1 #define _GNU_SOURCE
2 #include <stdint.h>
3 #include <string.h>
4 
twobyte_memmem(const unsigned char * h,size_t k,const unsigned char * n)5 static char* twobyte_memmem(const unsigned char* h, size_t k, const unsigned char* n) {
6     uint16_t nw = n[0] << 8 | n[1], hw = h[0] << 8 | h[1];
7     for (h += 2, k -= 2; k; k--, hw = hw << 8 | *h++)
8         if (hw == nw)
9             return (char*)h - 2;
10     return hw == nw ? (char*)h - 2 : 0;
11 }
12 
threebyte_memmem(const unsigned char * h,size_t k,const unsigned char * n)13 static char* threebyte_memmem(const unsigned char* h, size_t k, const unsigned char* n) {
14     uint32_t nw = n[0] << 24 | n[1] << 16 | n[2] << 8;
15     uint32_t hw = h[0] << 24 | h[1] << 16 | h[2] << 8;
16     for (h += 3, k -= 3; k; k--, hw = (hw | *h++) << 8)
17         if (hw == nw)
18             return (char*)h - 3;
19     return hw == nw ? (char*)h - 3 : 0;
20 }
21 
fourbyte_memmem(const unsigned char * h,size_t k,const unsigned char * n)22 static char* fourbyte_memmem(const unsigned char* h, size_t k, const unsigned char* n) {
23     uint32_t nw = n[0] << 24 | n[1] << 16 | n[2] << 8 | n[3];
24     uint32_t hw = h[0] << 24 | h[1] << 16 | h[2] << 8 | h[3];
25     for (h += 4, k -= 4; k; k--, hw = hw << 8 | *h++)
26         if (hw == nw)
27             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 
twoway_memmem(const unsigned char * h,const unsigned char * z,const unsigned char * n,size_t l)37 static char* twoway_memmem(const unsigned char* h, const unsigned char* z, const unsigned char* n,
38                            size_t l) {
39     size_t i, ip, jp, k, p, ms, p0, mem, mem0;
40     size_t byteset[32 / sizeof(size_t)] = {};
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], |=)
46     , shift[n[i]] = i + 1;
47 
48     /* Compute maximal suffix */
49     ip = -1;
50     jp = 0;
51     k = p = 1;
52     while (jp + k < l) {
53         if (n[ip + k] == n[jp + k]) {
54             if (k == p) {
55                 jp += p;
56                 k = 1;
57             } else
58                 k++;
59         } else if (n[ip + k] > n[jp + k]) {
60             jp += k;
61             k = 1;
62             p = jp - ip;
63         } else {
64             ip = jp++;
65             k = p = 1;
66         }
67     }
68     ms = ip;
69     p0 = p;
70 
71     /* And with the opposite comparison */
72     ip = -1;
73     jp = 0;
74     k = p = 1;
75     while (jp + k < l) {
76         if (n[ip + k] == n[jp + k]) {
77             if (k == p) {
78                 jp += p;
79                 k = 1;
80             } else
81                 k++;
82         } else if (n[ip + k] < n[jp + k]) {
83             jp += k;
84             k = 1;
85             p = jp - ip;
86         } else {
87             ip = jp++;
88             k = p = 1;
89         }
90     }
91     if (ip + 1 > ms + 1)
92         ms = ip;
93     else
94         p = p0;
95 
96     /* Periodic needle? */
97     if (memcmp(n, n + p, ms + 1)) {
98         mem0 = 0;
99         p = MAX(ms, l - ms - 1) + 1;
100     } else
101         mem0 = l - p;
102     mem = 0;
103 
104     /* Search loop */
105     for (;;) {
106         /* If remainder of haystack is shorter than needle, done */
107         if (z - h < l)
108             return 0;
109 
110         /* Check last byte first; advance by shift on mismatch */
111         if (BITOP(byteset, h[l - 1], &)) {
112             k = l - shift[h[l - 1]];
113             if (k) {
114                 if (mem0 && mem && k < p)
115                     k = l - p;
116                 h += k;
117                 mem = 0;
118                 continue;
119             }
120         } else {
121             h += l;
122             mem = 0;
123             continue;
124         }
125 
126         /* Compare right half */
127         for (k = MAX(ms + 1, mem); k < l && n[k] == h[k]; k++)
128             ;
129         if (k < l) {
130             h += k - ms;
131             mem = 0;
132             continue;
133         }
134         /* Compare left half */
135         for (k = ms + 1; k > mem && n[k - 1] == h[k - 1]; k--)
136             ;
137         if (k <= mem)
138             return (char*)h;
139         h += p;
140         mem = mem0;
141     }
142 }
143 
memmem(const void * h0,size_t k,const void * n0,size_t l)144 void* memmem(const void* h0, size_t k, const void* n0, size_t l) {
145     const unsigned char *h = h0, *n = n0;
146 
147     /* Return immediately on empty needle */
148     if (!l)
149         return (void*)h;
150 
151     /* Return immediately when needle is longer than haystack */
152     if (k < l)
153         return 0;
154 
155     /* Use faster algorithms for short needles */
156     h = memchr(h0, *n, k);
157     if (!h || l == 1)
158         return (void*)h;
159     k -= h - (const unsigned char*)h0;
160     if (k < l)
161         return 0;
162     if (l == 2)
163         return twobyte_memmem(h, k, n);
164     if (l == 3)
165         return threebyte_memmem(h, k, n);
166     if (l == 4)
167         return fourbyte_memmem(h, k, n);
168 
169     return twoway_memmem(h, h + k, n, l);
170 }
171