bzoj3160(FFT+回文自动机)

    last=ch[last][s[i]-‘a‘];size[last]++;

#include<iostream>
#include<cstdio>
#include<cstring>
#include<cmath>
#define N 100009
using namespace std;
typedef long long ll;
const int mod=1e9+7;
const double pai=acos(-1.0);
ll ans;
int len[N],cnt,last,fail[N],ch[N][2],size[N];
int L,l,n,rev[N<<2];
char s[N];
inline int rd(){
    int x=0;char c=getchar();bool f=0;
    while(!isdigit(c)){if(c==‘-‘)f=1;c=getchar();}
    while(isdigit(c)){x=(x<<1)+(x<<3)+(c^48);c=getchar();}
    return f?-x:x;
}
inline int power(ll x,int y){
    ll ans=1;
    while(y){
        if(y&1)ans=(ans*x)%mod;
        x=(x*x)%mod;
        y>>=1;
    }
    return ans;
}
struct fs{
    double x,y;
    fs(){}
    fs(double xx,double yy){x=xx;y=yy;}
    fs operator +(const fs &b)const{return fs{x+b.x,y+b.y};}
    fs operator -(const fs &b)const{return fs{x-b.x,y-b.y};}
    fs operator *(const fs &b)const{return fs{x*b.x-y*b.y,x*b.y+y*b.x};}
}a[N<<2],b[N<<2];
inline void FFT(fs *a,int tag){
    for(int i=0;i<l;++i)if(rev[i]>i)swap(a[i],a[rev[i]]);
    for(int i=1;i<l;i<<=1){
        fs wn(cos(pai/i),tag*sin(pai/i));
        for(int j=0;j<l;j+=(i<<1)){
            fs w(1,0);
            for(int k=0;k<i;++k,w=w*wn){
                fs x=a[j+k],y=w*a[k+i+j];
                a[j+k]=x+y;a[i+j+k]=x-y;
            }
        }
    }
}
int main(){
    scanf("%s",s+1);n=strlen(s+1);
    l=1;L=0;while(l<=(n<<1))l<<=1,++L;
    for(int i=0;i<l;++i)rev[i]=(rev[i>>1]>>1)|((i&1)<<(L-1));
    for(int i=1;i<=n;++i)if(s[i]==‘a‘)a[i].x=1;else b[i].x=1;
    FFT(a,1);FFT(b,1);
    for(int i=0;i<l;++i)a[i]=a[i]*a[i]+b[i]*b[i];FFT(a,-1);
    for(int i=0;i<l;++i)a[i].x=(int)(a[i].x/l+0.1);
    for(int i=0;i<l;++i)ans=(ans+power(2,ceil((double)a[i].x/2))-1)%mod;
    cnt=last=1;len[1]=-1;fail[0]=1;
    for(int i=1;i<=n;++i){
        while(s[i-len[last]-1]!=s[i])last=fail[last];
        if(!ch[last][s[i]-‘a‘]){
            len[++cnt]=len[last]+2;
            int y=fail[last];
            while(s[i-len[y]-1]!=s[i])y=fail[y];
            fail[cnt]=ch[y][s[i]-‘a‘];ch[last][s[i]-‘a‘]=cnt;
        }
        last=ch[last][s[i]-‘a‘];size[last]++;
    }
    for(int i=cnt;i;--i)(size[fail[i]]+=size[i])%=mod;
    for(int i=2;i<=cnt;++i)ans=(ans-size[i]+mod)%mod;
    cout<<ans;
    return 0;
}