#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;
}