From 525946569532251877ad55dbd203ea99d20171ef Mon Sep 17 00:00:00 2001
From: MITSUNARI Shigeo <herumi@nifty.com>
Date: Mon, 30 Sep 2019 12:45:58 +0900
Subject: [PATCH] add mclBnF{r,p}_isNegative

---
 include/mcl/bn.h               |  5 +++++
 include/mcl/impl/bn_c_impl.hpp |  8 ++++++++
 test/bn_c_test.hpp             | 25 +++++++++++++++++++++++++
 3 files changed, 38 insertions(+)

diff --git a/include/mcl/bn.h b/include/mcl/bn.h
index 0a5c3b4..e381c49 100644
--- a/include/mcl/bn.h
+++ b/include/mcl/bn.h
@@ -286,6 +286,11 @@ MCLBN_DLL_API int mclBnFp2_isEqual(const mclBnFp2 *x, const mclBnFp2 *y);
 MCLBN_DLL_API int mclBnFp2_isZero(const mclBnFp2 *x);
 MCLBN_DLL_API int mclBnFp2_isOne(const mclBnFp2 *x);
 
+// return 1 if half <= x < r, where half = (r + 1) / 2 else 0
+MCLBN_DLL_API int mclBnFr_isNegative(const mclBnFr *x);
+// return 1 if half <= x < p, where half = (p + 1) / 2 else 0
+MCLBN_DLL_API int mclBnFp_isNegative(const mclBnFp *x);
+
 #ifndef MCL_DONT_USE_CSRPNG
 // return 0 if success
 MCLBN_DLL_API int mclBnFr_setByCSPRNG(mclBnFr *x);
diff --git a/include/mcl/impl/bn_c_impl.hpp b/include/mcl/impl/bn_c_impl.hpp
index 18280e6..f9c3f13 100644
--- a/include/mcl/impl/bn_c_impl.hpp
+++ b/include/mcl/impl/bn_c_impl.hpp
@@ -175,6 +175,10 @@ int mclBnFr_isOne(const mclBnFr *x)
 {
 	return cast(x)->isOne();
 }
+int mclBnFr_isNegative(const mclBnFr *x)
+{
+	return cast(x)->isNegative();
+}
 
 #ifndef MCL_DONT_USE_CSRPNG
 int mclBnFr_setByCSPRNG(mclBnFr *x)
@@ -727,6 +731,10 @@ int mclBnFp_isOne(const mclBnFp *x)
 {
 	return cast(x)->isOne();
 }
+int mclBnFp_isNegative(const mclBnFp *x)
+{
+	return cast(x)->isNegative();
+}
 
 int mclBnFp_setHashOf(mclBnFp *x, const void *buf, mclSize bufSize)
 {
diff --git a/test/bn_c_test.hpp b/test/bn_c_test.hpp
index e1f3ff0..45095b7 100644
--- a/test/bn_c_test.hpp
+++ b/test/bn_c_test.hpp
@@ -327,6 +327,31 @@ CYBOZU_TEST_AUTO(GT_inv)
 	CYBOZU_TEST_ASSERT(mclBnGT_isOne(&e4));
 }
 
+CYBOZU_TEST_AUTO(Fr_isNegative)
+{
+	mclBnFr a, half, one;
+	mclBnFr_setInt(&half, 2);
+	mclBnFr_inv(&half, &half); // half = (r + 1) / 2
+	mclBnFr_setInt(&one, 1);
+	mclBnFr_sub(&a, &half, &one);
+	CYBOZU_TEST_ASSERT(!mclBnFr_isNegative(&a));
+	mclBnFr_add(&a, &a, &one);
+	CYBOZU_TEST_ASSERT(mclBnFr_isNegative(&a));
+}
+
+CYBOZU_TEST_AUTO(Fp_isNegative)
+{
+	mclBnFp a, half, one;
+	mclBnFp_setInt(&half, 2);
+	mclBnFp_inv(&half, &half); // half = (p + 1) / 2
+	mclBnFp_setInt(&one, 1);
+	mclBnFp_sub(&a, &half, &one);
+	CYBOZU_TEST_ASSERT(!mclBnFp_isNegative(&a));
+	mclBnFp_add(&a, &a, &one);
+	CYBOZU_TEST_ASSERT(mclBnFp_isNegative(&a));
+}
+
+
 CYBOZU_TEST_AUTO(pairing)
 {
 	mclBnFr a, b, ab;