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

size_t c16rtomb(char* restrict s, char16_t c16, mbstate_t* restrict ps) {
  static unsigned internal_state;
  if (!ps)
    ps = (void*)&internal_state;
  unsigned* x = (unsigned*)ps;
  wchar_t wc;

  if (!s) {
    if (*x)
      goto ilseq;
    return 1;
  }

  if (!*x && c16 - 0xd800u < 0x400) {
    *x = (c16 - 0xd7c0) << 10;
    return 0;
  }

  if (*x) {
    if (c16 - 0xdc00u >= 0x400)
      goto ilseq;
    else
      wc = *x + c16 - 0xdc00;
    *x = 0;
  } else {
    wc = c16;
  }
  return wcrtomb(s, wc, 0);

ilseq:
  *x = 0;
  errno = EILSEQ;
  return -1;
}