49 lines
1.3 KiB
C++
49 lines
1.3 KiB
C++
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void fft(vector<complex<ld>>& y, bool idft) {
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int n = y.size();
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vector<int> rev(n);
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for (int i = 0; i < n; ++i) {
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rev[i] = rev[i >> 1] >> 1;
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if (i & 1) {
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rev[i] |= n >> 1;
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}
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}
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for (int i = 0; i < n; ++i) {
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if (i < rev[i]) {
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swap(y[i], y[rev[i]]);
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}
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}
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for (int h = 2; h <= n; h <<= 1) {
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complex<ld> wn(cos(2 * M_PI / h), sin(2 * M_PI / h));
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for (int j = 0; j < n; j += h) {
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complex<ld> w(1);
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for (int k = j; k < j + h / 2; ++k) {
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complex<ld> u = y[k], t = w * y[k + h / 2];
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y[k] = u + t;
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y[k + h / 2] = u - t;
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w *= wn;
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}
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}
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}
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if (idft) {
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reverse(y.begin() + 1, y.end());
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for (int i = 0; i < n; ++i) {
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y[i] /= n;
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}
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}
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}
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vector<int> multiply(const vector<int>& a, const vector<int>& b) {
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vector<complex<ld>> A(a.begin(), a.end()), B(b.begin(), b.end());
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int n = 1;
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while (n < a.size() + b.size()) n <<= 1;
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A.resize(n), B.resize(n);
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fft(A, false), fft(B, false);
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for (int i = 0; i < n; ++i) {
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A[i] *= B[i];
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}
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fft(A, true);
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vector<int> res(n);
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transform(A.begin(), A.end(), res.begin(), expr(int(round(x.real())), auto&& x));
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return res;
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}
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