bm.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <limits.h>
#include <ctype.h>

const char * boyer_moore(const char *haystack, const char *needle) {
  size_t plen = strlen(needle), slen = strlen(haystack);

  if (plen > slen) {
    return NULL;
  }

  int skip[UCHAR_MAX + 1];
  int i, j, k, * next;

  /* calc skip table („bad rule“) */
  for (i = 0; i <= UCHAR_MAX; i++) {
     skip[i] = plen;
  }

  for (i = 0; i < plen; i++) {
     skip[tolower((unsigned char)needle[i])] = plen - i - 1;
  }


  /* calc next table („good rule“) */
  next = (int*)malloc((plen + 1) * sizeof(int));

  for (j = 0; j <= plen; j++) {
    for (i = plen - 1; i >= 1; i--) {
      for (k = 1; k <= j; k++) {
        if (i - k < 0) {
          break;
        }
        if (tolower((unsigned char)needle[plen - k]) != tolower((unsigned char)needle[i - k])) {
          goto nexttry;
        }
      }
      goto matched;
nexttry:
      ;
    }
matched:
    next[j] = plen - i;
  }

  plen--;
  i = plen; /* position of last p letter in s */

  while (i < slen) {
    j = 0; /* matched letter count */
    while (j <= plen) {
      if (tolower((unsigned char)haystack[i - j]) == tolower((unsigned char)needle[plen - j])) {
        j++;
      } else {
        i += skip[tolower((unsigned char)haystack[i - j])] > next[j] ? skip[tolower((unsigned char)haystack[i - j])] - j : next[j];
        goto newi;
      }
    }
    free(next);
    return haystack + i - plen;
newi:
    ;
  }
  free(next);
  return NULL;
}