FFT模板

我终于下定决心学习FFT了。

orzCHX，得出模板：

#include<cstdio>
#include<cctype>
#include<queue>
#include<cmath>
#include<cstring>
#include<algorithm>
#define rep(i,s,t) for(int i=s;i<=t;i++)
#define dwn(i,s,t) for(int i=s;i>=t;i--)
#define ren for(int i=first[x];i!=-1;i=next[i])
using namespace std;
inline int read() {
    int x=0,f=1;char c=getchar();
    for(;!isdigit(c);c=getchar()) if(c=='-') f=-1;
    for(;isdigit(c);c=getchar()) x=x*10+c-'0';
    return x*f;
}
const int maxn=300010;
const double PI=acos(-1.0);
struct FFT {
    struct cox {
        double r,i;
        cox(double _r=0.0,double _i=0.0) {r=_r;i=_i;}
        cox operator + (const cox& b) const {return cox(r+b.r,i+b.i);}
        cox operator - (const cox& b) const {return cox(r-b.r,i-b.i);}
        cox operator * (const cox& b) const {return cox(r*b.r-i*b.i,r*b.i+i*b.r);}
    }f[maxn];
    int len;
    void init(int* A,int Len,int L) {
        len=L;rep(i,0,Len-1) f[i]=cox(A[Len-i-1],0);
    }
    void cal(int tp) {
        int j=len>>1;
        rep(i,1,len-2) {
            if(i<j) swap(f[i],f[j]);int k=len>>1;
            while(j>=k) j-=k,k>>=1;j+=k;
        }
        double lm=-2*tp*PI;
        for(int i=2;i<=len;i<<=1) {
            cox wn(cos(lm/i),sin(lm/i));
            for(int j=0;j<len;j+=i) {
                cox w(1,0);
                for(int k=j;k<j+(i>>1);k++) {
                    cox u=f[k],v=w*f[k+(i>>1)];
                    f[k]=u+v;f[k+(i>>1)]=u-v;w=w*wn;
                }
            }
        }
        if(tp<0) rep(i,0,len-1) f[i].r/=len;
    }
}a,b;
void mul(int* A,int* B,int L1,int L2,int& L,int* ans) {
    L=1;while(L<L1<<1||L<L2<<1) L<<=1;
    a.init(A,L1,L);b.init(B,L2,L);
    a.cal(1);b.cal(1);
    rep(i,0,L-1) a.f[i]=a.f[i]*b.f[i];
    a.cal(-1);rep(i,0,L-1) ans[i]=int(a.f[i].r+0.5);
}
int A[maxn],B[maxn],ans[maxn];
int main() {
    int L1=read()+1,L2=read()+1,L;
    rep(i,0,L1-1) A[i]=read();
    rep(i,0,L2-1) B[i]=read();
    mul(A,B,L1,L2,L,ans);
    while(L>L1+L2-1&&!ans[L-1]) L--;
    dwn(i,L-1,0) printf("%d ",ans[i]);
    return 0;
}