1 files changed, 28 insertions, 0 deletions
diff --git a/math/transforms/ntt.cpp b/math/transforms/ntt.cpp
new file mode 100644
index 0000000..e1e4588
--- /dev/null
+++ b/math/transforms/ntt.cpp
@@ -0,0 +1,28 @@
+constexpr ll mod = 998244353;
+constexpr ll root = 3;
+
+void fft(vector<ll>& a, bool inverse = 0) {
+	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]);
+	}
+	for (int s = 1; s < n; s *= 2) {
+		ll ws = powMod(root, (mod - 1) / s >> 1, mod);
+		if (inverse) ws = powMod(ws, mod - 2, mod);
+		for (int j = 0; j < n; j+= 2 * s) {
+			ll w = 1;
+			for (int k = 0; k < s; k++) {
+				ll u = a[j + k], t = a[j + s + k] * w;
+				t %= mod;
+				a[j + k] = (u + t) % mod;
+				a[j + s + k] = (u - t + mod) % mod;
+				w *= ws;
+				w %= mod;
+	}}}
+	if (inverse) {
+		ll div = powMod(n, mod - 2, mod);
+		for (ll i = 0; i < n; i++) {
+			a[i] *= div;
+			a[i] %= mod;
+}}}