UVA - 350 Pseudo-Random Numbers

#include<cstdio>
#include<cstring>
const int maxn = 1000000;
int f[maxn],s[maxn];
int main()
{
//freopen("a.txt","r",stdin);
int a,b,c,d,x,i,j=1;
while(~scanf("%d%d%d%d",&a,&b,&c,&d)&&a+b+c+d)
{
memset(f,0,sizeof(f));
i=1;
x=d;
while(1)
{
x=(a*x+b)%c;
if(f[x]) break;
f[x]=1;s[x]=i++;
}
printf("Case %d: %d\n",j++,i-s[x]);
}
return 0;
}