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New strstr() implementation from musl libc by Rich Felker.  This
version uses the two-way string matching algorithm and is faster
than the old implementation.  With this change, ports that check
for strstr having linear complexity time strstr will no longer
replace the libc strstr with a private version.
OK deraadt@ espie@

/*	$OpenBSD: strstr.c,v 1.7 2017/04/12 16:06:12 millert Exp $ */

/*
 * Copyright (c) 2005-2014 Rich Felker
 *
 * Permission is hereby granted, free of charge, to any person obtaining
 * a copy of this software and associated documentation files (the
 * "Software"), to deal in the Software without restriction, including
 * without limitation the rights to use, copy, modify, merge, publish,
 * distribute, sublicense, and/or sell copies of the Software, and to
 * permit persons to whom the Software is furnished to do so, subject to
 * the following conditions:
 *
 * The above copyright notice and this permission notice shall be
 * included in all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
 * IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
 * CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
 * TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
 * SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 */

#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))))

/*
 * Maxime Crochemore and Dominique Perrin, Two-way string-matching,
 * Journal of the ACM, 38(3):651-675, July 1991.
 */
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]];
#ifdef DEBUG
			printf("adv by %zu (on %c) at [%s] (%zu;l=%zu)\n", k, h[l-1], h, shift[h[l-1]], l);
#endif
			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);
}
DEF_STRONG(strstr);