strstr.c raw
1 #include <string.h>
2 #include <stdint.h>
3
4 static 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
11 static char *threebyte_strstr(const unsigned char *h, const unsigned char *n)
12 {
13 uint32_t nw = (uint32_t)n[0]<<24 | n[1]<<16 | n[2]<<8;
14 uint32_t hw = (uint32_t)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
19 static char *fourbyte_strstr(const unsigned char *h, const unsigned char *n)
20 {
21 uint32_t nw = (uint32_t)n[0]<<24 | n[1]<<16 | n[2]<<8 | n[3];
22 uint32_t hw = (uint32_t)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
33 static 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 MAX(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 if (k) {
112 if (k < mem) k = mem;
113 h += k;
114 mem = 0;
115 continue;
116 }
117 } else {
118 h += l;
119 mem = 0;
120 continue;
121 }
122
123 /* Compare right half */
124 for (k=MAX(ms+1,mem); n[k] && n[k] == h[k]; k++);
125 if (n[k]) {
126 h += k-ms;
127 mem = 0;
128 continue;
129 }
130 /* Compare left half */
131 for (k=ms+1; k>mem && n[k-1] == h[k-1]; k--);
132 if (k <= mem) return (char *)h;
133 h += p;
134 mem = mem0;
135 }
136 }
137
138 char *strstr(const char *h, const char *n)
139 {
140 /* Return immediately on empty needle */
141 if (!n[0]) return (char *)h;
142
143 /* Use faster algorithms for short needles */
144 h = strchr(h, *n);
145 if (!h || !n[1]) return (char *)h;
146 if (!h[1]) return 0;
147 if (!n[2]) return twobyte_strstr((void *)h, (void *)n);
148 if (!h[2]) return 0;
149 if (!n[3]) return threebyte_strstr((void *)h, (void *)n);
150 if (!h[3]) return 0;
151 if (!n[4]) return fourbyte_strstr((void *)h, (void *)n);
152
153 return twoway_strstr((void *)h, (void *)n);
154 }
155