From 5ab8a5088b729a9953b8dff1b2a985dc8fb2098b Mon Sep 17 00:00:00 2001
From: mzuenni <michi.zuendorf@gmail.com>
Date: Mon, 27 Jun 2022 17:19:28 +0200
Subject: updated tcr

---
 math/modSqrt.cpp | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)
 create mode 100644 math/modSqrt.cpp

(limited to 'math/modSqrt.cpp')

diff --git a/math/modSqrt.cpp b/math/modSqrt.cpp
new file mode 100644
index 0000000..367c6c7
--- /dev/null
+++ b/math/modSqrt.cpp
@@ -0,0 +1,23 @@
+ll sqrtMod(ll a, ll p) {
+	assert(powMod(a, (p + 1)/2, p) == 1); //a ist ein quadrat mod p?
+	if (p % 4 == 3) return powMod(a, (p + 1)/2, p);
+	if (p % 8 == 5) return powMod(a, (p + 3)/8, p);
+	ll s = p - 1;
+	ll r = 0;
+	while (s % 2 == 0) s /= 2, r++;
+	ll n = 2;
+	while (powMod(n, (p - 1)/2, p) != p - 1) n++;
+	ll x = powMod(a, (s + 1)/2, p);
+	ll b = powMod(a, s, p);
+	ll g = powMod(n, s, p);
+	while (true) {
+		ll t = b;
+		ll m = 0;
+		for (;m < r && t != 1; m++) t = (t * t) % p;
+		if (t == 1) return x;
+		ll gs = powMod(g, 1ll << (r - m - 1), p);
+		g = (gs * gs) % p;
+		x = (x * gs) % p;
+		b = (b * g) % p;
+		r = m;
+}}
-- 
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