/*
 * This code was written by Rich Felker in 2010; no copyright is claimed.
 * This code is in the public domain. Attribution is appreciated but
 * unnecessary.
 */

#include <wchar.h>

size_t mbsnrtowcs(wchar_t* restrict wcs,
                  const char** restrict src,
                  size_t n,
                  size_t wn,
                  mbstate_t* restrict st) {
  size_t l, cnt = 0, n2;
  wchar_t *ws, wbuf[256];
  const char* s = *src;

  if (!wcs)
    ws = wbuf, wn = sizeof wbuf / sizeof *wbuf;
  else
    ws = wcs;

  /* making sure output buffer size is at most n/4 will ensure
   * that mbsrtowcs never reads more than n input bytes. thus
   * we can use mbsrtowcs as long as it's practical.. */

  while (s && wn && ((n2 = n / 4) >= wn || n2 > 32)) {
    if (n2 >= wn)
      n2 = wn;
    n -= n2;
    l = mbsrtowcs(ws, &s, n2, st);
    if (!(l + 1)) {
      cnt = l;
      wn = 0;
      break;
    }
    if (ws != wbuf) {
      ws += l;
      wn -= l;
    }
    cnt += l;
  }
  if (s)
    while (wn && n) {
      l = mbrtowc(ws, s, n, st);
      if (l + 2 <= 2) {
        if (!(l + 1)) {
          cnt = l;
          break;
        }
        if (!l) {
          s = 0;
          break;
        }
        /* have to roll back partial character */
        *(unsigned*)st = 0;
        break;
      }
      s += l;
      n -= l;
      /* safe - this loop runs fewer than sizeof(wbuf)/8 times */
      ws++;
      wn--;
      cnt++;
    }
  if (wcs)
    *src = s;
  return cnt;
}