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

static char* twobyte_strstr(const unsigned char* h, const unsigned char* n) {
    uint16_t nw = n[0] << 8 | n[1], hw = h[0] << 8 | h[1];
    for (h++; *h && hw != nw; hw = hw << 8 | *++h)
        ;
    return *h ? (char*)h - 1 : 0;
}

static char* threebyte_strstr(const unsigned char* h, 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 += 2; *h && hw != nw; hw = (hw | *++h) << 8)
        ;
    return *h ? (char*)h - 2 : 0;
}

static char* fourbyte_strstr(const unsigned char* h, 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 += 3; *h && hw != nw; hw = hw << 8 | *++h)
        ;
    return *h ? (char*)h - 3 : 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_strstr(const unsigned char* h, const unsigned char* n) {
    const unsigned char* z;
    size_t l, 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 (l = 0; n[l] && h[l]; l++)
        BITOP(byteset, n[l], |=)
    , shift[n[l]] = l + 1;
    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 (memcmp(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 unsigned char* z2 = memchr(z, 0, grow);
            if (z2) {
                z = z2;
                if (z - h < l)
                    return 0;
            } else
                z += grow;
        }

        /* Check last byte first; advance by shift on mismatch */
        if (BITOP(byteset, h[l - 1], &)) {
            k = l - shift[h[l - 1]];
            // printf("adv by %zu (on %c) at [%s] (%zu;l=%zu)\n", k, h[l-1], h, shift[h[l-1]], l);
            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); 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 (char*)h;
        h += p;
        mem = mem0;
    }
}

char* strstr(const char* h, const char* n) {
    /* Return immediately on empty needle */
    if (!n[0])
        return (char*)h;

    /* Use faster algorithms for short needles */
    h = strchr(h, *n);
    if (!h || !n[1])
        return (char*)h;
    if (!h[1])
        return 0;
    if (!n[2])
        return twobyte_strstr((void*)h, (void*)n);
    if (!h[2])
        return 0;
    if (!n[3])
        return threebyte_strstr((void*)h, (void*)n);
    if (!h[3])
        return 0;
    if (!n[4])
        return fourbyte_strstr((void*)h, (void*)n);

    return twoway_strstr((void*)h, (void*)n);
}