github.com/afumu/libc@v0.0.6/musl/src/string/memmem.c (about)

     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  }