memmem.c raw
1 #define _GNU_SOURCE
2 #include <string.h>
3 #include <stdint.h>
4
5 static char *twobyte_memmem(const unsigned char *h, size_t k, const unsigned char *n)
6 {
7 uint16_t nw = n[0]<<8 | n[1], hw = h[0]<<8 | h[1];
8 for (h+=2, k-=2; k; k--, hw = hw<<8 | *h++)
9 if (hw == nw) return (char *)h-2;
10 return hw == nw ? (char *)h-2 : 0;
11 }
12
13 static char *threebyte_memmem(const unsigned char *h, size_t k, const unsigned char *n)
14 {
15 uint32_t nw = (uint32_t)n[0]<<24 | n[1]<<16 | n[2]<<8;
16 uint32_t hw = (uint32_t)h[0]<<24 | h[1]<<16 | h[2]<<8;
17 for (h+=3, k-=3; k; k--, hw = (hw|*h++)<<8)
18 if (hw == nw) return (char *)h-3;
19 return hw == nw ? (char *)h-3 : 0;
20 }
21
22 static char *fourbyte_memmem(const unsigned char *h, size_t k, const unsigned char *n)
23 {
24 uint32_t nw = (uint32_t)n[0]<<24 | n[1]<<16 | n[2]<<8 | n[3];
25 uint32_t hw = (uint32_t)h[0]<<24 | h[1]<<16 | h[2]<<8 | h[3];
26 for (h+=4, k-=4; k; k--, hw = hw<<8 | *h++)
27 if (hw == nw) return (char *)h-4;
28 return hw == nw ? (char *)h-4 : 0;
29 }
30
31 #define MAX(a,b) ((a)>(b)?(a):(b))
32 #define MIN(a,b) ((a)<(b)?(a):(b))
33
34 #define BITOP(a,b,op) \
35 ((a)[(size_t)(b)/(8*sizeof *(a))] op (size_t)1<<((size_t)(b)%(8*sizeof *(a))))
36
37 static char *twoway_memmem(const unsigned char *h, const unsigned char *z, const unsigned char *n, size_t l)
38 {
39 size_t i, ip, jp, k, p, ms, p0, mem, mem0;
40 size_t byteset[32 / sizeof(size_t)] = { 0 };
41 size_t shift[256];
42
43 /* Computing length of needle and fill shift table */
44 for (i=0; i<l; i++)
45 BITOP(byteset, n[i], |=), shift[n[i]] = i+1;
46
47 /* Compute maximal suffix */
48 ip = -1; jp = 0; k = p = 1;
49 while (jp+k<l) {
50 if (n[ip+k] == n[jp+k]) {
51 if (k == p) {
52 jp += p;
53 k = 1;
54 } else k++;
55 } else if (n[ip+k] > n[jp+k]) {
56 jp += k;
57 k = 1;
58 p = jp - ip;
59 } else {
60 ip = jp++;
61 k = p = 1;
62 }
63 }
64 ms = ip;
65 p0 = p;
66
67 /* And with the opposite comparison */
68 ip = -1; jp = 0; k = p = 1;
69 while (jp+k<l) {
70 if (n[ip+k] == n[jp+k]) {
71 if (k == p) {
72 jp += p;
73 k = 1;
74 } else k++;
75 } else if (n[ip+k] < n[jp+k]) {
76 jp += k;
77 k = 1;
78 p = jp - ip;
79 } else {
80 ip = jp++;
81 k = p = 1;
82 }
83 }
84 if (ip+1 > ms+1) ms = ip;
85 else p = p0;
86
87 /* Periodic needle? */
88 if (memcmp(n, n+p, ms+1)) {
89 mem0 = 0;
90 p = MAX(ms, l-ms-1) + 1;
91 } else mem0 = l-p;
92 mem = 0;
93
94 /* Search loop */
95 for (;;) {
96 /* If remainder of haystack is shorter than needle, done */
97 if (z-h < l) return 0;
98
99 /* Check last byte first; advance by shift on mismatch */
100 if (BITOP(byteset, h[l-1], &)) {
101 k = l-shift[h[l-1]];
102 if (k) {
103 if (k < mem) k = mem;
104 h += k;
105 mem = 0;
106 continue;
107 }
108 } else {
109 h += l;
110 mem = 0;
111 continue;
112 }
113
114 /* Compare right half */
115 for (k=MAX(ms+1,mem); k<l && n[k] == h[k]; k++);
116 if (k < l) {
117 h += k-ms;
118 mem = 0;
119 continue;
120 }
121 /* Compare left half */
122 for (k=ms+1; k>mem && n[k-1] == h[k-1]; k--);
123 if (k <= mem) return (char *)h;
124 h += p;
125 mem = mem0;
126 }
127 }
128
129 void *memmem(const void *h0, size_t k, const void *n0, size_t l)
130 {
131 const unsigned char *h = h0, *n = n0;
132
133 /* Return immediately on empty needle */
134 if (!l) return (void *)h;
135
136 /* Return immediately when needle is longer than haystack */
137 if (k<l) return 0;
138
139 /* Use faster algorithms for short needles */
140 h = memchr(h0, *n, k);
141 if (!h || l==1) return (void *)h;
142 k -= h - (const unsigned char *)h0;
143 if (k<l) return 0;
144 if (l==2) return twobyte_memmem(h, k, n);
145 if (l==3) return threebyte_memmem(h, k, n);
146 if (l==4) return fourbyte_memmem(h, k, n);
147
148 return twoway_memmem(h, h+k, n, l);
149 }
150