String search algorithm BM algorithm (Boyer-Moore) Algorithm

The two most famous character string search algorithms are KMP algorithm (knuth-Morris-Pratt) and BM algorithm (Boyer-Moore). In the worst case, both algorithms have linear search time. However, in practice, the KMP algorithm is not much faster than the simplest C-database function strstr (), while the BM algorithm is usually 3-5 times faster than the KMP algorithm.

However, in the worst case, the time complexity of BM seems to be n × n.

Not to mention the specifics. The BM algorithm uses the main text string to quickly perform non-backtracking search. The beating algorithm is implemented using this sentence in the program. below:

1. I = I + M-min (J, 1 + last (P, T [I]);

Last is the final position of the character in the search string.

This algorithm is very troublesome. If you can, Baidu.

The Code is as follows:

1. # Include <string. h>
2. Int last (char * P, char c) {// locate the position where c finally matches in P. If no position is found,-1 is returned.
3. Int length = strlen (P), Count = 0;
4. Char * PP = P + Length-1;
5. While (PP> = P)
6. {
7. If (* PP = C)
8. {
9. Return length-count-1;
10. }
11. PP --;
12. Count ++;
13. }
14. Return-1;
15. }
17. Int min (int A, int B ){
18. Return (A <= B )? A: B;
19. }
21. Int bm_index (char * t, char * P ){
22. Int n = strlen (t );
23. Int M = strlen (P );
24. Int I = s-1, j = m-1;
25. While (I <= N-1)
26. {
27. If (T [I] = P [J])
28. {
29. If (j = 0)
30. {
31. Return I;
32. }
33. Else
34. I --, j --;
35. }
36. Else {
37. I = I + M-min (J, 1 + last (P, T [I]); // jump back, depending on the position of the last matched character
38. J = m-1;
39. }
40. }
41. Return-1;
42. }
44. Int _ tmain (INT argc, _ tchar * argv [])
45. {
46. Char * P = "woainizz! Izzzzzz -- zzzzzzut ";
47. Int A = bm_index (P, "zzzzut"); // result 18, no problem
48. Return 0;
49. }