source: azure_iot_hub/trunk/musl-1.1.18/src/string/memmem.c@ 388

#define _GNU_SOURCE
#include <string.h>
#include <stdint.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)] = { 0 };
        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);
}