#define _GNU_SOURCE
#include <stdint.h>
#include <string.h>

static char* twobyte_memmem(const unsigned char* h, size_t k, const unsigned char* n) {
  uint16_t nw = n[0] << 8 | n[1], hw = h[0] << 8 | h[1];
  for (h += 2, k -= 2; k; k--, hw = hw << 8 | *h++)
    if (hw == nw)
      return (char*)h - 2;
  return hw == nw ? (char*)h - 2 : 0;
}

static char* threebyte_memmem(const unsigned char* h, size_t k, const unsigned char* n) {
  uint32_t nw = ((uint32_t)n[0]) << 24 | n[1] << 16 | n[2] << 8;
  uint32_t hw = ((uint32_t)h[0]) << 24 | h[1] << 16 | h[2] << 8;
  for (h += 3, k -= 3; k; k--, hw = (hw | *h++) << 8)
    if (hw == nw)
      return (char*)h - 3;
  return hw == nw ? (char*)h - 3 : 0;
}

static char* fourbyte_memmem(const unsigned char* h, size_t k, const unsigned char* n) {
  uint32_t nw = ((uint32_t)n[0]) << 24 | n[1] << 16 | n[2] << 8 | n[3];
  uint32_t hw = ((uint32_t)h[0]) << 24 | h[1] << 16 | h[2] << 8 | h[3];
  for (h += 4, k -= 4; k; k--, hw = hw << 8 | *h++)
    if (hw == nw)
      return (char*)h - 4;
  return hw == nw ? (char*)h - 4 : 0;
}

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

#define BITOP(a, b, op) \
  ((a)[(size_t)(b) / (8 * sizeof *(a))] op(size_t) 1 << ((size_t)(b) % (8 * sizeof *(a))))

static char* twoway_memmem(const unsigned char* h, const unsigned char* z, const unsigned char* n,
                           size_t l) {
  size_t i, ip, jp, k, p, ms, p0, mem, mem0;
  size_t byteset[32 / sizeof(size_t)] = {};
  size_t shift[256];

  /* Computing length of needle and fill shift table */
  for (i = 0; i < l; i++)
    BITOP(byteset, n[i], |=)
  , shift[n[i]] = i + 1;

  /* 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 (memcmp(n, n + p, ms + 1)) {
    mem0 = 0;
    p = MAX(ms, l - ms - 1) + 1;
  } else
    mem0 = l - p;
  mem = 0;

  /* Search loop */
  for (;;) {
    /* If remainder of haystack is shorter than needle, done */
    if (z - h < l)
      return 0;

    /* Check last byte first; advance by shift on mismatch */
    if (BITOP(byteset, h[l - 1], &)) {
      k = l - shift[h[l - 1]];
      if (k) {
        if (mem0 && mem && k < p)
          k = l - p;
        h += k;
        mem = 0;
        continue;
      }
    } else {
      h += l;
      mem = 0;
      continue;
    }

    /* Compare right half */
    for (k = MAX(ms + 1, mem); k < l && n[k] == h[k]; k++)
      ;
    if (k < l) {
      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 (char*)h;
    h += p;
    mem = mem0;
  }
}

void* memmem(const void* h0, size_t k, const void* n0, size_t l) {
  const unsigned char *h = h0, *n = n0;

  /* Return immediately on empty needle */
  if (!l)
    return (void*)h;

  /* Return immediately when needle is longer than haystack */
  if (k < l)
    return 0;

  /* Use faster algorithms for short needles */
  h = memchr(h0, *n, k);
  if (!h || l == 1)
    return (void*)h;
  k -= h - (const unsigned char*)h0;
  if (k < l)
    return 0;
  if (l == 2)
    return twobyte_memmem(h, k, n);
  if (l == 3)
    return threebyte_memmem(h, k, n);
  if (l == 4)
    return fourbyte_memmem(h, k, n);

  return twoway_memmem(h, h + k, n, l);
}