[Boost-bugs] [Boost C++ Libraries] #12559: sqrt for cpp_int_backend behaves incorrect for small argument values

Subject: [Boost-bugs] [Boost C++ Libraries] #12559: sqrt for cpp_int_backend behaves incorrect for small argument values
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2016-10-27 09:51:04

#12559: sqrt for cpp_int_backend behaves incorrect for small argument values
-------------------------------------------+----------------------------
 Reporter:  Rüdiger Brünner <rbruenner@…>  |      Owner:  johnmaddock
     Type:  Bugs                           |     Status:  new
Milestone:  To Be Determined               |  Component:  multiprecision
  Version:  Boost 1.62.0                   |   Severity:  Problem
 Keywords:  multiprecision integer sqrt    |
-------------------------------------------+----------------------------
 The sqrt function for multiprecision integers does not work for small
 values.
 Using this code:
 {{{
 checked_int128_t int1;
 for (int i = 10; i >= 0; i--)
 {
         int1 = i;
         try
         {
                 int1 = sqrt(int1);
                 std::cout << "sqrt(" << i << ") = " << int1 << std::endl;
         }
         catch (std::exception& ex)
         {
                 std::cout << "sqrt(" << i << ") -> " << typeid(ex).name()
 << ": " << ex.what() << std::endl;
         }
 }
 }}}

 I get the following output:
 {{{
 sqrt(10) = 3
 sqrt(9) = 3
 sqrt(8) = 2
 sqrt(7) = 2
 sqrt(6) = 2
 sqrt(5) = 2
 sqrt(4) = 2
 sqrt(3) -> class boost::exception_detail::clone_impl<struct
 boost::exception_detail::error_info_injector<class std::overflow_error> >:
 Unable to allocate sufficient storage for the value of the result: value
 overflows the maximum allowable magnitude.
 sqrt(2) -> class boost::exception_detail::clone_impl<struct
 boost::exception_detail::error_info_injector<class std::overflow_error> >:
 Unable to allocate sufficient storage for the value of the result: value
 overflows the maximum allowable magnitude.
 sqrt(1) = 0
 sqrt(0) = 0
 }}}

 While sqrt(3) and sqrt(2) are raising an exception, sqrt(1) gives a wrong
 result.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/12559>
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