随机数生成器

#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#include<cstdio>
#include "immintrin.h"
#include "emmintrin.h"
typedef double db __attribute__ ((__vector_size__(32),__may_alias__));
int n;
int main(){
    scanf("%d",&n);
    if(n==1)return puts("0.00000"),0;
    
    db x={1,1,1,1};
    db ans={0,0,0,0};
    db bs={8,8,8,8};
    double p=0;
    unsigned a=1;
    for(;a<n&&a<=1000000;++a)p+=1./a;
    db i={a,a+1,a+2,a+3};
    for(;a+8<n;a+=8)
        ans+=x/i,i+=bs;
    p += (ans[0]+ans[1]+ans[2]+ans[3])*2+1;
    for(;a<n;++a)p+=1./a;
    printf("%.5lf\n",p);
}