Merge pull request #605 from boostorg/issue604

Fix up default fmod to perform post-condition checking.
boostorg · Mar 1, 2024 · 42a3edf · 42a3edf
2 parents f0319ec + 062503f
commit 42a3edf
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diff --git a/include/boost/multiprecision/detail/default_ops.hpp b/include/boost/multiprecision/detail/default_ops.hpp
@@ -1295,6 +1295,30 @@ inline BOOST_MP_CXX14_CONSTEXPR void eval_fmod(T& result, const T& a, const T& b
       eval_floor(n, result);
    eval_multiply(n, b);
    eval_subtract(result, a, n);
+   if (eval_get_sign(result) != 0)
+   {
+      //
+      // Sanity check, that due to rounding errors in division, 
+      // we haven't accidently calculated the wrong value:
+      // See https://github.com/boostorg/multiprecision/issues/604 for an example.
+      //
+      if (eval_get_sign(result) == eval_get_sign(b))
+      {
+         if (result.compare(b) >= 0)
+         {
+            eval_subtract(result, b);
+         }
+      }
+      else
+      {
+         n = b;
+         n.negate();
+         if (result.compare(n) >= 0)
+         {
+            eval_subtract(result, n);
+         }
+      }
+   }
 }
 template <class T, class A>
 inline BOOST_MP_CXX14_CONSTEXPR typename std::enable_if<boost::multiprecision::detail::is_arithmetic<A>::value, void>::type eval_fmod(T& result, const T& x, const A& a)

diff --git a/test/Jamfile.v2 b/test/Jamfile.v2
@@ -1235,6 +1235,11 @@ test-suite misc :
       [ run git_issue_573.cpp : : : <toolset>msvc:<cxxflags>-sdl ]
       [ run git_issue_576.cpp : : : [ check-target-builds ../config//has_float128 : <define>TEST_FLOAT128 <source>quadmath : ] ]
       [ run git_issue_595.cpp : : : [ check-target-builds ../config//has_mpfr : <source>gmp <source>mpfr : <build>no ] ]
+      [ run git_issue_604.cpp : : : [ check-target-builds ../config//has_mpfr : <source>gmp <source>mpfr <define>TEST_MPFR_50 ]
+                                    [ check-target-builds ../config//has_gmp : <source>gmp <define>TEST_MPF_50 ]
+                                    [ check-target-builds ../config//has_float128 : <source>quadmath <define>TEST_FLOAT128 ]
+                                    <define>TEST_CPP_DEC_FLOAT 
+                                    <define>TEST_CPP_BIN_FLOAT ]
       [ compile git_issue_98.cpp : 
          [ check-target-builds ../config//has_float128 : <define>TEST_FLOAT128 <source>quadmath : ]
          [ check-target-builds ../config//has_gmp : <define>TEST_GMP <source>gmp : ]

diff --git a/test/git_issue_604.cpp b/test/git_issue_604.cpp
@@ -0,0 +1,96 @@
+///////////////////////////////////////////////////////////////////////////////
+//  Copyright 2024 John Maddock. Distributed under the Boost
+//  Software License, Version 1.0. (See accompanying file
+//  LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#if !defined(TEST_MPF_50) && !defined(TEST_CPP_DEC_FLOAT) && !defined(TEST_MPFR_50) && !defined(TEST_FLOAT128) && !defined(TEST_CPP_BIN_FLOAT)
+#define TEST_MPF_50
+#define TEST_MPFR_50
+#define TEST_CPP_DEC_FLOAT
+#define TEST_FLOAT128
+#define TEST_CPP_BIN_FLOAT
+
+#ifdef _MSC_VER
+#pragma message("CAUTION!!: No backend type specified so testing everything.... this will take some time!!")
+#endif
+#ifdef __GNUC__
+#pragma warning "CAUTION!!: No backend type specified so testing everything.... this will take some time!!"
+#endif
+
+#endif
+
+#if defined(TEST_MPF_50)
+#include <boost/multiprecision/gmp.hpp>
+#endif
+#if defined(TEST_MPFR_50)
+#include <boost/multiprecision/mpfr.hpp>
+#endif
+#ifdef TEST_CPP_DEC_FLOAT
+#include <boost/multiprecision/cpp_dec_float.hpp>
+#endif
+#ifdef TEST_FLOAT128
+#include <boost/multiprecision/float128.hpp>
+#endif
+#ifdef TEST_CPP_BIN_FLOAT
+#include <boost/multiprecision/cpp_bin_float.hpp>
+#endif
+
+#include <iostream>
+#include "test.hpp"
+
+template <class T>
+void test()
+{
+   {
+      const T val1{6};
+      const T val2{3};
+      T       r = boost::multiprecision::fmod(val1, val2);
+      BOOST_CHECK_EQUAL(r, T(0));
+   }
+   {
+      const T val1{-6};
+      const T val2{3};
+      T       r = boost::multiprecision::fmod(val1, val2);
+      BOOST_CHECK_EQUAL(r, T(0));
+   }
+   {
+      const T val1{6};
+      const T val2{-3};
+      T       r = boost::multiprecision::fmod(val1, val2);
+      BOOST_CHECK_EQUAL(r, T(0));
+   }
+   {
+      const T val1{-6};
+      const T val2{-3};
+      T       r = boost::multiprecision::fmod(val1, val2);
+      BOOST_CHECK_EQUAL(r, T(0));
+   }
+}
+
+
+
+int main()
+{
+   using namespace boost::multiprecision;
+
+#ifdef TEST_MPF_50
+   test<boost::multiprecision::mpf_float_50>();
+   test<boost::multiprecision::mpf_float_100>();
+#endif
+#ifdef TEST_MPFR_50
+   test<boost::multiprecision::mpfr_float_50>();
+   test<boost::multiprecision::mpfr_float_100>();
+#endif
+#ifdef TEST_CPP_DEC_FLOAT
+   test<boost::multiprecision::cpp_dec_float_50>();
+   test<boost::multiprecision::cpp_dec_float_100>();
+#endif
+#ifdef TEST_FLOAT128
+   test<boost::multiprecision::float128>();
+#endif
+#ifdef TEST_CPP_BIN_FLOAT
+   test<boost::multiprecision::cpp_bin_float_50>();
+   test<boost::multiprecision::number<boost::multiprecision::cpp_bin_float<35, boost::multiprecision::digit_base_10, std::allocator<char>, long long> > >();
+#endif
+   return boost::report_errors();
+}