蒟蒻刚学 OI，四次 DFT WA 65 求助

拆系数然后通过系数共轭得到点值表达，为什么 65 \kel \kel \kel
怀疑是精度问题

代码：

#include <cstdio>
#include <cmath>
#include <algorithm>
using namespace std;
const int N = 1 << 18;
const long double pi = acos(-1);
const int W = 1 << 15;
int lena,lenb,mod,n;
int lg2[N + 5],rev[N + 5];
int ans[N + 5];
struct cp
{
    long double a,b;
    inline void operator+=(const cp &o)
    {
        a += o.a,b += o.b;
    }
    inline cp operator+(const cp &o) const
    {
        return (cp){a + o.a,b + o.b};
    }
    inline cp operator-(const cp &o) const
    {
        return (cp){a - o.a,b - o.b};
    }
    inline cp operator*(const cp &o) const
    {
        return (cp){a * o.a - b * o.b,a * o.b + b * o.a};
    }
    inline cp operator*(const double &o) const
    {
        return (cp){a * o,b * o};
    }
    inline cp operator~() const
    {
        return (cp){a,-b};
    }
} f[N + 5],g[N + 5],h[N + 5],a[N + 5],b[N + 5],rt[N + 5];
inline void init(int len)
{
    for(n = 1;n < len;n <<= 1);
    for(register int i = 2;i <= n;++i)
        lg2[i] = lg2[i >> 1] + 1;
    cp w = (cp){cos(2 * pi / n),sin(2 * pi / n)};
    rt[n >> 1] = (cp){1,0};
    for(register int i = (n >> 1) + 1;i <= n;++i)
        rt[i] = rt[i - 1] * w;
    for(register int i = (n >> 1) - 1;i;--i)
        rt[i] = rt[i << 1];
}
inline void fft(cp *a,int type,int n)
{
    type == -1 && (reverse(a + 1,a + n),1);
    int lg = lg2[n] - 1;
    for(register int i = 0;i < n;++i)
        rev[i] = (rev[i >> 1] >> 1) | ((i & 1) << lg),
        i < rev[i] && (swap(a[i],a[rev[i]]),1);
    for(register int w = 2,m = 1;w <= n;w <<= 1,m <<= 1)
        for(register int i = 0;i < n;i += w)
            for(register int j = 0;j < m;++j)
            {
                cp t = rt[m | j] * a[i | j | m];
                a[i | j | m] = a[i | j] - t,a[i | j] += t;
            }
    if(type == -1)
        for(register int i = 0;i < n;++i)
            a[i].a /= n,a[i].b /= n;
}
int main()
{
    scanf("%d%d%d",&lena,&lenb,&mod),init(max(++lena,++lenb) << 1);
    int x;
    for(register int i = 0;i < lena;++i)
        scanf("%d",&x),f[i] = (cp){x / W,x % W};
    for(register int i = 0;i < lenb;++i)
        scanf("%d",&x),g[i] = (cp){x / W,x % W};
    fft(f,1,n),fft(g,1,n);
    for(register int i = 0;i < n;++i)
        h[i] = ~f[(n - i) % n];
    for(register int i = 0;i < n;++i)
        a[i] = f[i] * g[i],b[i] = g[i] * h[i];
    fft(a,-1,n),fft(b,-1,n);
    for(register int i = 0;i < n;++i)
    {
        long long ac = (a[i].a + b[i].a) / 2 + 0.5;
        long long bd = b[i].a - ac + 0.5;
        long long bcad = a[i].b + 0.5;
        ans[i] = ((ac % mod * W % mod * W % mod) % mod + (bcad % mod * W % mod) % mod + bd % mod) % mod;
    }
    for(register int i = 0;i < lena + lenb - 1;++i)
        printf("%d ",ans[i]);
}