Baby Step Giant Step model

 1 /* ***********************************************
 2 Author        :kuangbin
 3 Created Time  :2013/8/24 0:06:54
 4 File Name     :F:\2013ACM练习\专题学习\数学\Baby_step_giant_step\POJ2417.cpp
 5 ************************************************ */
 6 
 7 #include <stdio.h>
 8 #include <string.h>
 9 #include <iostream>
10 #include <algorithm>
11 #include <vector>
12 #include <queue>
13 #include <set>
14 #include <map>
15 #include <string>
16 #include <math.h>
17 #include <stdlib.h>
18 #include <time.h>
19 using namespace std;
20 //baby_step giant_step
21 // a^x = b (mod n) n为素数，a,b < n
22 // 求解上式 0<=x < n的解
23 #define MOD 76543
24 int hs[MOD],head[MOD],next[MOD],id[MOD],top;
25 void insert(int x,int y)
26 {
27     int k = x%MOD;
28     hs[top] = x, id[top] = y, next[top] = head[k], head[k] = top++;
29 }
30 int find(int x)
31 {
32     int k = x%MOD;
33     for(int i = head[k]; i != -1; i = next[i])
34         if(hs[i] == x)
35             return id[i];
36     return -1;
37 }
38 
39 
40 
41 int BSGS(int a,int b,int n)
42 {                                                                                                                                                                                                                               
43     memset(head,-1,sizeof(head));
44     top = 1;
45     if(b == 1)return 0;
46     int m = sqrt(n*1.0), j;
47     long long x = 1, p = 1;
48     for(int i = 0; i < m; ++i, p = p*a%n)insert(p*b%n,i);
49     for(long long i = m; ;i += m)
50     {
51         if( (j = find(x = x*p%n)) != -1 )return i-j;
52         if(i > n)break;
53     }
54     return -1;
55 }
56