diff options
Diffstat (limited to 'content/math/transforms/fft.cpp')
| -rw-r--r-- | content/math/transforms/fft.cpp | 23 |
1 files changed, 23 insertions, 0 deletions
diff --git a/content/math/transforms/fft.cpp b/content/math/transforms/fft.cpp new file mode 100644 index 0000000..2bd95b2 --- /dev/null +++ b/content/math/transforms/fft.cpp @@ -0,0 +1,23 @@ +using cplx = complex<double>; + +void fft(vector<cplx>& a, bool inv = false) { + int n = sz(a); + for (int i = 0, j = 1; j < n - 1; ++j) { + for (int k = n >> 1; k > (i ^= k); k >>= 1); + if (j < i) swap(a[i], a[j]); + } + static vector<cplx> ws(2, 1); + for (static int k = 2; k < n; k *= 2) { + ws.resize(n); + cplx w = polar(1.0, acos(-1.0) / k); + for (int i=k; i<2*k; i++) ws[i] = ws[i/2] * (i % 2 ? w : 1); + } + for (int s = 1; s < n; s *= 2) { + for (int j = 0; j < n; j += 2 * s) { + for (int k = 0; k < s; k++) { + cplx u = a[j + k], t = a[j + s + k]; + t *= (inv ? conj(ws[s + k]) : ws[s + k]); + a[j + k] = u + t; + a[j + s + k] = u - t; + if (inv) a[j + k] /= 2, a[j + s + k] /= 2; +}}}} |