#include <wchar.h>

#define MAX(a, b) ((a) > (b) ? (a) : (b))
#define MIN(a, b) ((a) < (b) ? (a) : (b))

static wchar_t* twoway_wcsstr(const wchar_t* h, const wchar_t* n) {
  const wchar_t* z;
  size_t l, ip, jp, k, p, ms, p0, mem, mem0;

  /* Computing length of needle */
  for (l = 0; n[l] && h[l]; l++)
    ;
  if (n[l])
    return 0; /* hit the end of h */

  /* Compute maximal suffix */
  ip = -1;
  jp = 0;
  k = p = 1;
  while (jp + k < l) {
    if (n[ip + k] == n[jp + k]) {
      if (k == p) {
        jp += p;
        k = 1;
      } else
        k++;
    } else if (n[ip + k] > n[jp + k]) {
      jp += k;
      k = 1;
      p = jp - ip;
    } else {
      ip = jp++;
      k = p = 1;
    }
  }
  ms = ip;
  p0 = p;

  /* And with the opposite comparison */
  ip = -1;
  jp = 0;
  k = p = 1;
  while (jp + k < l) {
    if (n[ip + k] == n[jp + k]) {
      if (k == p) {
        jp += p;
        k = 1;
      } else
        k++;
    } else if (n[ip + k] < n[jp + k]) {
      jp += k;
      k = 1;
      p = jp - ip;
    } else {
      ip = jp++;
      k = p = 1;
    }
  }
  if (ip + 1 > ms + 1)
    ms = ip;
  else
    p = p0;

  /* Periodic needle? */
  if (wmemcmp(n, n + p, ms + 1)) {
    mem0 = 0;
    p = MAX(ms, l - ms - 1) + 1;
  } else
    mem0 = l - p;
  mem = 0;

  /* Initialize incremental end-of-haystack pointer */
  z = h;

  /* Search loop */
  for (;;) {
    /* Update incremental end-of-haystack pointer */
    if (z - h < l) {
      /* Fast estimate for MIN(l,63) */
      size_t grow = l | 63;
      const wchar_t* z2 = wmemchr(z, 0, grow);
      if (z2) {
        z = z2;
        if (z - h < l)
          return 0;
      } else
        z += grow;
    }

    /* Compare right half */
    for (k = MAX(ms + 1, mem); n[k] && n[k] == h[k]; k++)
      ;
    if (n[k]) {
      h += k - ms;
      mem = 0;
      continue;
    }
    /* Compare left half */
    for (k = ms + 1; k > mem && n[k - 1] == h[k - 1]; k--)
      ;
    if (k <= mem)
      return (wchar_t*)h;
    h += p;
    mem = mem0;
  }
}

wchar_t* wcsstr(const wchar_t* restrict h, const wchar_t* restrict n) {
  /* Return immediately on empty needle or haystack */
  if (!n[0])
    return (wchar_t*)h;
  if (!h[0])
    return 0;

  /* Use faster algorithms for short needles */
  h = wcschr(h, *n);
  if (!h || !n[1])
    return (wchar_t*)h;
  if (!h[1])
    return 0;

  return twoway_wcsstr(h, n);
}