1 files changed, 13 insertions, 0 deletions
diff --git a/content/math/recover.cpp b/content/math/recover.cpp
new file mode 100644
index 0000000..1a593f0
--- /dev/null
+++ b/content/math/recover.cpp
@@ -0,0 +1,13 @@
+ll sq(ll x) {return x*x;}
+
+array<ll, 2> recover(ll c, ll m) {
+	array<ll, 2> u = {m, 0}, v = {c, 1};
+	while (m <= 2 * sq(v[0])) {
+		ll q = u[0] / v[0];
+		u[0] -= q * v[0];
+		u[1] -= q * v[1];
+		swap(u, v);
+	}
+	if (v[1] <= 0 || 2 * sq(v[1]) >= m) return {-1, -1};
+	return v;
+}