memmem.c

/*
 * memmem.c
 *
 * Find a byte string inside a longer byte string
 *
 * This uses the "Not So Naive" algorithm, a very simple but
 * usually effective algorithm, see:
 *
 * http://www-igm.univ-mlv.fr/~lecroq/string/
 */

#include <string.h>

void *memmem(const void *haystack, size_t n, const void *needle, size_t m)
{
  const unsigned char *y = (const unsigned char *)haystack;
  const unsigned char *x = (const unsigned char *)needle;

  int j, k, l;

  if ( x[0] == x[1] ) {
    k = 2;
    l = 1;
  } else {
    k = 1;
    l = 2;
  }

  j = 0;
  while ( j <= n-m ) {
    if (x[1] != y[j+1]) {
      j += k;
    } else {
      if ( !memcmp(x+2, y+j+2, m-2) && x[0] == y[j] )
	return (void *)&y[j];
      j += l;
    }
  }

  return NULL;
}