrogate, c16);
//     }
//     else goto invalid_char;
//     // Write the codepoint that we decided to write to multibyte string
//     int written_len = utf8_chenc((char8_t *)s, 4, codepoint_to_write);
//     if(written_len < 0) {
//         goto invalid_char;
//     }
//     s[written_len] = 0;
//     return (size_t)written_len;
// invalid_char:
//     errno = EILSEQ;
//     return (size_t)(-1);
// }

// size_t mbrtoc32(
//     char32_t   *restrict pc32,
//     char const *restrict s,
//     size_t               n,
//     mbstate_t  *restrict ps
// ) {
//     if(s == NULL) {
//         return 0;
//     }
//     char32_t code_point;
//     int mblen = utf8_chdec((char8_t *)s, n, &code_point);
//     if(mblen == UNI_ESTRLN) return (size_t)(-2);
//     if(mblen <= 0) {
//         errno = EILSEQ;
//         return (size_t)(-1);
//     }
//     *pc32 = code_point;
//     if(code_point == 0) return 0;
//     return (size_t)mblen;
// }

// size_t c32rtomb(
//     char *restrict      s,
//     char32_t            c32,
//     mbstate_t *restrict ps
// ) {
//     if(s == NULL) {
//         *ps = (mbstate_t) {0};
//         return 0;
//     }
//     int mblen = utf8_chenc((char8_t *)s, 4, c32);
//     if(mblen <= 0) {
//         errno = EILSEQ;
//         return (size_t)(-1);
//     }
//     return (size_t)mblen;
// }