From 34207640064313481e9aa801309921708024727d Mon Sep 17 00:00:00 2001
From: MITSUNARI Shigeo <herumi@nifty.com>
Date: Fri, 5 Aug 2016 05:33:23 +0900
Subject: [PATCH] add getWeierstrass

---
 include/mcl/ec.hpp | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/include/mcl/ec.hpp b/include/mcl/ec.hpp
index d751187..42597f3 100644
--- a/include/mcl/ec.hpp
+++ b/include/mcl/ec.hpp
@@ -670,14 +670,18 @@ public:
 	{
 		compressedExpression_ = compressedExpression;
 	}
-	static inline void getYfromX(Fp& y, const Fp& x, bool isYodd)
+	static inline void getWeierstrass(Fp& yy, const Fp& x)
 	{
 		Fp t;
 		Fp::sqr(t, x);
 		t += a_;
 		t *= x;
-		t += b_;
-		Fp::squareRoot(y, t);
+		Fp::add(yy, t, b_);
+	}
+	static inline void getYfromX(Fp& y, const Fp& x, bool isYodd)
+	{
+		getWeierstrass(y, x);
+		if (!Fp::squareRoot(y, y)) throw cybozu::Exception("EcT:getYfromX") << x << isYodd;
 		if (y.isOdd() ^ isYodd) {
 			Fp::neg(y, y);
 		}