POJ 2417/BZOJ 3239(Discrete Logging-BSGS)[Template:数论]

BL == N (mod P)

5 2 1
5 2 2
5 2 3
5 2 4
5 3 1
5 3 2
5 3 3
5 3 4
5 4 1
5 4 2
5 4 3
5 4 4
12345701 2 1111111
1111111121 65537 1111111111

0
1
3
2
0
3
1
2
0
no solution
no solution
1
9584351
462803587

B(P-1) == 1 (mod P)

B(-m) == B(P-1-m) (mod P) .

#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<algorithm>
#include<functional>
#include<iostream>
#include<cmath>
#include<cctype>
#include<ctime>
using namespace std;
#define For(i,n) for(int i=1;i<=n;i++)
#define Fork(i,k,n) for(int i=k;i<=n;i++)
#define Rep(i,n) for(int i=0;i<n;i++)
#define ForD(i,n) for(int i=n;i;i--)
#define RepD(i,n) for(int i=n;i>=0;i--)
#define Forp(x) for(int p=pre[x];p;p=next[p])
#define Forpiter(x) for(int &p=iter[x];p;p=next[p])
#define Lson (x<<1)
#define Rson ((x<<1)+1)
#define MEM(a) memset(a,0,sizeof(a));
#define MEMI(a) memset(a,127,sizeof(a));
#define MEMi(a) memset(a,128,sizeof(a));
#define INF (2139062143)
#define F (100000007)
#define MAXN (1000000)
typedef long long ll;
ll mul(ll a,ll b){return (a*b)%F;}
ll add(ll a,ll b){return (a+b)%F;}
ll sub(ll a,ll b){return (a-b+(a-b)/F*F+F)%F;}
void upd(ll &a,ll b){a=(a%F+b%F)%F;}
char s[]="no solution\n";

class Math
{
public:
ll gcd(ll a,ll b){if (!b) return a;return gcd(b,a%b);}
ll abs(ll x){if (x>=0) return x;return -x;}
ll exgcd(ll a,ll b,ll &x, ll &y)
{
if (!b) {x=1,y=0;return a;}
ll g=exgcd(b,a%b,x,y);
ll t=x;x=y;y=t-a/b*y;
return g;
}
ll pow2(ll a,int b,ll p)
{
if (b==0) return 1;
if (b==1) return a;
ll c=pow2(a,b/2,p);
c=c*c%p;
if (b&1) c=c*a%p;
return c;
}
ll Modp(ll a,ll b,ll p)
{
ll x,y;
ll g=exgcd(a,p,x,y),d;
if (b%g) {return -1;}
d=b/g;x*=d,y*=d;
x=(x+abs(x)/p*p+p)%p;
return x;
}
int h[MAXN];
ll hnum[MAXN];
int hash(ll x)
{
int i=x%MAXN;
while (h[i]&&hnum[i]!=x) i=(i+1)%MAXN;
hnum[i]=x;
return i;
}
ll babystep(ll a,ll b,int p)
{
MEM(h) MEM(hnum)
int m=sqrt(p);while (m*m<p) m++;
ll res=b,ans=-1;

ll uni=pow2(a,m,p);
if (!uni) if (!b) ans=1;else ans=-1; //特判
else
{

Rep(i,m+1)
{
int t=hash(res);
h[t]=i+1;
res=(res*a)%p;
}
res=uni;

For(i,m+1)
{
int t=hash(res);
if (h[t]) {ans=i*m-(h[t]-1);break;}else hnum[t]=0;
res=res*uni%p;
}

}
return ans;
}
}S;

int main()
{
//	freopen("poj2471.in","r",stdin);
//	freopen(".out","w",stdout);

ll p,b,n;
while(cin>>p>>b>>n)
{
ll ans=S.babystep(b,n,p);
if (ans==-1) cout<<s;
else cout<<ans<<endl;
}

return 0;
}