1#include <string.h>
2#include <stdint.h>
3
4static char *twobyte_strstr(const unsigned char *h, const unsigned char *n)
5{
6	uint16_t nw = n[0]<<8 | n[1], hw = h[0]<<8 | h[1];
7	for (h++; *h && hw != nw; hw = hw<<8 | *++h);
8	return *h ? (char *)h-1 : 0;
9}
10
11static char *threebyte_strstr(const unsigned char *h, const unsigned char *n)
12{
13	uint32_t nw = n[0]<<24 | n[1]<<16 | n[2]<<8;
14	uint32_t hw = h[0]<<24 | h[1]<<16 | h[2]<<8;
15	for (h+=2; *h && hw != nw; hw = (hw|*++h)<<8);
16	return *h ? (char *)h-2 : 0;
17}
18
19static char *fourbyte_strstr(const unsigned char *h, const unsigned char *n)
20{
21	uint32_t nw = n[0]<<24 | n[1]<<16 | n[2]<<8 | n[3];
22	uint32_t hw = h[0]<<24 | h[1]<<16 | h[2]<<8 | h[3];
23	for (h+=3; *h && hw != nw; hw = hw<<8 | *++h);
24	return *h ? (char *)h-3 : 0;
25}
26
27#define MAX(a,b) ((a)>(b)?(a):(b))
28#define MIN(a,b) ((a)<(b)?(a):(b))
29
30#define BITOP(a,b,op) \
31 ((a)[(size_t)(b)/(8*sizeof *(a))] op (size_t)1<<((size_t)(b)%(8*sizeof *(a))))
32
33static char *twoway_strstr(const unsigned char *h, const unsigned char *n)
34{
35	const unsigned char *z;
36	size_t l, ip, jp, k, p, ms, p0, mem, mem0;
37	size_t byteset[32 / sizeof(size_t)] = { 0 };
38	size_t shift[256];
39
40	/* Computing length of needle and fill shift table */
41	for (l=0; n[l] && h[l]; l++)
42		BITOP(byteset, n[l], |=), shift[n[l]] = l+1;
43	if (n[l]) return 0; /* hit the end of h */
44
45	/* Compute maximal suffix */
46	ip = -1; jp = 0; k = p = 1;
47	while (jp+k<l) {
48		if (n[ip+k] == n[jp+k]) {
49			if (k == p) {
50				jp += p;
51				k = 1;
52			} else k++;
53		} else if (n[ip+k] > n[jp+k]) {
54			jp += k;
55			k = 1;
56			p = jp - ip;
57		} else {
58			ip = jp++;
59			k = p = 1;
60		}
61	}
62	ms = ip;
63	p0 = p;
64
65	/* And with the opposite comparison */
66	ip = -1; jp = 0; k = p = 1;
67	while (jp+k<l) {
68		if (n[ip+k] == n[jp+k]) {
69			if (k == p) {
70				jp += p;
71				k = 1;
72			} else k++;
73		} else if (n[ip+k] < n[jp+k]) {
74			jp += k;
75			k = 1;
76			p = jp - ip;
77		} else {
78			ip = jp++;
79			k = p = 1;
80		}
81	}
82	if (ip+1 > ms+1) ms = ip;
83	else p = p0;
84
85	/* Periodic needle? */
86	if (memcmp(n, n+p, ms+1)) {
87		mem0 = 0;
88		p = MAX(ms, l-ms-1) + 1;
89	} else mem0 = l-p;
90	mem = 0;
91
92	/* Initialize incremental end-of-haystack pointer */
93	z = h;
94
95	/* Search loop */
96	for (;;) {
97		/* Update incremental end-of-haystack pointer */
98		if (z-h < l) {
99			/* Fast estimate for MIN(l,63) */
100			size_t grow = l | 63;
101			const unsigned char *z2 = memchr(z, 0, grow);
102			if (z2) {
103				z = z2;
104				if (z-h < l) return 0;
105			} else z += grow;
106		}
107
108		/* Check last byte first; advance by shift on mismatch */
109		if (BITOP(byteset, h[l-1], &)) {
110			k = l-shift[h[l-1]];
111			//printf("adv by %zu (on %c) at [%s] (%zu;l=%zu)\n", k, h[l-1], h, shift[h[l-1]], l);
112			if (k) {
113				if (mem0 && mem && k < p) k = l-p;
114				h += k;
115				mem = 0;
116				continue;
117			}
118		} else {
119			h += l;
120			mem = 0;
121			continue;
122		}
123
124		/* Compare right half */
125		for (k=MAX(ms+1,mem); n[k] && n[k] == h[k]; k++);
126		if (n[k]) {
127			h += k-ms;
128			mem = 0;
129			continue;
130		}
131		/* Compare left half */
132		for (k=ms+1; k>mem && n[k-1] == h[k-1]; k--);
133		if (k <= mem) return (char *)h;
134		h += p;
135		mem = mem0;
136	}
137}
138
139char *strstr(const char *h, const char *n)
140{
141	/* Return immediately on empty needle */
142	if (!n[0]) return (char *)h;
143
144	/* Use faster algorithms for short needles */
145	h = strchr(h, *n);
146	if (!h || !n[1]) return (char *)h;
147	if (!h[1]) return 0;
148	if (!n[2]) return twobyte_strstr((void *)h, (void *)n);
149	if (!h[2]) return 0;
150	if (!n[3]) return threebyte_strstr((void *)h, (void *)n);
151	if (!h[3]) return 0;
152	if (!n[4]) return fourbyte_strstr((void *)h, (void *)n);
153
154	return twoway_strstr((void *)h, (void *)n);
155}
156