D Parity of Tuples

题意

给定n行m列数，对于 [0,2^k-1] 内的数x求

分析

1. 求 Count(x)
来自Qls,代表 1的个数的奇偶性，如果连乘式中有一个为偶， 整个就为0

2. 把连乘展开
得到
我们知道

3. FWT_XOR 正好就是我们需要的

(C1表示i&j中1的个数奇偶性为0，C2表示i&j的1的个数的奇偶性为1)

参考代码

const LL     mod = 1e9 + 7;
LL qpow(LL a, LL b) {LL s = 1; while (b > 0) {if (b & 1)s = s * a % mod; a = a * a % mod; b >>= 1;} return s;}
LL gcd(LL a, LL b) {return b ? gcd(b, a % b) : a;}
int dr[2][4] = {1, -1, 0, 0, 0, 0, -1, 1};
typedef pair P;
// 异或
void FWT(int  *a, int N, int opt) {
    const int inv2 = qpow(2, mod - 2);
    // j是区间开始点，i是区间距离，k是具***置，j+k,i+j+k就是在a数组中的坐标
    for (int i = 1; i < N; i <<= 1) {
        for (int p = i << 1, j = 0; j < N; j += p) {
            for (int k = 0; k < i; ++k) {
                LL X = a[j + k], Y = a[i + j + k];
                a[j + k] = (X + Y) % mod;
                a[i + j + k] = (X + mod - Y) % mod;
                if (opt == -1) a[j + k] = 1ll * a[j + k] * inv2 % mod, a[i + j + k] = 1ll * a[i + j + k] * inv2 % mod;
            }
        }
    }
}

const int maxn = 1 << 21;
int a[maxn];
int  v[maxn];
void dfs(int *a, int x, int  m, int sign, int t) {
    if (x > m) {
        v[t] += sign;
        return ;
    }
    dfs(a, x + 1, m, sign, t);
    dfs(a, x + 1, m, -sign, t ^ a[x]);
}
int main(void)
{
    int n, m, k;
    while (cin >> n >> m >> k) {
        for (int i = 0; i < (1 << k); ++i)
            v[i] = 0;
        for (int i = 1; i <= n; ++i) {
            for (int j = 1; j <= m; ++j) {
                scanf("%d", &a[j]);
            }
            dfs(a, 1, m, 1, 0);
        }
        int N = 1 << k;
        FWT(v, N, 1);
        LL sum = 0;
        LL inv = qpow(1<<m, mod - 2);
        LL tmp = 1;
        for (int i = 0; i < N; ++i) {
            sum ^= v[i] * tmp % mod * inv % mod;
            tmp = tmp*3%mod;
        }
        cout << sum << endl;
    }

    return 0;
}