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

size_t mbrtoc16(char16_t* restrict pc16, const char* restrict s, size_t n, mbstate_t* restrict ps) {
  static unsigned internal_state;
  if (!ps)
    ps = (void*)&internal_state;
  unsigned* pending = (unsigned*)ps;

  if (!s)
    return mbrtoc16(0, "", 1, ps);

  /* mbrtowc states for partial UTF-8 characters have the high bit set;
   * we use nonzero states without high bit for pending surrogates. */
  if ((int)*pending > 0) {
    if (pc16)
      *pc16 = *pending;
    *pending = 0;
    return -3;
  }

  wchar_t wc;
  size_t ret = mbrtowc(&wc, s, n, ps);
  if (ret <= 4) {
    if (wc >= 0x10000) {
      *pending = (wc & 0x3ff) + 0xdc00;
      wc = 0xd7c0 + (wc >> 10);
    }
    if (pc16)
      *pc16 = wc;
  }
  return ret;
}