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

#include "internal.h"

int mbtowc(wchar_t* restrict wc, const char* restrict src, size_t n) {
  unsigned c;
  const unsigned char* s = (const void*)src;
  wchar_t dummy;

  if (!s)
    return 0;
  if (!n)
    goto ilseq;
  if (!wc)
    wc = &dummy;

  if (*s < 0x80)
    return !!(*wc = *s);
  if (MB_CUR_MAX == 1)
    return (*wc = CODEUNIT(*s)), 1;
  if (*s - SA > SB - SA)
    goto ilseq;
  c = bittab[*s++ - SA];

  /* Avoid excessive checks against n: If shifting the state n-1
   * times does not clear the high bit, then the value of n is
   * insufficient to read a character */
  if (n < 4 && ((c << (6 * n - 6)) & (1U << 31)))
    goto ilseq;

  if (OOB(c, *s))
    goto ilseq;
  c = c << 6 | (*s++ - 0x80);
  if (!(c & (1U << 31))) {
    *wc = c;
    return 2;
  }

  if (*s - 0x80u >= 0x40)
    goto ilseq;
  c = c << 6 | (*s++ - 0x80);
  if (!(c & (1U << 31))) {
    *wc = c;
    return 3;
  }

  if (*s - 0x80u >= 0x40)
    goto ilseq;
  *wc = c << 6 | (*s++ - 0x80);
  return 4;

ilseq:
  errno = EILSEQ;
  return -1;
}