串匹配算法之BM算法
参考资料:
http://blog.csdn.net/eric491179912/article/details/6210009
http://blog.163.com/pengfeicui@yeah/blog/static/106403250201092681158650/
这里,我们仅仅用了坏字符规则
定义一个滑动距离函数:
dist(C)= 1) m – j j = max{j| tj = c && 1<= j <= m - 1}
2) m 若C不出现在模式中或tm = c
比如 T = pattern dist(a)= 7 – 2 ; dist(t) = 7 – 4 (算最后出现的) ; dist(e) = 7 – 5 ; dist(r) = 7 - 6;
最后一个dist(n) = 7 (m ,模式串T的总长度) ,其他的未出现的字符, dist(ch) = 7;
程序如下:
1: void BMDist(string str,int dist[])
2: {
3: int i;
4: for (i = 0; i < 256; i++)
5: {
6: dist[i] = str.length();
7: }
8: for (i = 0; i < str.length() - 1; i++)//最后一个字符取不到
9: {
10: dist[str[i]] = str.length() - i - 1;
11: }
12: dist[str[str.length()-1]] = str.length();
13:
14: }
15:
16: int BM(string s, string t,int dist[])
17: {
18: int i = t.length();
19: while(i <= s.length())
20: {
21: int j = t.length();
22: while(j>0 && s[i-1]==t[j-1])
23: {
24: j--;
25: i--;
26: }
27: if (j == 0)
28: {
29: return i + 1;
30: }
31: else
32: {
33: i = i + dist[s[i-1]];
34: }
35: }
36: return -1;
37: }
测试代码:
1: int main()
2:
3: {
4:
5: while(1)
6: {
7: clock_t start,stop;
8: string Str,Tsr;
9:
10: int flag1,flag2,flag3;
11: int dist[256];
12:
13: int next[1000]={0,};
14: cout <<"请输入S串与T串:" <<endl;
15: cin >> Str >> Tsr;
16: cout << endl;
17:
18: start = clock();
19: for (int i=0; i< 1000; i++)
20: {
21: flag1 = BF(Str,Tsr);
22: }
23: stop = clock();
24: cout << "BF算法的执行时间是:"<< stop - start << endl;
25:
26:
27: start = clock();
28: for (int i=0; i< 1000; i++)
29: {
30: kmpNext(Tsr,next);
31: flag2 = kmp(Str,Tsr,next);
32: }
33: stop = clock();
34: cout << "KMP算法的执行时间是:"<< stop - start << endl;
35:
36:
37: start = clock();
38: for (int i=0; i< 1000; i++)
39: {
40: BMDist(Tsr,dist);
41: flag3 = BM(Str,Tsr,dist);
42: }
43: stop = clock();
44: cout << "BM算法的执行时间是:"<< stop - start << endl;
45:
46: if (flag1 == -1)
47: {
48: cout << "没有找到子串"<<endl;
49: }
50: else
51: {
52: cout << "找到子串的位置为"<< flag1 <<endl;
53: }
54: if (flag2 == -1)
55: {
56: cout << "没有找到子串"<<endl;
57: }
58: else
59: {
60: cout << "找到子串的位置为"<< flag2 <<endl;
61: }
62: if (flag3 == -1)
63: {
64: cout << "没有找到子串"<<endl;
65: }
66: else
67: {
68: cout << "找到子串的位置为"<< flag3 <<endl;
69: }
70: }
71:
72: return 0;
73: }