content/math/transforms/xorTransform.cpp


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void fft(vector<ll>& a, bool inv = false) {
	int n = ssize(a);
	for (int s = 1; s < n; s *= 2) {
		for (int i = 0; i < n; i += 2 * s) {
			for (int j = i; j < i + s; j++) {
				ll &u = a[j], &v = a[j + s];
				tie(u, v) = pair(u + v, u - v);
	}}}
	if (inv) for (ll& x : a) x /= n;
}