bmsearch.c

/* Note: In this example needle is a Cstring. The ending
 * 0x00 will be cut off, so you could call this example with
 * boyermoore_horspool_memmem(haystack, hlen, "abc", sizeof("abc"))
 */


//#define UCHAR_MAX 255
#define CHAR_BIT	32
#define	CHAR_MAX	512
#define UCHAR_MAX	768
#define INT_MAX		2147483647
#define UINT_MAX	4294967295u
#define SHRT_MAX	32767

const unsigned char *
boyermoore_horspool_memmem(const unsigned char* haystack, ssize_t hlen,
						   const unsigned char* needle,   ssize_t nlen)
{
	size_t scan = 0;
	size_t bad_char_skip[UCHAR_MAX + 1]; /* Officially called:
										  * bad character shift */

	/* Sanity checks on the parameters */
	if (nlen <= 0 || !haystack || !needle)
		return NULL;

	/* ---- Preprocess ---- */
	/* Initialize the table to default value */
	/* When a character is encountered that does not occur
	 * in the needle, we can safely skip ahead for the whole
	 * length of the needle.
	 */
	for (scan = 0; scan <= UCHAR_MAX; scan = scan + 1)
		bad_char_skip[scan] = nlen;

	/* C arrays have the first byte at [0], therefore:
	 * [nlen - 1] is the last byte of the array. */
	size_t last = nlen - 1;

	/* Then populate it with the analysis of the needle */
	for (scan = 0; scan < last; scan = scan + 1)
		bad_char_skip[needle[scan]] = last - scan;

	/* ---- Do the matching ---- */

	/* Search the haystack, while the needle can still be within it. */
	while (hlen >= nlen)
	{
		/* scan from the end of the needle */
		for (scan = last; haystack[scan] == needle[scan]; scan = scan - 1)
			if (scan == 0) /* If the first byte matches, we've found it. */
				return haystack;

		/* otherwise, we need to skip some bytes and start again.
		   Note that here we are getting the skip value based on the last byte
		   of needle, no matter where we didn't match. So if needle is: "abcd"
		   then we are skipping based on 'd' and that value will be 4, and
		   for "abcdd" we again skip on 'd' but the value will be only 1.
		   The alternative of pretending that the mismatched character was
		   the last character is slower in the normal case (Eg. finding
		   "abcd" in "...azcd..." gives 4 by using 'd' but only
		   4-2==2 using 'z'. */
		hlen	 -= bad_char_skip[haystack[last]];
		haystack += bad_char_skip[haystack[last]];
	}

	return NULL;
}