From fdbeedc6090b44adeccc098381f2ddc890d6cf15 Mon Sep 17 00:00:00 2001
From: jzmaddock <john@johnmaddock.co.uk>
Date: Fri, 11 Sep 2015 17:13:34 +0100
Subject: [PATCH] Fix behaviour of fmod for negative divisor. And add more
 tests, see https://svn.boost.org/trac/boost/ticket/11641.

---
 .../multiprecision/detail/default_ops.hpp     |  2 +-
 test/test_arithmetic.hpp                      | 25 +++++++++++++++++++
 2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/include/boost/multiprecision/detail/default_ops.hpp b/include/boost/multiprecision/detail/default_ops.hpp
index 01fbe181..e3f38fc2 100644
--- a/include/boost/multiprecision/detail/default_ops.hpp
+++ b/include/boost/multiprecision/detail/default_ops.hpp
@@ -930,7 +930,7 @@ inline void eval_fmod(T& result, const T& a, const T& b)
    }
    T n;
    eval_divide(result, a, b);
-   if(eval_get_sign(a) < 0)
+   if(eval_get_sign(result) < 0)
       eval_ceil(n, result);
    else
       eval_floor(n, result);
diff --git a/test/test_arithmetic.hpp b/test/test_arithmetic.hpp
index 4f68b807..edb379c9 100644
--- a/test/test_arithmetic.hpp
+++ b/test/test_arithmetic.hpp
@@ -742,6 +742,7 @@ void test_float_funcs(const boost::mpl::true_&)
    a = 0.5;
    a = tanh(a);
    BOOST_CHECK_CLOSE_FRACTION(a, Real(tanh(Real(0.5))), tol);
+   // fmod, need to check all the sign permutations:
    a = 4;
    b = 2;
    a = fmod(a, b);
@@ -749,6 +750,30 @@ void test_float_funcs(const boost::mpl::true_&)
    a = 4;
    b = fmod(a, b);
    BOOST_CHECK_CLOSE_FRACTION(b, Real(fmod(Real(4), Real(2))), tol);
+   a = 4;
+   b = 2;
+   a = fmod(-a, b);
+   BOOST_CHECK_CLOSE_FRACTION(a, Real(fmod(-Real(4), Real(2))), tol);
+   a = 4;
+   b = fmod(-a, b);
+   BOOST_CHECK_CLOSE_FRACTION(b, Real(-fmod(Real(4), Real(2))), tol);
+   a = 4;
+   b = 2;
+   a = fmod(a, -b);
+   BOOST_CHECK_CLOSE_FRACTION(a, Real(fmod(Real(4), -Real(2))), tol);
+   a = 4;
+   b = fmod(a, -b);
+   BOOST_CHECK_CLOSE_FRACTION(b, Real(fmod(Real(4), -Real(2))), tol);
+   a = 4;
+   b = 2;
+   a = fmod(-a, -b);
+   BOOST_CHECK_CLOSE_FRACTION(a, Real(fmod(-Real(4), -Real(2))), tol);
+   a = 4;
+   b = fmod(-a, -b);
+   BOOST_CHECK_CLOSE_FRACTION(b, Real(fmod(-Real(4), -Real(2))), tol);
+
+
+
    b = 2;
    a = atan2(a, b);
    BOOST_CHECK_CLOSE_FRACTION(a, Real(atan2(Real(4), Real(2))), tol);