【HDOJ6217】BBP Formula（公式）

题意：给定一个无穷项的分式，它的和等于π，问π的十六进制表示的小数点后第n位是多少

1 ≤ n ≤ 100000

思路：From https://blog.csdn.net/meopass/article/details/78327614

C++double写起来细节真是烦

#include<cstdio>
#include<cstring>
#include<string>
#include<cmath>
#include<iostream>
#include<algorithm>
#include<map>
#include<set>
#include<queue>
#include<vector>
using namespace std;
typedef long long ll;
typedef unsigned int uint;
typedef unsigned long long ull;
typedef pair<int,int> PII;
typedef vector<int> VI;
#define fi first
#define se second 
#define MP make_pair
#define N   11000
#define M   41
#define eps 1e-8
#define pi acos(-1)



int read()
{ 
   int v=0,f=1;
   char c=getchar();
   while(c<48||57<c) {if(c=='-') f=-1; c=getchar();}
   while(48<=c&&c<=57) v=(v<<3)+v+v+c-48,c=getchar();
   return v*f;
}

ll Pow(ll a,ll b,ll MOD)
{
    ll ans=1;
    while(b)
    {
        if(b&1) ans=ans*a%MOD;
        a=a*a%MOD;
        b>>=1;
    }
    return ans;
}

double BBP(int n,ll k,ll b)
{
    double ans=0;
    for(int i=0;i<=n;i++) ans+=(Pow(16,n-i,8*i+b)*1.0/(8*i+b));
    for(int i=n+1;i<=n+1001;i++) ans+=(powf(16,n-i)/(8*i+b));
    return k*ans;
}

int main()
{
    //freopen("hdoj6217.in","r",stdin);
    //freopen("hdoj6217.out","w",stdout);
    int cas;
    scanf("%d",&cas);
    for(int v=1;v<=cas;v++)
    {
        int n;
        scanf("%d",&n);
        n--;
        double ans=0.0;
        ans=BBP(n,4,1)+BBP(n,-2,4)+BBP(n,-1,5)+BBP(n,-1,6);
        ans-=int(ans);
        if(ans<0) ans++;
        ans*=16;
        char ch;
        int p=int(ans);
        if(0<=p&&p<=9) ch=p+'0';
         else if(p==10) ch='A';
         else if(p==11) ch='B';
         else if(p==12) ch='C';
         else if(p==13) ch='D';
         else if(p==14) ch='E';
         else if(p==15) ch='F';
        printf("Case #%d: %d %c
",v,n+1,ch);
        
    }
    return 0;
}