#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 = n[0] << 24 | n[1] << 16 | n[2] << 8;
    uint32_t hw = 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 = n[0] << 24 | n[1] << 16 | n[2] << 8 | n[3];
    uint32_t hw = 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);
}