strstr.c

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