From 7ebd9dfd9b0bc463642b065036a3cd6947a74431 Mon Sep 17 00:00:00 2001
From: jzmaddock <john@johnmaddock.co.uk>
Date: Mon, 25 Jul 2016 18:57:39 +0100
Subject: [PATCH] Fix fencepost error in rational->float conversion. Add some
 test cases for the issue. Fixes
 https://svn.boost.org/trac/boost/ticket/12327.

---
 .../detail/generic_interconvert.hpp           |  6 ++--
 test/test_rat_float_interconv.cpp             | 28 +++++++++++++++++++
 2 files changed, 31 insertions(+), 3 deletions(-)

diff --git a/include/boost/multiprecision/detail/generic_interconvert.hpp b/include/boost/multiprecision/detail/generic_interconvert.hpp
index d1fa028d..aa6d3bf5 100644
--- a/include/boost/multiprecision/detail/generic_interconvert.hpp
+++ b/include/boost/multiprecision/detail/generic_interconvert.hpp
@@ -310,7 +310,7 @@ typename enable_if_c<is_number<To>::value || is_floating_point<To>::value>::type
       num = -num;
    }
    int denom_bits = msb(denom);
-   int shift = std::numeric_limits<To>::digits + denom_bits - msb(num) + 1;
+   int shift = std::numeric_limits<To>::digits + denom_bits - msb(num);
    if(shift > 0)
       num <<= shift;
    else if(shift < 0)
@@ -318,7 +318,7 @@ typename enable_if_c<is_number<To>::value || is_floating_point<To>::value>::type
    Integer q, r;
    divide_qr(num, denom, q, r);
    int q_bits = msb(q);
-   if(q_bits == std::numeric_limits<To>::digits)
+   if(q_bits == std::numeric_limits<To>::digits - 1)
    {
       //
       // Round up if 2 * r > denom:
@@ -334,7 +334,7 @@ typename enable_if_c<is_number<To>::value || is_floating_point<To>::value>::type
    }
    else
    {
-      BOOST_ASSERT(q_bits == 1 + std::numeric_limits<To>::digits);
+      BOOST_ASSERT(q_bits == std::numeric_limits<To>::digits);
       //
       // We basically already have the rounding info:
       //
diff --git a/test/test_rat_float_interconv.cpp b/test/test_rat_float_interconv.cpp
index d7a99f46..ea81adfd 100644
--- a/test/test_rat_float_interconv.cpp
+++ b/test/test_rat_float_interconv.cpp
@@ -233,12 +233,40 @@ void test_random_rationals()
 #endif
 }
 
+#if defined(TEST2)
+
+void double_spot_tests()
+{
+   boost::multiprecision::cpp_rational rat = 1;
+   boost::multiprecision::cpp_rational twiddle(boost::multiprecision::cpp_int(1), boost::multiprecision::cpp_int(boost::multiprecision::cpp_int(1) << 54));
+   rat += boost::multiprecision::cpp_rational(boost::multiprecision::cpp_int(1), boost::multiprecision::cpp_int(boost::multiprecision::cpp_int(1) << 50));
+
+   double d = rat.convert_to<double>();
+
+   rat += twiddle;
+   BOOST_CHECK_EQUAL(d, rat.convert_to<double>());
+   rat += twiddle;
+   // tie: round to even rounds down
+   BOOST_CHECK_EQUAL(d, rat.convert_to<double>());
+   rat += twiddle;
+   BOOST_CHECK_NE(d, rat.convert_to<double>());
+   rat -= twiddle;
+   BOOST_CHECK_EQUAL(d, rat.convert_to<double>());
+   rat += boost::multiprecision::cpp_rational(boost::multiprecision::cpp_int(1), boost::multiprecision::cpp_int(boost::multiprecision::cpp_int(1) << 52));
+   // tie, but last bit is now a 1 so we round up:
+   BOOST_CHECK_NE(d, rat.convert_to<double>());
+
+}
+
+#endif
+
 int main()
 {
    using namespace boost::multiprecision;
 #if defined(TEST1) && !defined(BOOST_MSVC)
    test_round_trip<number<cpp_bin_float<113, digit_base_2, void, boost::int16_t> >, cpp_rational>();
 #elif defined(TEST2)
+   double_spot_tests();
    test_round_trip<double, cpp_rational>();
 #elif defined(TEST3) && !defined(BOOST_MSVC)
    test_random_rationals<number<cpp_bin_float<113, digit_base_2, void, boost::int16_t> >, cpp_rational>();